Description

The present invention relates to a process for cultivating algae in a photobioreactor, comprising the following steps: a) providing a photobioreactor which comprises on its surface an adsorbed layer of a polymer P which is a copolymer of at least one monomer M selected from alkyl (meth)acrylate, aryloxyalkyl (meth)acrylate, alkyl (meth)acrylamide or aryl (meth)acrylamide; and at least one zwitterionic monomer selected from zwitterionic ethylenically unsaturated monomers, and b) cultivating the algae in a supernatant medium above the surface of the photobioreactor. The invention also relates to a photobioreactor for cultivating algae which comprises on its surface an adsorbed layer of the polymer P; and to a process for making a photobioreactor, which comprises on its surface an adsorbed layer of a polymer P as defined in the proceeding claims, where the process comprises the following steps: providing a photobioreactor; and applying to the surface of the photobioreactor a solution S of the polymer P in a solvent L. Combinations of preferred embodiments with other preferred embodiments are within the scope of the present invention.

Photobioreactors for cultivation of algae play an important role in many technologies. An important issue with the application of photobioreactors is the unwanted deposition of biological or organic material on a surface, also known as fouling. For example, when cultivating algae, some types tend to attach to the surface or proteins may be deposited. Several approaches have been tried to solve these problems and to prevent the formation and deposition of organic or biological materials on a surface of photobioreactors.

The objectives were achieved by aprocess for cultivating algae in a photobioreactor, comprising the following steps: a) providing a photobioreactor which comprises on its surface an adsorbed layer of a polymer P which is a copolymer of

- at least one monomer M selected from alkyl (meth)acrylate, aryloxyalkyl (meth)acrylate, alkyl (meth)acrylamide or aryl (meth)acrylamide, and

- at least one zwitterionic monomer selected from zwitterionic ethylenically unsaturated monomers, and b) cultivating the algae in a supernatant medium above the surface of the photobioreactor.

The objects were also achieved by a photobioreactor for cultivating algae which comprises on its surface an adsorbed layer of the polymer P. The objects were also achieved by a process for making a photobioreactor, which comprises on its surface an adsorbed layer of a polymer P as defined in the proceeding claims, where the process comprises the following steps:

- providing a photobioreactor, and

- applying to the surface of the photobioreactor a solution S of the polymer P in a solvent L.

A photobioreactor is usually a device which provides a biologically active environment in which algae can grow. Typically, a photobioreactor utilizes a light source to cultivate the algae.

The biologically active environment preferably is a liquid medium, preferably water, comprising a carbon source and one or more nutrients.

Preferably the photobioreactor is a closed system (e.g. instead of an open system). In particular, a photobioreactor can be configured as a raceway comprising tubes or channels and can comprise a vessel, for example one or more bags and/or larger containers. Photobioreactor vessels preferably comprise volumes from liters to cubic meters. The photobioreactor preferably is or comprises, for contact with the algae, components of concrete, stainless steel, glass or polymer materials.

Preferably the photobioreactor comprises at least one irradiation section for transmission of light to the algae. The irradiation section preferably comprises a light transmission surface provided by a transparent material, preferably glass or a polymer material. Preferably the light transmission surface is provided on the outside of a tube or vessel component of the bioreactor. Particularly preferred are transparent tubes or bags as irradiation sections.

Preferably light will be provided to an irradiation section by a light source. A suitable light source is sunlight and/or by irradiation using one or more aritificial lights, preferably LEDs or incandescent light sources.

For photosynthetic algae, a preferred carbon source is carbon dioxide, and a preferred energy source is light, most preferably sunlight.

Furthermore, a preferred photobioreactor comprises a circulation system to transport algae.

To provide carbon dioxide as a carbon source, a preferred photobioreactor comprises a gas supply, preferably a bubble column and/or a gas-permeable membrane for pressing carbon dioxide into the medium. The carbon dioxide may be provided from the ambient atmosphere, preferably after sterile filtration or sterilization or from a specific source of preferably sterile gas. Preferably carbon dioxide is obtained in a post-combustion carbon capture or integrated gasification combined cycle process using flue gas emissions, or in the form of purified industrial carbon dioxide pressured gas.

For harvesting, a preferred photobioreactor comprises an outlet and preferably a filter to separate biomass from the medium.

Optionally, the photobioreactor is subjected to a sterilization prior to cultivating algae in step b), e.g. by exposing the photobioreactor to gaseous ethylene oxide, electron-beam, x-ray or gamma-irradiation.

