Method for harvesting biomass from a photobioreactor Field of invention The present invention relates to a method for harvesting biomass from a

photobioreactor and to a system comprising a plurality of photobioreactors. A method of manufacturing a photobioreactor that can be used in such methods and systems is also disclosed. Background of invention

Phototrophic microorganisms are already today found in many commercial applications. Algae are produced to manufacture β-carotene, astaxanthin, etc., or the complete algae biomass is sold as nutritional supplement. Today, the production of algae biomass faces two main difficulties. First, a large part of the current production results from open systems (e.g., so-called open ponds). These open systems are sensitive to contaminations by other algae strains or by pest, therefore only algae with very specific growth requirements can be grown in these systems. Thus, for instance the alga Dunaliella is cultured for the production of β-carotene under very saline conditions, which are not suited for most other organisms. Second, the production costs of algae biomass is rather high (> USD 2,000 per metric ton), so that a commercial production for many applications, especially in the energy sector or the transportation sector, is not profitable. In particular, production costs are often increasing even more if closed systems are used instead of open systems to avoid contaminations. Besides the open ponds, a large number of various photobioreactor types are presently in use. Tube reactors, which can consist of one or more horizontal tubes, or wherein a tube is helically wound around a cylinder or cone (biocoil), are among the best known. Furthermore, flat panel reactors are often used, such reactors providing a vertical liquid layer for cultivation of algae.

The main challenges in the production of chemicals and energy from algae are the risk of contamination and the high cost for the manufacturing of the algae biomass.

Likewise, the main challenges in the production of fine chemicals, nutritional supplements, vitamins, omega-3-fatty acids, antioxidants (e.g. carotenoids), pharmaceutically active substances or dried biomass for nutritional supplementation from algae are thus the risk of contamination and the high cost for the manufacturing of the biomass. The same challenges apply when culturing algae for biofuels, animal feed, amino acids, methane production, etc. WO2009/090549 discloses a photobioreactor for cultivation of a phototrophic microorganism, where the photobioreactor is partially or completely surrounded by a body of water. The position of the photobioreactor is regulated by providing a density difference between the culture liquid comprised in the photobioreactor and the body of water. Harvesting occurs by conventional means via a valve, and the harvested biomass is directed to a tank. The photobioreactor comprises a closed algae compartment defined by walls of a water tight transparent, flexible and light weight material. It can be equipped with one or more additional compartments or tubes adapted to further control the buoyancy of the photobioreactor. The algae compartment of the photobioreactor can also comprise two or more sub-compartments adapted to comprise the culture liquid, to help stabilise the photobioreactor.

Although the photobioreactor disclosed in WO2009/090549 leads to a significant reduction in the costs for producing biomass from algae, large-scale cultivation with a plurality of photobioreactors gives rise to significant costs related to the harvesting. Indeed, a system comprising several photobioreactors where the biomass from each photobioreactor is harvested independently as described in WO2009/090549 requires a large amount of tubing to connect each reactor to the harvesting tank. Likewise, providing fresh culture liquid to each photobioreactor independently also leads to large amounts of materials to provide sufficient tubing to connect each reactor to the feeding tank.

There is thus a need for a method for harvesting biomass which further reduces the expenses generated by large-scale cultivation systems. Summary of invention

The present invention solves the problem of further reducing operating costs of operating a photobioreactor by taking advantage of the photobioreactor's structure itself. Photobioreactors can be made from a top sheet and a bottom sheet which are welded together at the edges, thereby defining a central compartment. By welding the top sheet and the bottom sheet twice on each side of the photobioreactor, an additional, peripheral compartment can be defined, which runs at least along part of the edge of the central compartment. This peripheral compartment can be connected via a first fluid connection to the central compartment, to allow fluid exchange between the two compartments when the first fluid connection is activated. The peripheral compartment is also connected to a harvester and/or to a feeder via a further fluid connection. The connection may be direct, i.e. the fluid to be harvested passes directly into the harvester, or indirect, i.e. the fluid to be harvested may need to pass through further peripheral compartments from further photobioreactors before reaching the harvester. This arrangement allows the peripheral compartment to be used as tubing for harvesting the biomass produced in the central compartment. The peripheral compartment can also be used as tubing for providing fresh culture liquid to the central compartment. The present method thus requires only limited tubing compared to what is required in previously known harvesting and feeding methods.

Herein is thus provided a method for harvesting biomass, said method comprising the steps of:

i. providing at least one photobioreactor partially or completely surrounded by a body of water, said photobioreactor comprising a central compartment holding a culture liquid comprising a mixotrophic microorganism such as a phototrophic microorganism, and at least one peripheral compartment along at least part of the edge of the central compartment, wherein said photobioreactor comprises at least one first fluid connection between the central compartment and the peripheral compartment, and wherein the peripheral compartment comprises at least one further fluid connection, where the opening and closing of the first fluid connection and optionally of the further fluid connection can be controlled;

ii. opening said first fluid connection, thereby allowing at least part of the

culture liquid to flow from the central compartment to the peripheral compartment;

iii. opening said further fluid connection and activating a harvester, thereby allowing the culture liquid to flow from the peripheral compartment through the further fluid connection, wherein the further fluid connection directly or indirectly connects the peripheral compartment of one photobioreactor to said harvester. Also provided is a system comprising at least one closed photobioreactor for large- scale cultivation of mixotrophic microorganisms such as phototrophic microorganisms, wherein:

- each photobioreactor is adapted to be partially or completely surrounded by a body of water;

- each photobioreactor comprises a central compartment for holding a culture liquid therein and a peripheral compartment along at least part of the edge of the central compartment;

- each photobioreactor comprises a first fluid connection between the central and the peripheral compartment, of which the opening and closing can be controlled;

- each peripheral compartment comprises at least one further fluid connection, of which the opening and closing can optionally be controlled;

- the peripheral compartment of each photobioreactor is connectable to a

harvester via said at least one further fluid connection.

Also provided is a method of manufacturing a photobioreactor, comprising the steps of: i. Providing a top sheet and a bottom sheet of a material which is flexible, water-tight and nutrient-impermeable;

ii. Providing at least one first fluid connection and at least one further fluid connection;

iii. Placing said top sheet on said bottom sheet and said first fluid connection and further fluid connection therebetween;

iv. Welding the top and bottom sheet together in a first position along their length, thereby defining a central compartment,

v. Welding the top and bottom sheet together in a second position along their length, where the first position is different from the first position, thereby defining at least one peripheral compartment along at least part of the edge of the central compartment,

in such a manner that the first fluid connection connects the central and the peripheral compartment and the further fluid connection can be configured to directly or indirectly connect the peripheral compartment to the harvester and/or feeder. Description of Drawings

Figure 1 a is a schematic view of a photobioreactor as described in WO2009/090549. Figure 1 b is a cross-sectional view of a photobioreactor as described in

WO2009/090549.

Figure 2a shows a three-dimensional view of a photobioreactor with a C0 ₂ supply tube floating on the culture liquid compartment due to its low density as described in WO2009/090549.

Figure 2b displays a vertical cross-section of a photobioreactor with a C0 ₂ supply tube floating on the culture liquid due to its low density as described in WO2009/090549.

Figure 3 shows a photobioreactor with an additional compartment for controlling vertical position and or the shape of the reactor as described in WO2009/090549.

Figure 4a is a vertical cross-section of a photobioreactor of the present disclosure with a central compartment, a peripheral compartment, two first fluid connections and two further fluid connections.

Figure 4b is a horizontal cross-section of part of the photobioreactor represented in fig.

4a.

Figure 5 shows a horizontal cross-section of a photobioreactor of the present disclosure with a central compartment and a peripheral compartment.

Figure 6 shows a horizontal cross-section of a system of the present disclosure comprising two photobioreactors, each comprising a central compartment and a peripheral compartment.

Figure 7 shows schematic representations of systems of the present disclosure comprising a plurality of photobioreactors in different arrangements. The central photobioreactor in each arrangement is connected to each of the surrounding photobioreactors (connections not shown on the figure).

Detailed description of the invention The invention is as defined in the claims.

In a first aspect, the invention relates to a method for harvesting biomass, said method comprising the steps of:

i. providing at least one photobioreactor partially or completely surrounded by a body of water, said photobioreactor comprising a central compartment „ holding a culture liquid comprising a mixotrophic microorganism such as a phototrophic microorganism, and at least one peripheral compartment along at least part of the edge of the central compartment, wherein said photobioreactor comprises at least one first fluid connection between the central compartment and the peripheral compartment, and wherein the peripheral compartment comprises at least one further fluid connection, where the opening and closing of the first fluid connection and optionally of the further fluid connection can be controlled;

ii. opening said first fluid connection, thereby allowing at least part of the

culture liquid to flow from the central compartment to the peripheral compartment;

iii. opening said further fluid connection and activating a harvester, thereby allowing the culture liquid to flow from the peripheral compartment through the further fluid connection, wherein the further fluid connection directly or indirectly connects the peripheral compartment of one photobioreactor to said harvester.

In a second aspect is provided a system comprising at least one closed

photobioreactor for large-scale cultivation of a mixotrophic microorganism such as a phototrophic microorganism, wherein:

- each photobioreactor is adapted to be partially or completely surrounded by a body of water;

- each photobioreactor comprises a central compartment for holding a culture liquid therein and a peripheral compartment along at least part of the edge of the central compartment;

- each photobioreactor comprises a first fluid connection between the central and the peripheral compartment, of which the opening and closing can be controlled;

- each peripheral compartment comprises at least one further fluid connection, of which the opening and closing can optionally be controlled;

- the peripheral compartment of each photobioreactor is connectable to a

harvester via said at least one further fluid connection.

In a third aspect is provided a method of manufacturing a photobioreactor, comprising the steps of: i. Providing a top sheet and a bottom sheet of a material which is flexible, water-tight and nutrient-impermeable;

ii. Providing at least one first fluid connection and at least one further fluid connection;

iii. Placing said top sheet on said bottom sheet and said first fluid connection and further fluid connection therebetween;

iv. Welding the top and bottom sheet together in a first position along their length, thereby defining a central compartment,

v. Welding the top and bottom sheet together in a second position along their length, where the first position is different from the first position, thereby defining at least one peripheral compartment along at least part of the edge of the central compartment,

in such a manner that the first fluid connection connects the central and the peripheral compartment and the further fluid connection can be configured to directly or indirectly connect the peripheral compartment to the harvester and/or feeder.