The term “algae” refers to macroalgae, microalgae and cyanobacteria. Preferably the algae are capable of photosynthesis. Photosynthetic algae are usually relatively simple and cost-effective to grow and maintain. Algae can grow photosynthetically using carbon dioxide and sunlight, plus a minimum amount of trace nutrients.

The terms "macroalgae" or "microalgae" refers to any eukaryotic, microscopic algae or phytoplankton. They also can alternatively or additionally grow heterotrophically on another carbon source, such as glucose or sucrose, or waste water. They are generally regarded as environmentally friendly and safe for human operators.

Preferred taxa of micro-algae are:

The term "cyanobacteria" refers to prokaryotic, photosynthetic organisms that are also know as blue-green algae. In contrast to macroalgae and microalgae that employ chlorophyll for photosynthesis, cyanobacteria generally employ phycobilins to absorb light for photosynthesis. Phycobilins in cyanobacteria include phycoerythrin, phycocyanin and allophycocyanin.

Preferred taxa of cyanobacteria are:

The photobioreactor comprises on its surface an adsorbed layer of the polymer P. The whole surface or parts of the surface comprises the adsorbed layer of the polymer P. Preferably, at least the part of the surface which usually comes into contact with biological material (e.g. algae) under normal operating conditions comprises the adsorbed layer of the polymer P.

The layer of the polymer P is adsorbed to the surface, which usually means that there are no covalent chemical bonds between the polymer and the surface. The term “adsorbed” usually refers to physisorption, and usually not to chemisorption.

Preferably, the surface is free of other adsorbed polymers beside the polymer P. Preferably, the surface is free of other layers (e.g. adsorbed, or covalently bound layers) beside the adsorbed layer of the polymer P.

The adsorbed layer may comprise at least one polymer P, such as one, two or three different polymers P. Preferably, the adsorbed layer may comprise at least one polymer P, such as one, two or three different polymers P, and at least one amphiphilic polyalkoxylate. In one form the adsorbed layer consists of the polymer P. In a preferred form the adsorbed layer consists of at least one polymer P and at least one amphiphilic polyalkoxylate. In another preferred form the adsorbed layer consists of the polymer P and the amphiphilic polyalkoxylate.

In one form the adsorbed layer is free of other polymers beside the polymer P. In another form the adsorbed layer is free of biological compounds, pharmaceuticals or biologically active compounds. The surface of photobioreactor can be made of any biocompatible material, e.g. material on which algae can be grown, be it with the algae being attached to the surface or not. For example the surface is at least partly made of glass, quartz, silicon, metals, metal oxides or organic polymers, preferably glass. Suitable glass can be fused quartz glass (also called fuse silica), soda-lime glass, borosilicate glass, lead glass, aluminosilicate glass, glass-ceramics and fiberglass. Preferred material is fused quartz glass and soda-lime glass.

In another form, the surface is at least partly made of organic polymers, such as polycarbonate, polystyrene, hydrophilized polystyrene, polyamide, poly(methyl methacrylate), polyesters, polysulfones (like polyethersulfones), polyvinylchloride, polyvinylidene chloride, fluorinated or partially fluorinated polyolefins (like fluorinated polyethylene or polypropylene), polyolefines [such as polyethylene (like low density polyethylene, ultralow density polyethylene, linear low density polyethylene, high density polyethylene, high molecular weight polyethylene, ultrahigh molecular weight polyethylene), polypropylene (like oriented polypropylene, biaxially oriented polypropylene), cyclic olefin polymers (COP, like polynorbornene) or cyclic olefin copolymers (COC, like copolymers of ethylene and norbornene)].

In another form, the surface is at least partly made of hydrophilized polystyrene.

In one especially preferred form the surface is at least partly made of glass.

The term “surfaces is at least partly made of a material” usually means that at least 50 %, preferably at least 80%, and in particular at least 95% of the surface is made of the material. The surface usually refers to that part of the surface of the photobioreactor which in general comes into contact with biological material (e.g. algae) under normal operating conditions. In a form the at least 50 %, preferably at least 80%, and in particular at least 95% of the surface is made of glass.

The polymer P is a copolymer of at least one monomer M and at least one zwitterionic monomer selected from zwitterionic ethylenically unsaturated monomers. In the context of this application, this shall mean that polymer P comprises these monomers in polymerized form. Preferably, the polymer P consists of the monomer M and the zwitterionic monomer. In another form the polymer P is free of other monomers beside the monomer M and the zwitterionic monomer.

The monomer M can be selected from alkyl (meth)acrylate, aryloxyalkyl (meth)acrylate, alkyl (meth)acrylamide, or aryl (meth)acrylamide. Mixtures of momoner M are also possible. Preferably, monomer M is C1-C18 alkyl (meth)acrylate, more preferably C1-C12 alkyl (meth) acrylate, and in particular Ci-Ce alkyl (meth)acrylate.

Suitable alkyl (meth)acrylates are C1-C18 alkyl (meth)acrylate, preferably C1-C12 alkyl (meth)- acrylate, in particular Ci-Ce alkyl (meth)acrylate. The alkyl unit of the alkyl (meth)acrylate may be linear, branched or cyclic, preferably linear or branched. In a preferred form alkyl (methacrylate is methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate. In particular the mononer M is selected from ethyl acrylate and ethyl methacrylate.

Suitable aryloxyalkyl (meth)acrylates are phenyloxyalkyl (meth)acrylates, such as phenoxyethyl acrylate and phenoxyethyl methacrylate, wherein phenoxyethyl acrylate is preferred.

Suitable alkyl (meth)acrylamides are N-C1-C22 alkyl (meth)acrylamides, preferably N-C4-C20 (meth)alkyl acrylamides, such as N-tert-butyl acrylamide, N-hexadecyl acrylamide, or N-octa- decyl acrylamide.

Suitable aryl (meth)acrylamides are N-aryl (meth)acrylamides, preferably N-phenyl acrylamide.

The polymer P usually comprises 15 - 75 mol%, preferably 25 - 60 mol%, and in particular 35 - 60 mol% of the monomer M, which may be a C1-C12 alkyl (meth)acrylate. The polymer P may comprise at least 10, 15, 20, 30, 35, 40 or 45 mol% of the monomer M, which may be a C1-C12 alkyl (meth)acrylate. The polymer P may comprise up to 90, 80, 70, 65, 60, or 55 mol% of the monomer M, which may be a C1-C12 alkyl (meth)acrylate.

The zwitterionic monomer can be selected from

- carboxybetaine (meth)acrylates and (meth)acrylamides, such as 3-[[2-(Methacryloyloxy)- ethyl]dimethylammonio]propionate, 3-[(3-Acrylamidopropyl)dimethylammonio]propanoate, 2- [[2-(Methacryloyloxy)ethyl]dimethylammonio]acetate;

- sulfobetaine (meth)acrylates and (meth)acrylamides, such as 3-[[2-(Methacryloyloxy)ethyl]- dimethylammonio]propane-1 -sulfonate, 4-[[2-(Methacryloyloxy)ethyl]dimethylammonio]- butane-1 -sulfonate, 3-[[2-(Acryloyloxy)ethyl]dimethylammonio]propane-1 -sulfonate, 3-[(3- Methacrylamidopropyl)dimethylammonio]propane-1 -sulfonate, 4-[(3-Methacrylamidopropyl)- dimethylammonio]butane-1 -sulfonate, 3-[(3-Acrylamidopropyl)dimethylammonio]propane-1- sulfonate;

- sulfobetaine-vinylimidazole, such as 3-(1-ethenyl-1 H-imidazol-. 3-ium-3-yl)propane-1- sulfonate; - carboxybetaine-vinylimidazole, such as 3-(2-Carboxylatoethyl)-1 -vinyl-1 H-imidazole-3-ium;

- phosphobetaine (meth)acrylates such as 2-Methacryloyloxyethyl-phosphorylcholin; or

- sulfobetaine based on vinylpyridine, such as 1-(3-Sulfopropyl)-2-Vinylpyridinium betaine. Mixtures of zwitterionic monomers are also possible.

The zwitterionic monomer is preferably selected from sulfobetaine (meth)acrylates, such as 3- [[2-methacryloyloxy)ethyl]dimethyhammonio]- ^, propane-1 -sulfonate, 4-[[2-(methacryloyloxy)- ethyl]dimethylammonio]butane-1 -sulfonate, 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propane- 1 -sulfonate, 3-[(3-methacryhamido- ^, propyl)-dimethylammonio]propane-1 -sulfonate, 4-[(3- methacrylamidopropyl)-dimethylammonio]butane-1 -sulfonate, 3-[(3-acrylamido- propyl)dimethylammonio]propane-1 -sulfonate.

The polymer P may comprise at least 1, 5, 10, 15, 20, 25, 30, 35, 40 or 45 mol% of the zwitterionic monomer, which may be preferably selected from sulfobetaine (meth)acrylates.

The polymer P may comprise up to 90, 80, 70, 65, 60, or 55 mol% of the zwitterionic monomer , which may be preferably selected from sulfobetaine (meth)acrylates.