Photobioreactor

The present method is particularly advantageous when photobioreactors partially or completely surrounded by a body of water are used. Non-limiting examples of such bioreactors are known from WO2009/090549, WO2014/209935 and WO2010/121 136. Such photobioreactors are advantageous for several reasons. First, operating costs are reduced because the body of water can be used to regulate temperature of the culture liquid comprised within the photobioreactor, thereby reducing energy consumption. The fact that the photobioreactor is floating on or is surrounded by a body of water automatically gives a homogenous distribution of the thickness of the culture liquid, thereby ensuring that there are no "dead volumes" in the photobioreactor - productivity is thus increased. Furthermore, the hydrostatic inner pressure of the photobioreactor being partly compensated by the surrounding water, the strength of the walls of the photobioreactor can be reduced or a less stable material can be used, thereby also reducing production costs.

The term photobioreactor, as used herein, generally refers to a photobioreactor comprising at least one central compartment adapted for holding a culture liquid comprising a mixotrophic microorganism, and at least one peripheral compartment along at least part of the edge of the central compartment. Preferably, the

photobioreactor is essentially flat. When the photobioreactor is operating, the central compartment holds culture liquid wherein a mixotrophic microorganism is cultured. The photobioreactor may also comprise additional compartments or tubes, sub- compartments or mechanical means for controlling the position and/or shape of the photobioreactor, as well as peripheral equipment such as e.g. pumps, hoses, tanks and other equipment required for operating the reactor, for example sensors.

The term mixotroph herein refers to an organism that can use a mix of different sources of energy and carbon. A mixotrophic microorganism may be phototrophic and chemotrophic; lithotrophic and organotrophic; autotrophic and heterotrophic;

autotrophic, heterotrophic and phototrophic, chemotrophic, lithotrophic or

organotrophic. Other combinations of the above trophisms are also envisaged.

Mixotrophs can be either eukaryotic or prokaryotic. In the present context, the mixotrophic microorganism is at least capable of performing photosynthesis when exposed to light. The phototrophic microorganism may also be capable of heterotrophic growth when not exposed to light. The mixotrophic microorganism as understood herein is thus at least capable of photosynthetic growth, but may also be any of a chemotroph, a lithotroph, an organotroph, an autotroph and a heterotroph. Each trophic mode may be obligate, i.e. necessary for sustaining growth and/or maintenance of the microorganism, or facultative.

In practice, the photobioreactor is manufactured from a top sheet and a bottom sheet of a flexible, nutrient-impermeable and water-tight material. The top sheet and bottom sheet are placed on each other, and the central compartment is defined by welding the sheets together close to the edge. By performing a first welding in a first position and a second welding in a second position, at least one peripheral compartment is defined between the central compartment and the edge of the photobioreactor. Both

compartments are water-tight and nutrient-impermeable. The peripheral compartment may run along the entire edge of the central compartment, or it may run along part of the edge of the central compartment.

This particular structure allows operations such as the harvesting of biomass from and/or the provision of fresh culture liquid medium to the central compartment by using the peripheral compartments instead of using extensive tubing. Particularly, a method for harvesting biomass is disclosed, said method comprising the steps of:

i. providing at least one photobioreactor partially or completely surrounded by a body of water, said photobioreactor comprising a central compartment holding a culture liquid comprising a mixotrophic microorganism, and at least one peripheral compartment along at least part of the edge of the central compartment, wherein said photobioreactor comprises at least one first fluid connection between the central compartment and the peripheral compartment, and wherein the peripheral compartment comprises at least one further fluid connection, where the opening and closing of the first fluid connection and optionally of the further fluid connection can be controlled; ii. opening said first fluid connection, thereby allowing at least part of the culture liquid to flow from the central compartment to the peripheral compartment;

iii. opening said further fluid connection and activating a harvester, thereby allowing the culture liquid to flow from the peripheral compartment through the further fluid connection, wherein the further fluid connection directly or indirectly connects the peripheral compartment of one photobioreactor to said harvester.

Connection between the central compartment and the peripheral compartment

In order to allow fluid connection between the central compartment and the peripheral compartment of a given photobioreactor, the compartments are connected via at least one first fluid connection, of which the opening and closing can be controlled.

Accordingly, when the first fluid connection is closed, no fluid exchange occurs between the central compartment and the peripheral compartment. When the first fluid connection is open, fluid exchange is enabled between the central compartment and the peripheral compartment. As understood herein, the term "fluid" preferably refers to a liquid. Thus opening the first fluid connection may allow culture liquid containing biomass to flow from the central compartment to the peripheral compartment. The peripheral compartment can thus be used to harvest biomass from a photobioreactor. Alternatively, or additionally, the opening may allow culture liquid to flow from the peripheral compartment to the central compartment. This can be used to replenish the culture liquid comprised in the central compartment upon nutrient depletion. The peripheral compartment may thus be used to provide fresh culture liquid to the central compartment.

The term "fresh culture liquid" shall be construed as any liquid which results in provision of nutrients to the central compartment. In some embodiments, the fresh culture liquid may differ from the liquid that was initially provided in the central compartment. The fresh culture liquid may vary and will depend on the nutrient requirements of the mixotrophic microorganism for optimal growth under given conditions.

The first fluid connection may be bidirectional, allowing fluid exchange from the central compartment to the peripheral compartment and from the peripheral compartment to the central compartment, or unidirectional, allowing fluid exchange either from the central compartment to the peripheral compartment or from the peripheral

compartment to the central compartment. The first fluid connection may be an automatic valve, for example an electrovalve, or a manual valve.

It will be understood that the peripheral compartment may run along the entire edge of the central compartment. For example, in the case of a rectangular photobioreactor comprising a rectangular central compartment, the peripheral compartment may run along all four sides of the central compartment. However, it is also possible for the peripheral compartment to run along one, two or three sides of the central

compartment. As non-limiting examples, the peripheral compartment may also run along part of the edge of the central compartment, e.g. along part of the edge of one side, part of the edge of two sides, along the whole of one side and part of the edge of one or two other sides. Similarly, in embodiments where the photobioreactor is round or elliptical, the peripheral compartment may run along the entire edge or along part of the edge.

Connection between the peripheral compartment and the environment

In order to allow fluid connection between the peripheral compartment and the environment, the peripheral compartment comprises at least one further fluid connection. Optionally, the opening and closing of the further fluid connection can be controlled. Accordingly, when the further fluid connection is closed, no fluid exchange occurs between the peripheral compartment and the environment. When the further fluid connection is opened, fluid exchange can occur. Accordingly, the liquid comprised within the peripheral compartment can flow from the peripheral compartment to the environment, or fluids from the environment can flow into the peripheral compartment. The term "environment" herein refers to devices such as a harvester or a feeder as described below, or generally any device which is not directly part of the

photobioreactor. Such harvester or feeder may comprise pumps or containers, as detailed below. The first fluid connection may be an automatic valve, for example an electrovalve, or a manual valve. The peripheral compartment may be directly or indirectly connected to a harvester. Activation of the harvester may automatically activate opening of the further fluid connection, thereby allowing the fluids comprised within the peripheral compartment to flow therefrom to the harvester via the further fluid connection. Preferably, the harvester are located downstream of the photobioreactor. In some embodiments, the harvester may also automatically activate opening of the first fluid connection, thereby allowing at least part of the culture liquid comprising biomass comprised within the central compartment to flow therefrom to the peripheral compartment via the first fluid connection, and allowing said part of the biomass-comprising liquid to flow further from the peripheral compartment to the harvester via the further fluid connection. The harvester may comprise a pump. The harvester may comprise a container for storing the harvested biomass. The harvester may further comprise or be connected to a device configured for separating the harvested biomass from the culture liquid. In embodiments where the microorganisms produce any of the products described herein elsewhere, the harvester further comprise or are connected to a device configured for separating and/or recovering such products from the biomass and/or from the culture liquid.

In some embodiments, the connection between the peripheral compartment and the harvester is a direct connection, i.e. the harvester are in direct continuation of the further fluid connection. In other embodiments, the connection between the peripheral compartment and the harvester is an indirect connection, i.e. the harvester are not in direct continuation of the further fluid connection. Instead, the further fluid connection may connect the peripheral compartment of a first photobioreactor to the peripheral compartment of a second photobioreactor, where the second photobioreactor comprises at least one further fluid connection which is directly connected to the harvester. Alternatively, the peripheral compartment of the second photobioreactor may in turn be connected via at least one further fluid connection to the peripheral compartment of a third photobioreactor, which comprises at least one further fluid connection which is directly connected to the harvester. The peripheral compartments of a plurality of photobioreactors may thus be connected together via their further fluid connection, where the photobioreactor which is located at the most downstream position is directly connected to the harvester and the remaining photobioreactors are all indirectly connected to the harvester via their peripheral compartments and the peripheral compartment of the photobioreactor in the most downstream position.

The skilled person will realise that when using photobioreactors of a rectangular shape and of similar sizes, each photobioreactor can be connected to up to eight

photobioreactors. In one embodiment, each photobioreactor is connected as described above to at least one other photobioreactor. In one embodiment, each photobioreactor is connected to at least two other photobioreactors. In one embodiment, each photobioreactor is connected to at least three other photobioreactors. In one embodiment, each photobioreactor is connected to at least four other photobioreactors. In one embodiment, each photobioreactor is connected to at least five other photobioreactors. In one embodiment, each photobioreactor is connected to at least six other photobioreactors. In one embodiment, each photobioreactor is connected to at least seven other photobioreactors. In one embodiment, each photobioreactor is connected to at least eight other photobioreactors. Non-limiting examples of such arrangements are shown in fig. 7 for illustrative purposes only. In systems comprising a plurality of photobioreactors, as detailed below, the photobioreactors may be arranged in series or in parallel, or in a grid.