The polymer P may comprise 20 - 80 mol%, preferably 30 - 70 mol%, and in particular 40 - 60 mol% of the zwitterionic monomer, which may be preferably selected from sulfobetaine (meth)acrylates.

In a preferred form the polymer P comprises

15 - 70 mol% of the monomer M, preferably 25 - 65 mol%, and in particular 35 - 60 mol%; and

15 - 70 mol% of the zwitterionic monomer, preferably 25 - 65 mol%, and in particular 35 - 60 mol%, and where the molar amounts of the monomers sums up to 100 %.

In another preferred form the polymer P comprises

15 - 70 mol% of the monomer M, which may be a C1-C12 alkyl (meth)acrylate; and 15 - 70 mol% of the zwitterionic monomer, which may be selected from sulfobetaine (meth)acrylates, and where the molar amounts of the monomers sums up to 100 %.

In another preferred form the polymer P comprises

25 - 65 mol% of the monomer M, which may be a Ci-' 12 alkyl (meth)acrylate; and 25 - 65 mol% of the zwitterionic monomer, which may be selected from sulfobetaine (meth)acrylates, and where the molar amounts of the monomers sums up to 100 %.

In another preferred form the polymer P comprises

35 - 60 mol% of the monomer M, which may be a C1-C12 alkyl (meth)acrylate; and 35 - 60 mol% of the zwitterionic monomer, which may be selected from sulfobetaine (meth)acrylates, and where the molar amounts of the monomers sums up to 100 %.

The polymer P has usually a number average molar mass Mn of 2,000 to 100,000 g/mol, preferably of 3,000 to 80,000, and in particular of 5,000 to 40,000. The polymer P has usually a weight average molar mass Mw of 3,000 to 200,000 g/mol, preferably of 4,000 to 100,000, and in particular of 5,000 to 80,000. All values for the average molar mass Mn or Mw given in this application may be determined by gel permeation chromatography (GPC), e.g. using the method as described in the experimental section of this application.

The polymer P is preferably a statistical copolymer in which monomer M and the zwitterionic monomer are distributed statistically.

The polymer P is normally prepared by radical polymerization of the monomer M and the zwitterionic monomer, e.g. by solution polymerization or emulsion polymerization.

Preferably the polymer P is prepared by solution polymerization. “Solution polymerization” means that all starting materials are at least partly dissolved in the same solvent and that the polymerization reaction takes place in homogenous phase, without additional surfactants having to be present. In one preferred embodiment, the monomer M and zwitterionic monomer are dissolved in suitable solvents like alcohols like methanol, ethanol, 1 -propanol, 2-propanol, butanol or mixtures thereof and are then polymerized. Preferably, such solvents for the solution polymerization comprise at least 50 % by weight, preferably 70% and more preferably 80 % by weight of alcohols like methanol, ethanol, 1-propanol, 2-propanol, butanol or mixtures thereof. Preferably, such solvents for the solution polymerization comprise 20 % by weight or less, preferably 10 % by weight or less of water.

The radical polymerization can be initiated by oxidative radical starters like organic peroxides (e.g. tert-butyl-2,2-dimethylpropaneperoxoate, sodium persulfate, potassium persulfate, metachloroperbenzoic acid) or azo starters like azo-bisisobutyrodinitrile or 2,2'-Azobis(2-methyl- butyronitrile).

The photobioreactor comprises on its surface an adsorbed layer of a polymer P, where the adsorbed layer may be a self-assembled monolayer of the polymer P.

A “self-assembled monolayer” means typically a molecular assembly formed spontaneously on a surface by adsorption. A self-assembled monolayer forms usually spontaneously on such surfaces without any further process step being required. Self-assembled monolayers can for example be characterized by atomic force microscopy (AFM) or X-ray photoelectron spectroscopy (XPS) or in situ methods such as quartz crystal microbalance or surface plasmon resonance spectroscopy.

The self-assembled monolayer of the polymer P normally has a thickness that correlates with the size of the individual molecules adsorbed to that surface, e.g. it is normally smaller than 100 nm, such as 1 to 50 nm, preferably 1 to 20 nm, and in particular 1 to 10 nm.

In a form the photobioreactor comprises on its surface an adsorbed layer of a polymer P which is a copolymer of at least one monomer M selected from alkyl (meth)acrylate, and at least one zwitterionic monomer, and where the surface is at least partly made of glass.

In another form the photobioreactor comprises on its surface an adsorbed layer of a polymer P which is a copolymer of at least one monomer M selected from alkyl (meth)acrylate, and at least one zwitterionic monomer selected from sulfobetaine (meth)acrylates, and where the surface is at least partly made of glass.

In another form the photobioreactor comprises on its surface an adsorbed layer of a polymer P which is a copolymer of at least one monomer M selected from alkyl (meth)acrylate, and at least one zwitterionic monomer, and where the surface is at least partly made of glass, and where the algae are selected from microalgae or cyanobacteria.

In another form the photobioreactor comprises on its surface an adsorbed layer of a polymer P which is a copolymer of at least one monomer M selected from alkyl (meth)acrylate, and at least one zwitterionic monomer selected from sulfobetaine (meth)acrylates, and where the surface is at least partly made of glass, and where the algae are selected from microalgae or cyanobacteria. In a preferred form the layer comprises the polymer P and an amphiphilic polyalkoxylate.

The amphiphilic polyalkoxylate is usually a nonionic amphiphilic polyalkoxylate, which usually means it is free of ionic groups. The polyalkoxylate is amphiphilic, which usually means that is has surfactant properties and lowers the surface tension of water. Usually, the amphiphilic polyalkoxylate is obtainable by alkoxylation using alkyleneoxides, such as C2-Ce-alkylene oxide, preferably ethylene oxide, propylene oxide, or butylene oxide. Examples of amphiphilic polyalkoxylates are block polymers or compounds such as alcohols, alkylphenols, amines, amides, arylphenols, fatty acids or fatty acid esters which have been alkoxylated with 1 to 50 equivalents.

The amphiphilic polyalkoxylate may have a melting point of at least 35 °C, preferably at least 43 °C, more preferably at least 48 °C and in particular at least 50 °C.

The amphiphilic polyalkoxylate is usually soluble in water at 20 °C, and e.g. at pH 7. Preferably, the solubility in water of the amphiphilic polyalkoxylate is at least 3 wt%, more preferably at least 7 wt%, and in particular at least 10 wt%.

The molecular weight of the amphiphilic polyalkoxylate is usually in the range of from 0,5 to 50 kDa, preferably from 2 to 35 kDa, and in particular from 5 to 20 kDa.

The amphiphilic polyalkoxylate is preferably a block polymer, which may contain a hydrophilic block and a hydrophobic block. Suitable block polymers are block polymers of the A-B or A-B-A type comprising blocks of polyethylene oxide and polypropylene oxide, or of the A-B-C type comprising alkanol, polyethylene oxide and polypropylene oxide.

Preferably, the amphiphilic polyalkoxylate is a block polymer comprising at least one polyethoxylate block and at least one poly-Cs-Cs-alkoxylate block (e.g. polypropoxylate or polybutoxylate).

The weight ratio of the amphiphilic polyalkoxylate to the polymer P can be from 30:1 to 1 :30, 20:1 to 1 :20, 15:1 to 1 :15, 20:1 to 1 :3, 20:1 to 1 :1 , 15:1 to 5:1.

The invention also relates to a process for making the photobioreactor, which comprises on its surface an adsorbed layer of the polymer P, where the process comprises the following steps: - providing a photobioreactor (also called step A), and

- applying to the surface of the photobioreactor a solution S of the polymer P in a solvent L (also called step B).

In another form the invention relates to a process for making the photobioreactor, which comprises on its surface an adsorbed layer of the polymer P, which is a copolymer of

- at least one monomer M selected from alkyl (meth)acrylate, aryloxyalkyl (meth)acrylate, alkyl (meth)acrylamide or aryl (meth) acrylamide, and

- at least one zwitterionic monomer selected from zwitterionic ethylenically unsaturated monomers, and where the process comprises the following steps:

- providing a photobioreactor, and

- applying to the surface of the photobioreactor a solution S of the polymer P in a solvent L.

Typically, the photobioreactor is obtainable by the process for making the photobioreactor.

The solvent L comprises usually water or an alcohol (such as methanol, ethanol, n/iso- propanol or n/sec/iso/tert-butanol), where water is preferred.

The solution S may comprise 0.01 to 5 wt% of the polymer P and 5 to 90 wt% of the solvent L. The solution S normally comprises 0.001 to 10 % by weight of polymer P based on the solution S, preferably 0.05 to 2 % by weight and even more preferably 0.05 to 0.3 % by weight.

The solution S may comprise in another form 0.01 to 5 wt% of the sum of the polymer P and the amphiphilic polyalkoxylate, and 5 to 90 wt% of the solvent L. The solution S normally comprises 0.001 to 10 % by weight of the sum of the polymer P the amphiphilic polyalkoxylate based on the solution S, preferably 0.05 to 2 % by weight and even more preferably 0.05 to 0.3 % by weight.