The peripheral compartment may also be connected to a feeder. Activation of the feeder may automatically activate opening of the further fluid connection, thereby allowing the fluids to flow from the feeder to the peripheral compartment via the further fluid connection. In some embodiments, the feeder may also automatically activate opening of the first fluid connection, thereby allowing fresh culture liquid to be provided from the peripheral compartment to the central compartment via the first fluid connection. The feeder preferably comprises fresh culture liquid as defined above. The fresh culture liquid may be stored in a container, connected to the peripheral compartment of a photobioreactor. Upon opening of the further fluid connection, the fresh culture liquid flows from the feeder to the peripheral compartment. Upon opening of the first fluid connection, the fresh culture liquid flows from the peripheral

compartment to the central compartment, thereby providing nutrients to the

microorganism.

In some embodiments, the connection between the peripheral compartment and the feeder is a direct connection, i.e. the further fluid connection is in direct continuation of the feeder. Preferably, the feeder are located upstream of the photobioreactor. In other embodiments, the connection between the peripheral compartment and the feeder is an indirect connection, i.e. the further fluid connection is not in direct continuation of the feeder. Instead, the further fluid connection may connect the peripheral compartment of a first photobioreactor to the peripheral compartment of a second photobioreactor, where the second photobioreactor comprises at least one further fluid connection which is directly connected to the feeder. Alternatively, the peripheral compartment of the second photobioreactor may in turn be connected via at least one further fluid connection to the peripheral compartment of a third photobioreactor, which comprises at least one further fluid connection which is directly connected to the feeder. The peripheral compartments of a plurality of photobioreactors may thus be connected together via their further fluid connection, where the photobioreactor which is located at the most upstream position is connected to the feeder.

The further fluid connection may be bidirectional, allowing fluid exchange from the peripheral compartment to the harvester and/or feeder and from the harvester and/or feeder to the peripheral compartment, or unidirectional, allowing fluid exchange from the peripheral compartment to the harvester and/or feeder or from the harvester and/or feeder to the peripheral compartment.

Harvesting

In some embodiments, the peripheral compartment is used for harvesting at least part of the biomass comprised in the central compartment. In order to determine when said biomass should be harvested, the photobioreactor may be equipped with a sensor providing information as to the density of the culture. The sensor is preferably placed in the central compartment. The sensor may be connected to a monitoring system, measuring and analysing process parameters such as temperature, salinity, pH, C0 ₂ concentration, 0 ₂ concentration and biomass concentration. Suitable sensors and monitoring systems will be readily recognised by the skilled person. Preferably, the harvester comprises or is connected to a container for holding the harvested biomass.

In order to determine when biomass is to be harvested, the present method may further comprise the step of measuring biomass density in the photobioreactor, comparing said measured value with a reference value, and harvesting at least part of the biomass from the photobioreactor where the measured value is equal to or greater than said reference value. The biomass may also be harvested at predefined time intervals. When biomass is to be harvested, the first fluid connection is opened, as detailed herein elsewhere. Fluid exchange is thus possible between the central compartment and the peripheral compartment, whereby at least part of the culture liquid comprising biomass flows to the peripheral compartment. Said fluid exchange can be facilitated by the means described below. The peripheral compartment now comprises culture liquid comprising biomass. Activation of the further fluid connection, resulting in its opening, and of the harvester results in fluid exchange between the peripheral compartment and the harvester. As described above, the fluid exchange between the peripheral compartment of one photobioreactor and the harvester may involve that the liquid to be harvested be transferred from the peripheral compartment of a first photobioreactor to the peripheral compartment of at least one further photobioreactor, in cases where the peripheral compartment is indirectly connected to the harvester. The peripheral compartment may be directly connected to the harvester. The harvester is preferably placed downstream of the photobioreactor from which biomass is to be harvested. The harvester preferably helps facilitate fluid exchange. In one embodiment, the harvester comprises a pump, such as an automatic pump or a manual pump, connected to a container for holding the harvested biomass. In other embodiments, gravity is taken advantage of, and the harvester merely comprises a container for holding the harvested biomass, but do not comprise a pump. For example, a siphon may be used. Alternatively, gravity and a pump are used together. In such

embodiments where gravity is taken advantage of, the peripheral compartment of the photobioreactor from which biomass is to be harvested may be divided in sub- compartments, wherein only the sub-compartments located on one same side of the photobioreactor are connected to the central compartment via a first fluid connection. When the first fluid connection is opened, the liquid entering the sub-compartment on one side of the photobioreactor has a higher gravity than the liquid in the sub- compartment on the opposite side of the photobioreactor. This difference may cause tilting of the photobioreactor, whereby the liquid, due to gravity, can flow to the container for holding the biomass via the further fluid connection more easily. In embodiments where the harvesting is facilitated by gravity, harvesting may be initiated or further supported by a pump to facilitate initiation of the fluid exchange.

In systems comprising a plurality of photobioreactors, the biomass of each

photobioreactor may be harvested independently of any other photobioreactor, simply by opening only the first fluid connection of the photobioreactor from which biomass is to be harvested while maintaining the first fluid connections of the other

photobioreactors closed.

The harvester may comprise or be connected to a harvesting compartment, such as a container. The harvesting compartment may be equipped with additional devices facilitating the separation of the harvested biomass from the culture liquid, and/or facilitating the conversion of the harvested biomass intro the product of interest.

In preferred embodiments, there is no direct connection between the harvester and the central compartment.

Providing fresh culture medium

The peripheral compartment of a given photobioreactor may be used not only for harvesting biomass but also for providing nutrients to the central compartment. This can be done essentially as described for harvesting, except that the direction of fluid exchange is reversed. Preferably, the feeder comprises or is connected to a container for holding fresh culture liquid or liquid comprising nutrients.

In some embodiments, the peripheral compartment is used for providing nutrients to the microorganism comprised in the culture liquid in the central compartment. In order to determine when to replenish the culture liquid of the central compartment, the photobioreactor may be equipped with a sensor providing information as to process parameters, such as nutrient concentration. The sensor is preferably placed in the central compartment. The sensor may be connected to a monitoring system, measuring and analysing process parameters such as temperature, salinity, pH, C0 ₂ concentration, 0 ₂ concentration and biomass concentration. Sensors and monitoring systems suitable in the present method will be readily recognised by the skilled person. In order to determine when the culture liquid within the central compartment should be replenished, the present method may further comprise the step of measuring process parameters of the culture liquid in the central compartment of the photobioreactor, comparing said measured value with a reference value, and providing fresh culture liquid to the photobioreactor where the measured value is different from said reference value. The fresh culture liquid may also be harvested at predefined time intervals.

When nutrients are to be provided to the culture liquid in the central compartment, the further fluid connection is activated, which results in its opening. The liquid comprising nutrients is provided from the feeder to the peripheral compartment via the further fluid opening. The fluid exchange can be facilitated by the means described below. The peripheral compartment now comprises fresh culture liquid. Activation of the first fluid connection, resulting in its opening, results in fluid exchange between the peripheral compartment and the central compartment. As described above, the fluid exchange between the feeder the peripheral compartment of one photobioreactor may involve that the liquid to be provided to the central compartment be first transferred from the peripheral compartment of a first photobioreactor to the peripheral compartment of at least one further photobioreactor, in the cases where the peripheral compartment of the photobioreactor to be provided with fresh culture liquid is indirectly connected to the feeder. The feeder is preferably placed upstream of the photobioreactor, the central compartment of which is to be provided with fresh culture liquid or nutrients.

The feeder preferably helps facilitate fluid exchange. In one embodiment, the feeder comprise a pump, such as an automatic pump or a manual pump, connected to a container for holding the liquid to be provided to the photobioreactor. In other embodiments, gravity is taken advantage of, and the feeder merely comprises a container for holding the culture liquid, but do not comprise a pump. Alternatively, gravity and a pump are used together.

In systems comprising a plurality of photobioreactors, the central compartment of each photobioreactor may be provided with fresh culture liquid independently of any other photobioreactor, simply by maintaining the first fluid connections of said other photobioreactors closed.

The steps of harvesting and/or feeding culture liquid medium may be performed in a continuous or in a semi-continuous manner.

Harvesting biomass and providing fresh culture medium

In some embodiments, the peripheral compartment of a photobioreactor is not divided in sub-compartments. In such embodiments, the step of harvesting biomass from the central compartment and the step of providing fresh culture liquid to the central compartment happen in a sequential manner.

In other embodiments, the peripheral compartment of a photobioreactor is divided in two sub-compartments or more. This is achieved e.g. by welding the top sheet and the bottom sheet of the photobioreactor all the way to the outer edge of the peripheral compartment on two sides, so that there are effectively two adjacent distinct peripheral sub-compartments, both directly adjacent to the central compartment. In some embodiments, fluid exchange between these compartments is not possible. In other embodiments, the sub-compartments may be connected via a fluid connection to allow fluid exchange in a controlled manner if this is desirable. The photobioreactor can thus be converted from a photobioreactor with one peripheral compartment (when said fluid connection is open) to a photobioreactor with two or more sub-compartments (when said fluid connection is closed). The fluid connection can be of the kind described herein elsewhere.

Some sub-compartments may thus be used preferably for harvesting, while other sub- compartments may be used preferably for providing fresh culture liquid to the central compartment. In such embodiments, the steps of harvesting biomass and providing fresh culture liquid may occur simultaneously, or they may occur sequentially and independently of each other.

Peripheral compartment

The at least one peripheral compartment may be a plurality of peripheral

compartments. In some embodiments, the photobioreactor comprises two or more peripheral compartments arranged concentrically around the central compartment. In such embodiments, the central compartment is surrounded by a first peripheral compartment running along is edge, and itself surrounded by a second peripheral compartment.

Fluid exchange between the central compartment and the first peripheral compartment is controlled by way of the first fluid connection, as described above. Fluid exchange between the outermost peripheral compartment and the environment is controlled as described above by a further fluid connection. Fluid exchange between two adjacent peripheral compartments is controlled by additional fluid connections, of which the opening and closing can be controlled, and which may be any fluid connection as described herein elsewhere for the first and the further fluid connection. In one embodiment, the photobioreactor comprises two peripheral compartments. In another embodiment, the photobioreactor comprises three peripheral compartments.

The photobioreactor used in the present method may be any photobioreactor known to the skilled person as being suitable for culture of microorganisms.

Supplying CO? to the culture liquid of the photobioreactor

Algae require for their growth large amounts of C0 ₂ since they use this as a key source of carbon. Furthermore, in the process of photosynthesis, oxygen is produced, which might be toxic to the algae. The mass transfer of these gases across the liquid-gas barrier is therefore crucial for high productivity. A number of possible ways of providing C0 ₂ to the algae culture, and for removing formed oxygen from the same will be described hereinbelow. The methods described herein should not be construed as limiting to the present invention. Other methods that may also be apparent to a person skilled in the art in the light of the present disclosure are also considered to be within the scope of the present invention. In an embodiment, mass transfer of C0 ₂ to the culture medium is achieved via passive diffusion of gaseous C0 ₂ over a large surface area of the culture medium. Assuming that the kinetics of diffusion processes as described by Fick's first and second law and the subsequent hydration and deprotonization processes are fast enough to provide the algae culture with enough C0 ₂ and to avoid toxic effects of 0 ₂ via photo-oxidation, a passive diffusion of C0 ₂ via a large surface would be sufficient. Passive diffusion has the advantage that no energy is needed to move water or to force C0 ₂ into the water. Additionally, investment costs will be reduced since no active aeration would be required. In such a case, C0 ₂ transfer will take place at the interfacial layer between water and the C0 ₂ gas without any more energy added. In a more specific embodiment this could be realized by generating a gas bubble of C0 ₂ rich gas above the culture medium inside the photobioreactor.

In another embodiment, C0 ₂ is bubbled through the culture medium. The gaseous C0 ₂ may preferably be supplied by a tube or a tube-like device, extending into the culture medium. Such a system could comprise holes, through which a C0 ₂ rich gas may be pushed by applying pressure from an external device. The tubes or tube like devices may for example be fixed at the bottom of the reactor and the typical direction of the holes would be into the direction of the water surface. During operation of the photobioreactor, the C0 ₂ rich gas may be supplied continuously to the culture medium. This embodiment also has the additional advantage that it leads to a continuous degassing of oxygen close to statu nascendi, i.e. the oxygen produced is removed from the culture medium shortly after it is formed. Alternatively, the C0 ₂ rich gas can be added in short pulses. Various means exist to determine the length of a pulse, the amount of gas pushed in, the pressure the gas is pushed and the time between pulses. In an embodiment the gas could be pulsed by a timer, which gives a regular signal, e.g. every 5 minutes for a pulse of 1 minute. In another embodiment, the pulse is controlled by a special unit which is capable of estimating the amount of C0 ₂ used by the algae and calculating the optimal length of the pulse, the amount of gas to be pushed in, the pressure the gas is to be pushed in with, and the time between the pulses. To estimate the amount of C0 ₂ required, the unit may comprise different sensors, e.g. a sensor measuring the light intensity, a sensor measuring the temperature and a sensor measuring the biomass density in the photobioreactor. Using the data points received by these sensors a process controller would calculate the optimal pulse pattern for the photobioreactor system.

The amount of added C0 ₂ may also be related to the pH in the reactor. A pH electrode is arranged in the culture medium, and this electrode continuously measures the voltage across a semi-permeable membrane allowing protons to pass the membrane against a defined redox-system, e.g. against an Ag/AgCI electrode. The voltage is registered by a process control unit. The process control unit will add a C0 ₂ pulse as soon as the voltage reaches a predefined point. The parameters of the pulse, such as time, amount of pulses per minute, voltage to stop the pulsing can be entered into the process control unit.

In another embodiment, shown in Figs. 2a and 2b, gaseous C0 ₂ may be supplied to the culture medium by a tube or a tube-like device extending into the photobioreactor and arranged to float on top of the surface of the culture medium due to its lower density. Bubbling of gaseous C0 ₂ is performed similarly as in the case described above, wherein the tube or tube-like device extends into the culture medium. However, the tube or tube-like device through which the C0 ₂ is supplied will be specifically designed to float on the surface of the culture medium in the photobioreactor. This is achieved by the density of the whole (16) aeration system being lower than the density of the algae culture medium (17). The holes (18) in the tube or tube-like device, through which the C0 ₂ is pushed into the culture medium may preferably point downwards in this embodiment to achieve the best possible gas transfer. The holes will thereby be positioned at, or slightly below, the surface of the culture medium. During operation of the photobioreactor, the C0 ₂ rich gas may be supplied continuously to the culture medium. This embodiment also has the additional advantage that it leads to a continuous degassing of oxygen close to statu nascendi, i.e. the oxygen produced is removed from the culture medium shortly after it is formed. Alternatively, the C0 ₂ rich gas can be added in short pulses. Various means exist to determine the length of a pulse, the amount of gas pushed in, the pressure with which the gas is pushed in and the time between pulses. In an embodiment the gas could be pulsed by a timer, which gives a regular signal, e.g. every 5 minutes for a pulse of 1 minute. In another embodiment, the pulse is controlled by a special unit which is capable of estimating the amount of C0 ₂ used by the algae and calculating the optimal length of the pulse, the amount of gas to be pushed in, the pressure the gas is to be pushed in with, and the time between the pulses. To estimate the amount of C0 ₂ required, the unit may comprise different sensors, e.g. a sensor measuring the light intensity, a sensor measuring the temperature and a sensor measuring the biomass density in the photobioreactor. Using the data points received by these sensors a process controller would calculate the optimal pulse pattern for the photobioreactor system.

The amount of added C0 ₂ may also be related to the pH in the reactor. A pH electrode is arranged in the culture medium, and this electrode continuously measures the voltage across a semi-permeable membrane allowing protons to pass the membrane against a defined redox-system, e.g. against an Ag/AgCI electrode. The voltage is registered by a process control unit. The process control unit will add a C0 ₂ pulse as soon as the voltage reaches a predefined point. The parameters of the pulse, such as time, amount of pulses per minute, voltage to stop the pulsing can be entered into the process control unit.

C0 ₂ does not necessarily have to be supplied to the culture medium in the form of gaseous C0 ₂ inside the photobioreactor. The C0 ₂ enriched medium may also be prepared outside the photobioreactor, e.g. by bubbling gaseous C0 ₂ through an aqueous medium. In other words, instead of supplying the C0 ₂ in the transparent part of the photobioreactor this could be done outside of the actual photobioreactor. In an embodiment, such a system may employ a vertical tank containing an aqueous medium wherein C0 ₂ rich gas is supplied at the bottom or close to the bottom of the tank. While bubbles of C0 ₂ rise up through the aqueous medium, C0 ₂ will transfer from the bubbles into the aqueous medium, and at the same time the oxygen may be removed from the culture medium. In a preferred embodiment, the aqueous medium which is enriched with C0 ₂ is culture medium from the photobioreactor which is enriched with C0 ₂ and subsequently returned into the photobioreactor. As the tank may have a height of several meters the residential time of C0 ₂ may be comparatively long, allowing for a good mass transfer. To bubble C0 ₂ into a vertical tank, energy is required to work for instance against the hydrostatic pressure. The energy, which is put in for pressurizing the gas, may also be used to move the aqueous medium from the algae compartment into the C0 ₂ enrichment device and back to the algae

compartment.

In another embodiment, instead of bubbling C0 ₂ through the algae culture medium inside or outside of the photobioreactor, the C0 ₂ supply is facilitated by the use of a semi-permeable membrane. The use of such a membrane would have various advantages compared to the bubbling: a) Such a membrane would work as a one-way-valve, meaning that the membrane would allow C0 ₂ to enter the culture medium, but prevent water from entering into the C0 ₂ supply system, as such a membrane would be permeable for C0 ₂ but not for water.

b) Lower energy consumption. Since no bubble generation is required, the membrane method allows C0 ₂ supply with lower energy consumption compared to the bubbling process.

c) Low shear-stress. By avoiding the bubbling, the shear-stress on the algae cells is reduced. Less shear stress on the algae results in less dead algae cells in the algae culture medium and therefore less organic material which is prone to decomposition which may reduce the efficiency of the photobioreactor. Furthermore, this would significantly reduce the risk of contamination by heterotrophic organisms.

d) Increased mass transfer rate. The use of a membrane allows a higher C0 ₂ pressure than the embodiment employing passive diffusion described above, since the C0 ₂ pressure against the membrane is not limited to the surrounding atmospheric air pressure as it would be the case of passive diffusion. Furthermore, the membrane might have a higher surface area than a flat surface, such as the surface of the culture medium, with the same size as the membrane. The mass transfer of C0 ₂ and/or oxygen may also be facilitated by moving the photobioreactor, e.g. by tilting the reactor.

Hybrid photobioreactor

The photobioreactor may be further adapted so that it not only supports

photosynthesis, but also fermentation. In some cases, it may be desirable to have a hybrid system, where photosynthesis occurs during the day or when the

photobioreactor, and in particular the culture liquid comprising the microorganisms, is illuminated, and heterotrophic fermentation occurs at night or when the photobioreactor is not illuminated. Such hybrid photobioreactors are known in the art. Such

photobioreactors may be further equipped with a nitrogen removal device connected with the central compartment. It may be desirable to remove nitrogen in the medium after photosynthesis, and before switching to heterotrophic fermentation. In some embodiments, at least 90% of the nitrogen is removed, such as at least 91 %, such as at least 92%, such as at least 93%, such as at least 94%, such as at least 95%, such as at least 96%, such as at least 97%, such as at least 98%, such as at least 99%, such as at least 99.5% or more.

In such embodiments, a microorganism capable of both phototrophic and heterotrophic growth is used for producing biomass. Such organisms are known to the skilled person. Other trophic modes may be facultative modes for each specific microorganism.