Preferably, the solution S is an aqueous solution. “Aqueous” in this context shall mean that said polymer P is dissolved in a solvent or solvent mixture that comprises at least 50 % by weight, preferably at least 70 % by weight, more preferably at least 90 % by weight and particularly preferably at least 99 % by weight of water. In a preferred embodiment, the solvent L in which said at least one polymer P is dissolved is an aqueous solution. The aqueous solution S is usually a clear solution without any turbidity. In another embodiment, the aqueous solution S comprises polymer P at least partly in dissolved state but shows turbidity. Preferably, solution S is an aqueous solution comprising at least 50 % of water. Preferably, the solution S is water, an aqueous buffer, or an aqueous medium for cultivating the algae.

The solution S may comprise further additives in addition to the polymer P and the solvent L, and optionally the amphiphilic polyalkoxylate. The solution S may comprise 0.001 to 10 % by weight, preferably 0.05 to 5 % by weight of the further additives.

Typically, the photobioreactor in step A) corresponds to the photobioreactor before it was treated by step B).

Optionally, the process may further comprise the following step:

C1) removing the supernatant solution S; or

C2) removing the solvent L; or

C3) cultivating the algae in the solution S.

In one form the process may further comprise the following step:

C1) removing the supernatant solution S.

In step C1 the solvent L is preferably water. In step C1 the solution S is an aqueous solution, such as an aqueous buffer.

Suitable aquous buffers are are based on TRIS (tris(hydroxymethyl)aminomethane), HEPES ((4-(2-hydroxyethyl)-1 -piperazineethanesulfonic acid) or PBS (Phosphate-buffered saline).

Upon the application of solution S to the surface, the polymer P will normally self-organize to form a layer, in many cases a self-assembled monolayer, of polymer P on the surface. In many cases it will be sufficient to apply solution S to surface O and wait for a short period of time, for example 1 second to 1 day, preferably 1 second to 4 hours, or 1 second to 30 min, or 1 second to 10 min, or 1 second to 5 min. Next, the supernatant solution S can be removed, for example mechanically (for example by wiping or using a pipette) or by exchanging solution S by water or an (aqueous) solution that does not comprise of polymer P.

The step C1) of removing the supernatant solution S may comprise exchanging solution S for a solution or for a pure solvent or solvent mixture that does not contain polymer P (for example a cell culture medium or water or a buffer solution).

The step C1) usually results in the formation of self-adsorbed monolayer of the polymer P on the surface. In another form the process may further comprise the following step:

C2) removing the solvent L.

In step C2 the solvent L preferably comprises the alcohol (such as methanol, ethanol, n/iso- propanol or n/sec/iso/tert-butanol).

The removing of the solvent L may be achieved by drying or evaporation of the solvent L, e.g. at ambient or elevated temperature, or at ambient or reduced pressure. Preferably, the removing of the solvent L is achieved by drying at ambient temperature and ambient pressure.

Step C2) may be considered as a forced deposition wherein polymer P is applied from a solution and subsequently the solution is not withdrawn as a whole, but only the solvent L is removed, for example by evaporation, leaving the formerly dissolved polymer P deposited on the surface. A forced deposition can be applied by filling wells of a plate with the solution of polymer P followed by drying, alternatively, a forced deposition can be applied by dip coating, spin-coating, spraying, draw-down bar application, and other methods.

In another form the process may further comprise the following step:

C3) cultivating the algae in the solution S.

In step C3 the solution S is preferably an aqueous buffer or aqueous medium.

Suibable aqueous medium for cultivating the algae usually comprise at least 50, 70 or 90 wt% water. The pH is often from 4-9, or 5 to 8.

The aqueous medium may comprise 0.001 to 30 % by weight, preferably 0.01 to 5 % by weight and even more preferably 0.05 to 1 % by weight of the polymer P based on the aqueous medium. It is assumed that through this process adsorbed layer of the polymer P is prepared in situ that allows for efficient cultivation of such algae.

Any kind of aqueous medium may be used, wherein aqueous cell culture mediums with the solvent L comprising water are preferred. The supernatant medium of step C) usually corresponds to the solution S which is a cell culture medium. The supernatant medium of step C) usually comprises the polymer P and the solvent L (preferably water) applied in step B).