The term "photosynthetic phase" refers to a period of time during which photosynthesis is performed by the microorganism. It does not exclude that other types of reactions may occur at the same time, for example the term "photosynthetic phase" does not exclude that chemotrophic growth, lithotrophic growth, autotrophic growth,

heterotrophic growth and organotrophic growth occur simultaneously with

photosynthetic growth. Rather, the photosynthetic phase herein refers to a period of time during which the culture liquid comprising the microorganism is exposed to light so that photosynthesis is possible.

Conversely, the term "heterotrophic phase" refers to a period of time during which the culture liquid comprising the microorganism is not exposed to light or is exposed to darkness, so that photosynthesis is not possible, but heterotrophic growth is possible. This does not exclude that chemotrophic growth, lithotrophic growth, autotrophic growth, and organotrophic growth occur simultaneously with heterotrophic growth.

When such hybrid photobioreactors are used, the 0 ₂ produced by the microorganism during photosynthesis is at least partly consumed by the microorganism during the fermentation phase. Conversely, the C0 ₂ produced during the fermentation phase is at least partly consumed by the microorganism during the photosynthetic phase.

The C0 ₂ or 0 ₂ produced during the fermentation phase and the photosynthetic phase, respectively, may be agitated in the culture liquid comprised in the central compartment to optimise growth and/or productivity of the microorganism.

In embodiments where fermentation is taken advantage of, the above section relating to the supply of C0 ₂ to the photobioreactor applies mutatis mutandis to the supply of 0 ₂ in the photobioreactor, as will be obvious to the skilled person. In such embodiments, the production of 0 ₂ and C0 ₂ by the microorganism itself may be at least partly recycled to be consumed by the microorganism.

The water body

The photobioreactor used in the present method is partially or completely surrounded by a body of water. The body of water may be a closed body of water or an open body of water. The water may be brackish water or salt water such as sea water. Reference is made in this respect to WO2009/090549, in particular to the sections entitled "The photobioreactor in a closed body of water" and "The closed photobioreactor in open water".

The term "closed water body" refers to well-defined systems of water allowing control of, e.g. the amount or type, such as fresh, brackish or salt, of water therein. Examples of closed bodies of water are natural or artificial ponds or pools.

The term "open water" refers to natural bodies of water, such as lakes, rivers or the sea, wherein an effective control of the chemical or physical properties of the water is difficult or impossible.

Control system

In one embodiment, process parameters such as pH, p0 ₂ , pC0 ₂ , salinity and temperature of the culture liquid, are regulated by a multi-purpose system. As decribed herein, the density of the culture liquid can e.g. be used to control the position of the photobioreactor in the water body. The multipurpose system is programmed with information related to the photobioreactor, such as the overall weight and density of the photobioreactor and the amount of biomass and culture liquid that is contained in the photobioreactor. Moreover, the system continuously measures the temperature, salinity and density of the culture liquid and the density of the surrounding water, thereby continuously determining the density difference between the photobioreactor and the surrounding water. The system also controls the concentration of the different components of the culture liquid, such as the salt concentration. The system may then automatically regulate the position of the photobioreactor in the surrounding water as a response to a change in the temperature of the culture liquid, so as to keep the algae culture at a constant temperature. The system may thus be equipped with known control circuits or algorithms, such as control algorithms with feedback mechanisms, to allow optimal stability when regulating the position of the photobioreactor.

In a further embodiment, when the photobioreactor is in a closed body of water, the salinity and temperature of the surrounding water are regulated by the multi-purpose system described above. In an embodiment of the photobioreactor having additional compartments or tubes, the control system also regulates the filling and emptying of gas, water and other liquids of the compartments or tubes. In one embodiment, the multi-purpose system controls not only parameters related to positioning of the photobioreactor but also parameters relevant to growth of the algae. Thus, the control system also measures and regulates 0 ₂ and C0 ₂ contents of the algae culture.

Density difference

In preferred embodiments, the method is a method for harvesting the biomass from a photobioreactor partially or completely surrounded by a body of water, wherein a density difference between the culture liquid and the surrounding water is provided so that the position of the photobioreactor in the water body is controlled.

Thus, the vertical position of a flexible and light weight photobioreactor in a surrounding water body can be controlled by controlling the density of the photobioreactor versus the density of the surrounding water, e.g. by providing different salinity concentrations inside and outside of the reactor. Such photobioreactors assume a perfect horizontal position regardless of their starting position and are thus very stable. Moreover, the thickness of the layer of culture liquid inside the reactor becomes very homogenous, again independent of the starting point.

Since small density differences in the water inside and outside of the photobioreactor caused by a difference in salinity and/or temperature are the only driving forces for moving the reactor, it is preferable to have a thin and flexible material in the walls of the photobioreactor. Having thin and flexible walls will optimize the capability of the photobioreactor to self-stabilize. An example of a material which is suitable for use in the photobioreactor walls is polyethylene or equivalent material with a thickness of about 0.1 mm. By provision of said density difference between the culture liquid and the surrounding water so that the position of the photobioreactor in the water body is controlled is thus created a change in buoyancy of the photobioreactor in relation to the surrounding water, this change in buoyancy being the driving force of a vertical position change of the reactor. Thus, the density difference provided takes into account the weight and buoyancy of the photobioreactor itself.

The density difference may be provided by provision of a salinity difference between the culture liquid and the surrounding water. Said salinity difference may be provided by increasing or decreasing the salinity of the culture liquid. Said salinity difference may also, or alternatively, be provided by increasing or decreasing the salinity of the surrounding water, in particular the surrounding water of a closed water body.

A salinity increase of the culture liquid may be provided simultaneously with a salinity decrease of the surrounding water is provided. A salinity decrease of the culture liquid may be provided simultaneously with a salinity increase of the surrounding water is provided.

The density difference may be provided by provision of a temperature difference between the culture liquid and the surrounding water. Said temperature difference may be provided by changing the temperature of the surrounding water, in particular the surrounding water of a closed water body. The meaning of "closed water body" is as above. Parameters influencing the density of the culture liquid and/or the surrounding water may be modified separately or simultaneously in order to provide a desirable density difference. The density of the culture liquid and the density of the surrounding water may be modified separately or simultaneously in order to provide a desirable density difference.

The density difference may be provided so that the density of the culture liquid is increased or so that the density of the surrounding water is decreased, whereby the position of the photobioreactor in the water body is lowered. The density difference may be provided so that the density of the culture liquid is decreased or so that the density of the surrounding water is increased, whereby the position of the

photobioreactor in the water body is raised. The density difference may be provided so that the position of the photobioreactor in the water body is maintained. Again, the density of the culture liquid and the density of the surrounding water may be modified simultaneously in order to provide a desirable density difference, either for lowering, raising or maintaining the position of the photobioreactor.

In one embodiment, the photobioreactor rests on the surface of open water as a starting position. When the photobioreactor rests, or floats, on the surface of open water, the density of the photobioreactor is lower compared to the density of the fresh water. If the position of the photobioreactor needs to be lowered, the density difference between the photobioreactor and the open water is regulated. As an example, the position of the photobioreactor may need to be lowered when the measured

temperature of the culture liquid is higher or expected to be higher than a

predetermined temperature value. To regulate the density difference between the photobioreactor and the surrounding water, the salinity of the culture liquid is increased. This is achieved by replacing or complementing the culture liquid by culture liquid of higher salinity, i.e. by pumping culture liquid of higher salinity into the photobioreactor. The flow rate of the culture liquid is set so as to allow the algae to adapt to the higher salt concentrations in the culture liquid and to minimize any loss of algae in the photobioreactor. As the culture liquid is replaced or complemented by culture liquid of higher salinity, the density of the photobioreactor increases. The photobioreactor sinks in the open water due to its higher density compared to the surrounding water and the position of the photobioreactor is lowered. The position of the photobioreactor may be lowered until the measured temperature of the culture liquid is within a desirable temperature range.

Other ways of operating such photobioreactors, where the position of the

photobioreactor is controlled by a density difference, are described in WO2009/090549, in particular in the sections entitled "The closed photobioreactor in open water" and "The photobioreactor in a closed body of water". _Q

Additional means for controlling the position and /or shape of the photobioreactor The vertical position and/or the shape of the photobioreactor according to the present disclosure may be controlled by providing a suitable density difference between the culture liquid and the surrounding water in which the reactor is suspended. However, sometimes additional means for controlling the position and /or shape of the photobioreactor may be useful. This may for instance be the case when the

photobioreactor needs to be submerged quickly. Such means may include additional compartments or tubes capable of being filled with high or low density medium in order to assist submersion or floatation of the photobioreactor, mechanical means for assisting submersion or floatation of the photobioreactor, and sub-compartments within the algae compartment of the photobioreactor for controlling the shape of the reactor when it is submerged. These three types of means are discussed in detail in

WO2009/090549, in particular in the sections entitled "Additional means for controlling the position and/or shape of the photobioreactor", "Additional compartments or tubes capable of being filled with high or low density medium" and "Mechanical means for assisting submersion or floatation of the photobioreactor".

Subcompartments

In some embodiments, the central compartment can be subdivided in two or more sub- compartments adapted to comprise the culture liquid. Said sub-compartments may be adapted to comprise a portion of the culture liquid present in the photobioreactor.

Preferably, when the photobioreactor comprises two or more such sub-compartments, the culture liquid may be distributed evenly between sub-compartments. The use of sub-compartments in the photobioreactor may help stabilize the reactor when it is partially or fully submerged as the sub-compartments help reduce the potential adverse effects of culture liquid agglomeration and large gas bubbles as will be discussed more in detail herein.

The sub-compartments may be sealed from each other. The sub-compartments may also be connected to allow limited liquid and/or gas transport between the sub- compartments. This will reduce problems with agglomeration and large gas bubbles when the photobioreactor is submerged, while retaining the flexibility of the

photobioreactor and the advantage of common distribution of C0 ₂ and other nutrients to the culture liquid and removal of oxygen from the reactor. The photobioreactor may further comprise means for temporarily dividing the algae compartment of the photobioreactor into two or more sub-compartments. The photobioreactor or the algae compartment thereof, when floating on the surface of the surrounding water will generally comprise at least a flexible top sheet facing the atmosphere, and a flexible bottom sheet facing the water, between which two sheets the algae culture is maintained. The means for temporarily dividing the algae compartment of the photobioreactor may for example comprise a member adapted for pressing a top sheet of the photobioreactor towards a bottom sheet of the

photobioreactor such that a sub-compartment is formed inside the photobioreactor on each side of the depression. In an embodiment, said means for dividing the algae compartment of the photobioreactor into two or more sub-compartments comprises at least one elongated member, such as a rope, a cable or a rod stretched above the photobioreactor and arranged to be brought down to press the top sheet of the photobioreactor towards the bottom sheet of the photobioreactor such that a sub- compartment is formed inside the photobioreactor on each side of said at least one elongated member.

In another embodiment, said means for dividing the photobioreactor into two or more sub-compartments comprises at least one additional compartment or tube, separate from the algae compartment and arranged in contact with the top sheet of the photobioreactor and adapted to be filled with a liquid having higher density than the culture liquid, such that when the additional compartment or tube is filled with the high density liquid, the filled compartment or tube is capable of pressing the top sheet of the photobioreactor towards the bottom sheet of the photobioreactor such that a sub- compartment is formed inside the algae compartment of the photobioreactor on each side of said filled compartment.

Dimensions and materials

Photobioreactors suitable for cultivation of mixotrophic microorganisms such as phototrophic microorganisms preferably let light pass through, so that the

microorganisms can perform photosynthesis. In one embodiment, at least the central compartment of the photobioreactor is manufactured from a transparent material. Preferably, the material is flexible. The water tight, transparent and flexible material may preferably further be a light weight, or low density, material. The material may preferably be a polymer based material, such as a thin film of a polyolefin based polymer, e.g. polyethylene or polypropylene. Other polymers suitable for use with the present invention will be readily recognized by a person skilled in the art of polymeric materials. The thickness of the walls should be selected depending on the properties, such as flexibility, transparency and durability, of the specific material used and may for example be in the range of 10-1000 μηι or in the range of 25-500 μηι or in the range of 50-150 μηι. It is preferred, with regard taken to the durability of the material, to make the walls of the photobioreactor as thin as possible in order to maximize the flexibility and transparency. As a non-limiting example, a polyethylene film having a thickness of about 100 μηι has been found to be suitable for use in the walls of the photobioreactor. It will be understood that the present methods can be adapted to be used with a wide variety of photobioreactors, over a wide range of sizes. The area of the body of water covered by the photobioreactor may range from one to several hundred square meters. In some embodiments, the area covered by the photobioreactor is between 1 and 1000 m ² , such as between 10 and 900 m ² , such as between 25 and 800 m ² , such as between 40 and 750 m ² , such as between 50 and 700 m ² , such as between 60 and 600 m ² , such as between 70 and 500 m ² , such as between 80 and 450 m ² , such as between 90 and 400 m ² , such as between 100 and 300 m ² , such as between 150 and 275 m ² , such as between 200 and 275 m ² , such as about 250 m ² . In a preferred embodiment, the area is 10 m ² . In another preferred embodiment, the area is 40 m ² . In another preferred embodiment, the area is 250 m ² .

The volume of the central compartment may also vary over a wide range. In some embodiments, the volume of the central compartment of a photobioreactor is between 100 and 20,000 L, such as between 500 and 15,000 L, such as between 1 ,000 and 12,500 L, such as between 2,000 and 10,000 L, such as between 2,500 and 9,000 L, such as between 3,000 and 8,000 L, such as between 3,500 and 7,000 L, such as between 4,000 and 6,000 L, such as between 4,500 and 5,500 L, such as between 5,000 L. In a preferred embodiment, the volume is 500 L. In another preferred embodiment, the volume is 2,000 L. In another preferred embodiment, the volume is 12,500 L.

The average thickness or height of the central compartment may also vary. As explained herein elsewhere, the culture liquid in the central compartment in a photobioreactor completely or partially surrounded by a body of water will assume a homogenous distribution, and will be essentially even, with the exception of zones close to the edges of the central compartment, where the walls of the photobioreactor may be round to some extent. In some embodiments, the height of the central compartment is between 1 and 30 cm, such as between 1 .5 and 29 cm, such as between 2 and 28 cm, such as between 2.5 and 27 cm, such as between 3 and 26 cm, such as between 4 and 25 cm, such as between 5 and 20 cm, such as between 6 and 19 cm, such as between 7 and 18 cm, such as between 8 and 17 cm, such as between 9 and 16 cm, such as between 10 and 15 cm, such as between 1 1 and 14 cm, such as between 12 and 13 cm. In some embodiments, the thickness of the culture liquid is between 1 and 30 cm, such as between 1 .5 and 29 cm, such as between 2 and 28 cm, such as between 2.5 and 27 cm, such as between 3 and 26 cm, such as between 4 and 25 cm, such as between 5 and 20 cm, such as between 6 and 19 cm, such as between 7 and 18 cm, such as between 8 and 17 cm, such as between 9 and 16 cm, such as between 10 and 15 cm, such as between 1 1 and 14 cm, such as between 12 and 13 cm.

In one embodiment, the area covered by the photobioreactor is 10 m ² , and the volume of the central compartment is 500 L. In another embodiment, the area covered by the photobioreactor is 40 m ² , and the volume of the central compartment is 2000 L. In another embodiment area covered by the photobioreactor is 250 m ² , and the volume of the central compartment is 12,500 L.

It will be understood that in order for the present methods to operate optimally, the volume of the central compartment is preferably much greater than the volume of the peripheral compartment. Harvesting biomass from a photobioreactor and/or feeding culture liquid to a photobioreactor is easier if the volume of the peripheral compartment is not too large. Too big volumes would slow down the liquid flow and might result in "dead zones" because it would be difficult to empty the peripheral compartments completely, particularly in embodiments relating to a system comprising a plurality of photobioreactors, as described below.

In some embodiments, the peripheral compartment has a breadth of at least 1 cm, such as at least 2 cm, such as at least 3 cm, such as at least 4 cm, such as at least 5 cm, such as at least 6 cm, such as at least 7 cm, such as at least 8 cm, such as at least 9 cm, such as at least 10 cm, such as at least 15 cm, such as at least 20 cm, such as at least 25 cm, such as at least 50 cm.

Microorganisms

Mixotrophic microorganisms that are useful for biomass production or the production of products as described herein are known to the skilled person. Preferably, the microorganism is capable of photosynthetic growth when exposed to light, but may additionally also be capable of chemotrophic growth, lithotrophic growth, autotrophic growth, heterotrophic growth and organotrophic growth. When exposed to darkness, the microorganism is capable of heterotrophic growth, but may additionally also be capable of chemotrophic growth, lithotrophic growth, autotrophic growth and organotrophic growth.

Non-limiting examples of suitable microorganisms are: Dunianella Salina,

Haematoccocus Pluvialis, Neochloris Oleoabundans, Chlorella Vulgaris, Isochrysis galbana, Pavlova lutheri, Nanochloropsis oculata, Phaeodactylum tricornutum, Skeletonema sp., Thalassiosira sp., Chaetoceros sp., Tetraselmis sp. and Spirulina Platensis.

Products

Mixotrophic microorganisms can be used to produce compounds as described herein, which can find numerous applications in various fields, such as cosmetics and beauty products, pharmaceutical products, neutriceutical and dietary supplements, packaging and bioplastics, soil and water treatment, biofuels, pet foods and fertilisers, food and snacks.

The microorganisms may either be able of producing the above products, or the microorganism may be converted to the above products. Thus in one embodiment, the method further comprises the step of converting the produced biomass to a biofuel, an animal feed, a protein, an amino acid, an ingredient for basic human nutrition, fine chemicals, nutritional supplements, vitamins, omega-2-fatty acids, antioxidants, such as carotenoids or beta-carotene, pharmaceutically active substances, amino acids or astaxanthin. System of photobioreactors

The present methods are particularly advantageous for large-scale cultivation of microorganisms, since they allow savings on the required length of tubing required for harvesting the produced biomass and/or feeding the photobioreactors with culture liquid. As will be recognised by the skilled person, these savings are multiplied by the number of photobioreactors used.

Accordingly, it is an aspect of the present disclosure to provide a system comprising at least one closed photobioreactor for large-scale cultivation of microorganisms, wherein:

- each photobioreactor is adapted to be partially or completely surrounded by a body of water;

- each photobioreactor comprises a central compartment for holding a culture liquid therein and a peripheral compartment along at least part of the edge of the central compartment;

- each photobioreactor comprises a first fluid connection between the central and the peripheral compartment, of which the opening and closing can be controlled;

- each peripheral compartment comprises at least one further fluid connection, of which the opening and closing can optionally be controlled;

- the peripheral compartment of each photobioreactor is connectable to a

harvester via said at least one further fluid connection.

The connection between the peripheral compartment and a given photobioreactor may be direct or indirect, as described above.

In some embodiments, each photobioreactor is further connected directly or indirectly to a feeder.

The features of the photobioreactor, of the first and further fluid connections, of the central compartment and of the peripheral compartment, of the harvester and of the feeder are as described herein above.

In such a system comprising several photobioreactors, each photobioreactor may be operated independently. The central compartment of each photobioreactor acts as a closed system, which can be opened upon activation of the first fluid connection as described above. The activation of the first fluid connection of each photobioreactor may however occur independently of the activation of the first fluid connection of any other photobioreactor of the system. It will be recognised that simultaneous harvesting from or providing of fresh culture liquid to the central compartment of different photobioreactors may occur at the same time.

Advantageously, when biomass is to be harvested from the central compartment of one photobioreactor, or when fresh culture liquid is to be provided to the central compartment of one photobioreactor, the peripheral compartments of all the photobioreactors are connected by activation of the further fluid connections as described above. Thereby, the plurality of the peripheral compartments together acts as one tube, either for harvesting the biomass or for feeding the culture liquid with fresh nutrients. For example, if the system comprises 50 photobioreactors, numbered from 1 (closest to the feeder) to 50 (closest to the harvester), said photobioreactors being monitored by a system as described herein, and biomass density has reached the desired level in photobioreactor 25 and 40, the peripheral compartments of photobioreactors 1 to 50 may be connected simultaneously by activation of the further fluid connections of all the photobioreactors, while only the first fluid connections on photobioreactor 25 and 40 are activated to allow fluid exchange, whereby the biomass of these two

photobioreactors is harvested. Alternatively, if fresh culture liquid is to be provided to photobioreactors 25 and 40, only the first fluid connections on these two

photobioreactors are activated, while the peripheral compartments of all

photobioreactors are connected as described above and are used for providing fresh culture liquid to the central compartments of photobioreactors 25 and 40.