Between the step B) of applying the solution S and the step C) of cultivating the algae is preferably no removing of the solution S or the solvent L. Usually, the amount of the solution S in the photobioreactor in step C) is at least the same as in step B). Usually, the solution S is identical in step B) and in step C). Usually, the supernatant medium in step C) is identical to the solution S of step B).

The thickness and the amount per area of the adsorbed layer usually depends on the process for making the photobioreactor, especially if step C1) or C2) or C3) were applied. In general, the adsorbed layer (including water bound to the adsorbed polymer molecules) may have a thickness of 1 nm to 10 pm. In general, the polymer P is adsorbed on the surface in an amount, given as the wet mass (i.e. including hydration water), of at least 50 ng/cm ² , preferably at least 100 ng/cm ² , which may be determined by quartz crystal microbalance.

When the photobioreactor was obtained by the process comprising step C1) of removing the supernatant solution S, then the adsorbed layer may have a thickness of 1 nm to 100 nm, preferably 1 to 20 nm, and in particular 1 to 10 nm.

When the photobioreactor was obtained by the process comprising step C1) of removing the supernatant solution S, then the polymer P is adsorbed in an amount of 50 to 5000 ng/cm ² , preferably 100 to 3000 ng/cm ² , and in particular 200 to 1000 ng/cm ² on the surface.

When the photobioreactor was obtained by the process comprising step C2) of removing the solvent L, then the adsorbed layer may have a thickness of 0.01 pm to 100 pm, preferably 0.1 to 20 pm, and in particular 0.5 to 10 pm.

When the photobioreactor was obtained by the process comprising step C2) of removing the solvent L, then the polymer P is adsorbed in an amount of 0.5 to 5000 pg/cm ² , preferably 5 to 500 pg/cm ² , and in particular 30 to 300 pg/cm ² on the surface.

When the photobioreactor was obtained by the process comprising step C3), then the adsorbed layer may have a thickness of 1 nm to 100 nm, preferably 1 to 20 nm, and in particular 1 to 10 nm.

When the photobioreactor was obtained by the process comprising step C3), then the polymer P is adsorbed in an amount of 50 to 5000 ng/cm ² , preferably 100 to 3000 ng/cm ² , and in particular 200 to 1000 ng/cm ² on the surface.

The invention has various advantages: an increasing biomass production due to less biofilm formation on the surface, an increased light transmission due to less biogil formation, decreasing cleaning cylces of the bioreactor, or better external biomass monitoring due to less biofilm formation on the surface.

Examples

Example 1 : Preparation of Zwitterionic polymer ZIP-1

208 parts by weight of demineralized water were mixed with 52.0 parts by weight of ethanol and heated under nitrogen to 75 °C. Subsequently, 28 parts by weight of [2-(methacryloyloxy)- ethyl]dimethyl-(3-sulfopropyl)-ammonium hydroxide and 12.0 parts by weight of ethyl acrylate were dissolved in 288.0 parts by weight of demineralized water and 72.0 parts by weight of ethanol and added to the reaction mixture within 3 h. Simultaneously, 0.8 parts by weight of Wako V59 were dissolved in 20.0 parts by weight of ethanol and added to the reaction mixture within 4 h. Afterwards, the reaction was kept for 2 h at 75 °C. Subsequently, the reaction mixture was cooled down to the ambient temperature and subjected to purification by water steam distillation.

ZIP-1 is a copolymer of ethyl acrylate and [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)- ammonium hydroxide (CAS 3637-26-1) in a molar ration of 50:50.

The resulting polymer had a Mn of 10700 g/mol and a Mw of 24700 g/mol. Molecular weights were determined by Size Exclusion Chromatography using a mixed bed scouting column for water soluble linear polymers at 35°C.The eluent used was 0. 5 g/L sodium azide in water. The polymer was used as 1.5 mg/mL concentrated solution in the eluent. Before injection all samples were filtered through a 0.2 pm filter. The calibration was carried out with narrow poly(ethylene glycol) samples having molecular weights between 106 and 1.378.000 g/mol.

Example 2: Preparation of antifoulinq polymer powder

A powdery polymer mixture was prepared by mixing the zwitterionic polymer ZIP-1 from Example 1 with a powder of the amphiphilic polyalkoxylate PAO-1 in a weight ratio of 1 :1.