As described above, the peripheral compartments may be physically divided in two or more subcompartments. In such embodiments, the first subcompartments may be connected to each other and to the feeder and are used for providing fresh culture liquid to the central compartments, while the second subcompartments may be connected to each other and to the harvester and are used for harvesting biomass. Such arrangements may be advantageous for large systems, where it might be desirable to provide fresh culture liquid to some photobioreactors, while simultaneously harvesting biomass from others. For large systems, it will be recognised that the systems may be subdivided in subsystems, wherein the peripheral compartments in each subsystem are connected together and to at least one harvester and/or feeder.

Method of manufacture

It is another aspect of the disclosure to provide a method of manufacturing a photobioreactor, comprising the steps of:

i. Providing a top sheet and a bottom sheet of a material which is flexible, water-tight and nutrient-impermeable;

ii. Providing at least one first fluid connection and at least one further fluid connection;

iii. Placing said top sheet on said bottom sheet and said first fluid connection and further fluid connection therebetween;

iv. Welding the top and bottom sheet together in a first position along their length, thereby defining a central compartment,

v. Welding the top and bottom sheet together in a second position along their length, where the first position is different from the first position, thereby defining at least one peripheral compartment along at least part of the edge of the central compartment,

in such a manner that the first fluid connection connects the central and the peripheral compartment and the further fluid connection can be configured to directly or indirectly connect the peripheral compartment to the harvester and/or feeder. Detailed description of the drawings

Fig. 1 a is a view of a complete photobioreactor that can be adapted to be used in the methods described herein. The figure does not show the fluid connections, and is used solely to illustrate the general concept of photobioreactors. The panel shaped photobioreactor 1 (also referred to herein as the "reactor") floats on a water body, here an artificial pond 2. The size of such a photobioreactor 1 can vary. The photobioreactor 1 is in this embodiment manufactured from a flexible transparent material and within the photobioreactor is the culture liquid, in which the algae are suspended. By solar radiation on the photobioreactor 1 , the algae are enabled to produce biomass via photosynthesis. Carbon dioxide is used during this process and oxygen is produced. Therefore the culture medium is preferably always moving while illuminated, in order to provide new carbon dioxide and to remove oxygen which can be toxic for the algae. The culture medium is in this embodiment moved via a pump 3. The culture medium is thus moving through the photobioreactor and is brought back via a tube 4. The gas exchange will take place in a tank 5, to which a tube system 6 will steadily provide a carbon dioxide rich gas mixture by means of a compressor 7. The carbon dioxide rich gas mixture can have its origin for instance from an electrical power plant using fossil fuels. The degassed oxygen will be lead out via a tube 8 equipped with a sterile filter. Culture liquid with algae biomass can be taken out of the system via a valve 9 and be stored in a tank 10 until this harvested volume is processed further. New medium is provided to the system via a further valve 1 1 from a storage tank 12. This serves to level out the loss of liquid caused by the harvest and to supply culture liquid with new nutrients.

In an alternative embodiment (not shown), carbon dioxide is provided to the growing algae from a tube or hose located in the reactor, the tube or hose having one or more outlet(s) for carbon dioxide. Thus, in this embodiment the cultivation liquid must not move to pass tank 5 in order to be supplied with carbon dioxide.

Sensors 13 for determination of process parameters such as pH, p0 ₂ , pC0 ₂ , salinity and temperature of the culture liquid, and sensors 14 for determination of the salinity and the temperature of the surrounding water, are connected to a control unit 15. The control unit 15 determines the density difference between the culture liquid and the surrounding water, based on information from sensors 13, 14 as well as other parameters and stored data. The control unit controls pumps (not shown) supplying seawater and fresh water, respectively, to the pond 2. In another embodiment (not shown), the control unit 15 controls means for changing the salinity of the culture liquid in the photobioreactor 1 .

Fig. 1 b shows a cross section through such a system. The photobioreactor 1 is cut in a lateral way, in this figure the photobioreactor floats on a water body 2. The vertical thickness of the culture liquid in the photobioreactor is typically between 1 and 30 cm. The depth of the water body 2 might vary significantly. The tube 4 which is used to circulate the culture liquid is seen in the lateral cut as well. In another embodiment, shown in Figs. 2a and 2b, gaseous C0 ₂ may be supplied to the culture medium by a tube or a tube-like device extending into the photobioreactor and arranged to float on top of the surface of the culture medium due to its lower density. Bubbling of gaseous C0 ₂ is performed similarly as in the case described above, wherein the tube or tube-like device extends into the culture medium. However, the tube or tube-like device through which the C0 ₂ is supplied will be specifically designed to float on the surface of the culture medium in the photobioreactor. This is achieved by the density of the whole (16) aeration system being lower than the density of the algae culture medium (17). The holes (18) in the tube or tube-like device, through which the C0 ₂ is pushed into the culture medium may preferably point downwards in this embodiment to achieve the best possible gas transfer. The holes will thereby be positioned at, or slightly below, the surface of the culture medium. In an embodiment shown in Fig 3, an additional compartment is arranged on top of the photobioreactor. In this embodiment, the density of the total reactor system can be changed by adding a liquid with high density, preferably salt water, in the additional compartment (19), which is separate from the algae compartment (20). The

compartment, when filled, would increase the density of the whole reactor system such that the sinking process is accelerated. In this embodiment, the additional compartment is arranged on top of the photobioreactor. The additional compartment comprises inner gluing points (21 ) to provide structural stability. The additional compartments or tubes may be connected to a supply of high density liquid by one or more hoses (22) provided with valves (23) at one side of the reactor and a similar connection at the opposite side of the reactor. When used for accelerating the submersion of the photobioreactor according to this embodiment, the additional compartment will be filled with water from one side and the valves at the other side will also be opened. By starting the filling process from one side this side will become submerged first.

Remaining air in the additional compartment may thereby be collected at one side of the photobioreactor and be pushed out more efficiently. The filling process will be continued until all air is out and the complete reactor starts to sink. The valves opposite to the filling hoses are then closed. The filling process may be stopped at this point or the filling process may be continued for a while. Continuing the filling process increases the pressure in the additional compartment, thus increasing the rigidity of this compartment and allowing it to provide additional structural stability to the

photobioreactor during submersion and in a partially or fully submerged mode.

When the reactor system should go up, the salt water of the additional compartment will be pumped out by a pump, having the valves opposite to the pump closed to avoid that air bubbles enter the new compartment. To accelerate the process of going up, the valves opposite to the pump will be opened and through the respective hoses pressurized air or flue gases will be pushed in.

Fig. 4a shows a cross-section of a photobioreactor resting on a body of water, here an open body of water, and comprising a central compartment 30 holding a culture liquid 17 with a phototrophic microorganism, and a peripheral compartment 31 . A first fluid connection (33) is placed between the central and the peripheral compartment and a further fluid connection 32 enables controlled fluid exchange between the peripheral compartment and the environment, e.g. a harvester or a feeder (not shown). Fig. 4b shows a part of a cross-section of the same photobioreactor.

Fig. 5 shows a schematic top-view of a photobioreactor to be used in the present methods. In this particular embodiment, 6 first fluid connections 33 connect the central compartment and the peripheral compartment. 4 further fluid connections 32a, 32b, 32c and 32d connect the peripheral compartment to the environment. Also shown in the figure are an aeration system 16 for pushing C0 ₂ in the culture liquid, a C0 ₂ outlet valve 35, and a sensor 13. In the embodiment shown, culture liquid can be provided from the feeder 36 to the peripheral compartment via the further fluid connections 32a and 32b. The culture liquid can be transferred to the central compartment via the first fluid connections 33 upon activation thereof, or it can flow further in the system via the further fluid connections 32c and 32d. When biomass is to be harvested, the first fluid connections are activated as detailed herein above, allowing culture liquid containing biomass to flow from the central compartment to the peripheral compartment. Upon activation of the further fluid connections, the biomass containing liquid can then flow from the peripheral compartment to the harvester 37 via 32c and 32d, and can be collected in e.g. a harvest container (not shown). The dashed lines at the corners of the peripheral compartment show examples of where the peripheral compartment may be sealed if it is desirable to divide it in subcompartments. Fig. 6 is a schematic view of a system comprising two photobioreactors P1 and P2, P1 being upstream of P2. When fresh culture liquid is to be provided to P2, the further fluid connections 321 a, 321 b, 321 d and 322a are opened. The first fluid connections 331 of P1 are maintained closed, while one or more of the first fluid connections 332 of P2 are opened, thereby allowing fresh culture liquid to be provided from the feeder 36 to the peripheral compartment of P1 , further to the peripheral compartment of P2, and finally to the central compartment of P2. Opening one or more of the first fluid connections 331 of P1 allows fresh culture liquid to be provided to the photobioreactor P1 also. If biomass is to be harvested from P1 only, the first fluid connections 331 of P1 are opened, the further fluid connections 321 d, 322c and 322d are also opened, while the first fluid connections 332 of P2 are maintained closed. This allows biomass-containing liquid to flow from the central compartment of P1 to the peripheral compartment of P1 , further to the peripheral compartment of P2, toward the harvester 37. Fig. 7 shows possible ways of arranging a plurality of photobioreactors in a system. The central photobioreactor is connected to the surrounding photobioreactors via connections as described herein (not shown).