The PAO-1 is a powdered nonionic amphiphilic polyalkoxylate of the A-B-A type (polyethylene oxide - polypropylene oxide - polyethylene oxide), solubility in water >10 wt% at 25 °C, melting point above 50 °C, HLB 18-23. Example 3: Coating of model surfaces and antifoulinq evaluation

Method: The amounts of polymer or fibrinogen absorbed on the surface were determined by Quartz-Crystal Microbalance (QCM), which measures the resonance frequency of a freely oscillating quartz sensor after excitation in contact with the liquid medium. QCM measurements were performed using standard flow-through setups with a flow rate of 50 pL/min at 23°C. A measured shift in resonance frequency scales inversely with mass changes at the sensor surface. The adsorbed amounts were calculated from the shift of the 7th overtone of the resonance frequency according to the method of Sauerbrey. The mass sensitivity or detection limit of mass changes is estimated to -10 ng/cm ² in all experiments.

Model foulant was fibrinogen (glycoprotein) from human plasma, commercially available from Sigma Aldrich (CAS 9001-32-5). Microalgae produce proteins, polysaccharides and glycoproteins which typically cause the adhesive properties. The adhesions of proteins to substrates supports algae settling on the substrate (Xiao, R. and Y. Zheng, Overview of microalgal extracellular polymeric substances (EPS) and their applications. Biotechnology Advances, 2016. 34(7): 1225-1244; Halaj, M., et al., Extracellular biopolymers produced by Dictyosphaerium family - Chemical and immunomodulative properties. International Journal of Biological Macromolecules, 2019. 121 : 1254-1263). Further, fibrinogen experiments to show anti-fouling properties in marine application is a common system (US10544312).

Measurements were performed on a quartz sensor coated with various surfaces (cf Table 1):

- Glass: borosilicate glass from Q-sense QS-QSX336

- PE: polyethylene, Mw -4000 by GPC, Mn -1700 by GPC, applied to standard Au-coated sensors via spin-coating from 1% solutions in xylene at 100 °C, 3 drops at 4000 rpm for 30 s, followed by curing at 220 °C for 2 h

- PMMA: polymethylmethacrylate (CAS: 9011-14-7), applied to standard Au-coated sensors via spin-coating from 1 % solutions in acetone at 23 °C, 40 pL at 4000 rpm and 30 s.

- PC: polycarbonate (Aldrich, GF56189559), applied to standard Au-coated sensors via dipcoating in 2% solutions in NMP, followed by drying by 5 min/200°C with hot air stream)

- PVC: polyvinylchloride, applied to standard Au-coated sensors via spin-coating from 1% solutions in THF at 23 °C, 40 pL at 4000 rpm and 30 s.

The experiments comprised the following steps and used an aqueous PBS buffer (10 mmol/L phosphate, 2.7 mmol/L pottassium chloride and 137 mmol/L NaCI concentration) with pH 7.4 (“buffer”): 1) buffer until a stable baseline reading in resonance frequency was achieved;

2) 2 h 0.1 wt% solution of antifouling polymer from Example 2 in buffer;

3) 6 h buffer;

4) 1.5 h 0.1 wt% fibrinogen in buffer (0,1 wt%); 5) 1 h buffer.

The amount of polymer adsorbed was measured at the end of step 2). Antifouling was monitored during exposure of the samples to 0.1 wt% solutions of fibrinogen powder for 1.5 h.

The final mass change (in ng per unit macroscopic surface area) was recorded after another 1 h of rinsing with buffer (steps 4) and 5) above). The results (at least based on duplicate determinations) are given in Table 1.

Table 1 : Polymer adsorption and subsequent protein adhesion on various surfaces (each in comparison to protein adhesion without prior polymer layer adsorption). Amounts per area of surface are given with respect to the macroscopic surface area (not considering microscopic roughness). Fibrinogen adhesion in wt% represents the relative change of protein deposition as compared to the reference experiment without prior polymer adsorption. The QCM mass changes reported in Table 1 rely on the assumption that all substrates are perfectly flat and do not show roughness at the micro- or nanometer scale. To account for such effects, the topography of all substrates was analyzed by Atomic Force Microscope (AFM). The real (projected) surface area of each substrate was determined from three images (each covering an area of 20 pm x 20 pm) and used to calculate the average surface area difference, which is the ratio of the measured projected area and the ideal macroscopic surface area. The resulting surface area difference values were used to calculate corrected amounts of adsorbed polymer and deposited protein (referenced to the projected surface area), which are listed in Table 2.

Table 2: Polymer adsorption and subsequent protein adhesion on various surfaces (each in comparison to protein adhesion without prior polymer adsorption). Amounts per area of surface are given with respect to roughness-corrected surface area (as determined from 20 pm x 20 pm AFM images of the substrates before adsorption). Fibrinogen adhesion in wt% represents the relative change of protein deposition as compared to the reference experiment without prior polymer adsorption.