Items

1 . A method for harvesting biomass, said method comprising the steps of:

i. providing at least one photobioreactor partially or completely surrounded by a body of water, said photobioreactor comprising a central compartment holding a culture liquid comprising a mixotrophic microorganism such as a phototrophic microorganism, and at least one peripheral compartment along at least part of the edge of the central compartment, wherein said photobioreactor comprises at least one first fluid connection between the central compartment and the peripheral compartment, and wherein the peripheral compartment comprises at least one further fluid connection, where the opening and closing of the first fluid connection and optionally of the further fluid connection can be controlled;

ii. opening said first fluid connection, thereby allowing at least part of the

culture liquid to flow from the central compartment to the peripheral compartment; opening said further fluid connection and activating a harvester, thereby allowing the culture liquid to flow from the peripheral compartment through the further fluid connection, wherein the further fluid connection directly or indirectly connects the peripheral compartment of one photobioreactor to said harvester.

The method of item 1 , wherein the peripheral compartment is optionally divided in two or more peripheral sub-compartments.

The method of any one of the preceding items, wherein each peripheral sub- compartment comprises at least one further fluid connection, where the opening and closing of the further fluid connection can optionally be controlled.

The method of item 2 to 3, wherein at least one of the two or more peripheral sub-compartments is directly or indirectly connected to the harvester, and wherein at least one other peripheral sub-compartment is directly or indirectly connected to the feeder.

The method of any one of the preceding items, wherein the step of harvesting and/or the step of providing fresh culture liquid are repeated.

The method of any one of the preceding items, wherein the harvester and/or the feeder comprise or consist of a pump and/or gravity forces.

The method of item 1 , wherein the microorganism is a microalga capable of photosynthetic growth when exposed to light.

The method of any one of the preceding items, further comprising the step of converting the produced biomass to a biofuel, an animal feed, a protein, an amino acid, an ingredient for basic human nutrition, fine chemicals, nutritional supplements, vitamins, omega-3-fatty acids, antioxidants, preferably carotenoids or beta-carotene, pharmaceutically active substances, amino acids or astaxanthin. The method of any one of the preceding items, wherein the first and/or the further fluid connection is an automatic valve such as an electrovalve or a manual valve. The method of any one of the preceding items, wherein the first and/or the further fluid connection is a unidirectional or a bidirectional fluid connection. The method of any one of the preceding items, wherein the harvester and/or the feeder comprises or consists of a pump directly connected to the further fluid connection. The method of any one of the preceding items, wherein the further fluid connection directly connects the peripheral compartment of one photobioreactor to the peripheral compartment of another photobioreactor. The method of any one of the preceding items, wherein at least one first fluid connection is at least two first fluid connections, such as at least three first fluid connections, such as at least four first fluid connections, such as at least five first fluid connections, such as at least six first fluid connections, such as at least seven first fluid connections, such as at least eight first fluid connections. The method of any one of the preceding items, wherein the at least one further fluid connection is at least two further fluid connections, such as at least three further fluid connections, such as at least four further fluid connections, such as at least five further fluid connections, such as at least six further fluid

connections, such as at least seven further fluid connections, such as at least eight further fluid connections. The method of any one of the preceding items, wherein the photobioreactor has a flat panel shape. The method of item 6, wherein the at least one further fluid connection is at least two further fluid connections on each side of the photobioreactor, such as at least three further fluid connections, such as at least four further fluid connections, such as at least five further fluid connections, such as at least six further fluid connections, such as at least seven further fluid connections, such as at least eight further fluid connections on each side of the photobioreactor. The method of any one of the preceding items, wherein the first fluid connection and the further fluid connection are independently dispersed at substantially regular intervals. The method of any one of the preceding items, wherein the method further comprises a step of providing a salinity difference between the culture liquid and the body of water. The method of any one of the preceding items, wherein the method further comprises maintaining or changing the vertical position of the photobioreactor by controlling the salinity difference. The method of any one of the preceding items, wherein the step of harvesting and/or the step of providing fresh culture medium is continuous or semi- continuous. The method according to any one of the preceding items, further comprising the step of measuring biomass density in the photobioreactor, comparing said measured value with a reference value, and harvesting at least part of the biomass from the photobioreactor where the measured value is equal to or greater than said reference value. The method according to any one of the preceding items, further comprising the step of cultivating the microorganism. The method according to any one of the preceding items, wherein the photobioreactor is a photosynthesis-fermentation hybrid photobioreactor.

The method according to any one of the preceding items, wherein the at least one photobioreactor is a plurality of photobioreactors, wherein at least part of the biomass within the central compartment of one photobioreactor can be harvested independently of the harvesting of the biomass within the central compartment of any other photobioreactor.

25. The method according to any one of the preceding items, wherein the at least one photobioreactor is a plurality of photobioreactors, wherein the central compartment of one photobioreactor can be provided with fresh culture liquid independently of the provision of fresh culture liquid to the central compartment of any other photobioreactor. 26. A system comprising at least one closed photobioreactor for large-scale

cultivation of mixotrophic microorganisms such as phototrophic

microorganisms, wherein:

- each photobioreactor is adapted to be partially or completely surrounded by a body of water;

- each photobioreactor comprises a central compartment for holding a culture liquid therein and a peripheral compartment along at least part of the edge of the central compartment;

- each photobioreactor comprises a first fluid connection between the central and the peripheral compartment, of which the opening and closing can be controlled;

- each peripheral compartment comprises at least one further fluid connection, of which the opening and closing can optionally be controlled;

- the peripheral compartment of each photobioreactor is connectable to a

harvester via said at least one further fluid connection.

27. The system of item 26, wherein the mixotrophic microorganisms a microalga capable of photosynthetic growth when exposed to light.

28. The system of any one of items 26 to 27, wherein the peripheral compartment is connectable to said harvester via at least one peripheral compartment of another photobioreactor.

29. The system of any one of items 26 to 28, wherein each photobioreactor is made of a flexible, water-tight, nutrient-impermeable material. The system of any one of items 26 to 29, wherein the photobioreactor consists of a top sheet and a bottom sheet joined by a first and a second welding, wherein the peripheral compartment is defined by said first and second welding. The system of any one of items 26 to 30, wherein the central compartment and the peripheral compartment are impermeably separated by said welded edges. The system of any one of items 26 to 31 , wherein the central compartment has a rectangular shape, a circular shape or an elliptic shape. The system of item 32, wherein the peripheral compartment is a rim essentially surrounding the welded edges of the central compartment. The system of any one items 32 or 33, wherein the peripheral compartment has a breadth of at least 1 cm, such as at least 2 cm, such as at least 3 cm, such as at least 4 cm, such as at least 5 cm, such as at least 6 cm, such as at least 7 cm, such as at least 8 cm, such as at least 9 cm, such as at least 10 cm, such as at least 15 cm, such as at least 20 cm, such as at least 25 cm, such as at least 50 cm. The system of any one items 26 to 34, wherein the harvester comprises a pump for removing the content of the peripheral compartment. The system of any one items 26 to 35, wherein each photobioreactor is further equipped with the feeder for providing culture liquid to the central compartment. The system of any one of items 26 to 36, wherein the first fluid connection and/or the further fluid connection is an automatic valve such as an

electrovalve, or a manual valve. The system of any one of items 26 to 37, wherein the first, the second and/or the further fluid connection is a unidirectional or a bidirectional fluid connection.

The system of any one of items 26 to 38, wherein the first fluid connection can be adapted to enable fluid exchange from the central compartment to the peripheral compartment in an open position and to prevent fluid exchange from the peripheral compartment to the central compartment in a closed position. The system of any one of items 26 to 39, wherein the first fluid connection opens when the harvester or the feeder is activated. The system of any one of items 26 to 40, wherein each photobioreactor further comprises means for supplying gas such as C0 ₂ to the central compartment. The system of item 41 , wherein the first fluid connection opens when the means for supplying gas to the central compartment are activated. The system of any one of items 26 to 42, wherein a density difference between the culture liquid and the surrounding water is provided so that the position of each photobioreactor in the water body is controlled. The system of item 43, wherein the density difference is provided by provision of a salinity and/or temperature difference between the culture liquid and the body of water. The system of any one of items 43 to 44, wherein the density difference is provided so that the position of each photobioreactor in the water body can be maintained. The system of any one of items 26 to 45, wherein the photobioreactor is further equipped with means for releasing gas or liquid from the central compartment into the environment. The system of any one of items 26 to 46, wherein part of the cultured microorganism of the central compartment of any single photobioreactor can be harvested independently of any other photobioreactor of the system.

The system of any one of items 26 to 47, wherein each photobioreactor can be provided with culture liquid independently of any other photobioreactor of the system. 49. The system of any one of items 26 to 48, wherein the photobioreactor further comprises means for monitoring the biomass density in the central

compartment.

50. The system of any one of items 26 to 49, wherein the photobioreactor further comprises means for monitoring process parameters such as temperature and salinity of the culture liquid. 51 . The system of any one of items 26 to 50, wherein the top sheet and/or the

bottom sheet of the central compartment comprise C0 ₂ -permeable membranes.

52. A method of manufacturing a photobioreactor, comprising the steps of:

i. Providing a top sheet and a bottom sheet of a material which is flexible, water-tight and nutrient-impermeable;

ii. Providing at least one first fluid connection and at least one further fluid connection;

iii. Placing said top sheet on said bottom sheet and said first fluid connection and further fluid connection therebetween;

iv. Welding the top and bottom sheet together in a first position along their length, thereby defining a central compartment,

v. Welding the top and bottom sheet together in a second position along their length, where the first position is different from the first position, thereby defining at least one peripheral compartment along at least part of the edge of the central compartment,

in such a manner that the first fluid connection connects the central and the peripheral compartment and the further fluid connection can be configured to directly or indirectly connect the peripheral compartment to the harvester and/or feeder.

53. The method of item 52, where the material comprises or consists of

polyethylene.

54. The method of any one of items 52 to 53, wherein the top sheet and the bottom sheet have a thickness between 10 and 1000 μηι. The method of any one of items 52 to 54, where the material is transparent. The method of any one of items 52 to 55, further comprising the step of dividing the peripheral compartment in at least two sub-compartments. The method of any one of items 52 to 56, further comprising the step of providing one or more connecting means in the top sheet of the central compartment for connecting additional fluid connections and/or a sensor. The method of any one of items 52 to 57, wherein the material comprises or consists of C0 ₂ -permeable material.