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Title:
PROCESS FOR THE CO-CULTURE OF A BACTERIUM OF THE MESOTOGA LINEAGE AND AT LEAST ONE HYDROGENOTROPHIC SULFATE REDUCING BACTERIUM
Document Type and Number:
WIPO Patent Application WO/2017/220819
Kind Code:
A1
Abstract:
The present invention concerns a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium.

Inventors:
OLLIVIER, Bernard (Quartier Valcros, Roquevaire, 13360, FR)
FARDEAU, Marie-Laure (1653, Chemin de Bellepeire, Les Pennes Mirabeau, 13170, FR)
BEN HANIA, Wajdi (2 boulevard des Alisiers, Marseille, Marseille, 13009, FR)
HAMDI, Moktar (Gozlene 2 - Bloc C02, Ariana, 2094, 2094, TN)
FADHLAOUI, Khaled (Cité Ettahrir Supérieur, N° 5 rue 686, Tunis 2042, 2042, TN)
Application Number:
EP2017/065748
Publication Date:
December 28, 2017
Filing Date:
June 26, 2017
Export Citation:
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Assignee:
INSTITUT DE RECHERCHE POUR LE DEVELOPPEMENT (I.R.D.) (44 Boulevard de Dunkerque, CS 90009, Marseille Cedex 02, F-13572, FR)
INSTITUT NATIONAL SCIENCES APPLIQUEES TECHNOLOGIE (INSAT) (BP 676, Tunis, 1080, 1080, TN)
International Classes:
C12P1/04; B09C1/00; B09C1/10; C02F3/28; C02F3/34; C12P3/00; C12P7/54
Domestic Patent References:
WO2011061300A22011-05-26
Foreign References:
US20060205051A12006-09-14
Other References:
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AZABOU S; MECHICHI T; PATEL BK; SAYADI, S.: "Isolation and characterization of a mesophilic heavy-metals-tolerant sulfate-reducing bacterium Desulfomicrobium sp. from an enrichment culture using phosphogypsum as a sulfate source.", J HAZARD MATER, vol. 140, 2007, pages 264 - 270, XP022384514, DOI: doi:10.1016/j.jhazmat.2006.07.073
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BEN HANIA W; FADHLAOUI K; ARMANET CB; PERSILLON C; POSTEC A; HAMDI M; DOLLA A; OLLIVIER B; FARDEAU ML; LE MER J: "Draft genome sequence of Mesotoga strain PhosAC3, a mesophilic member of the bacterial order Thermotogales, isolated from a digestor treating phosphogypsum in Tunisia", STAND IN GENOMIC SCI, vol. 12, 2015, pages 1186 - 1198
BERLENDIS S; RANCHOU-PEYRUSE M; FARDEAU ML; LASCOURREGES JF; JOSEPH M; OLLIVIER B; AIILLO T; DEQUIDT D; MAGOT M; RANCHOU-PEYRUSE A: "Desulfotomaculum aquiferis sp. nov. and Desulfotomaculum profundi sp. nov., isolated from a deep natural gas storage aquifer", INT J SYST EVOL MICROBIOL, vol. 66, 2016, pages 4329 - 4938
BORREGO E; MAS JL; MARTIN JE; BOLIVAR JP; VACA F; AGUADO JL: "Radioactivity levels in aerosol particles surrounding a large TENORM waste repository after application of preliminary restoration work.", SCI TOTAL ENVIRON, vol. 377, 2007, pages 27 - 35, XP005932699, DOI: doi:10.1016/j.scitotenv.2007.01.098
BURNETT WC; SCHULTZ MK; HULL CD: "Radionuclide flow during the conversion of phosphogypsum to ammonium sulfate", J ENVIRON RADIOACT, vol. 32, 1996, pages 33 - 51
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DEGIRMENCI N: "Utilization of phosphogypsum as raw and calcined material in manufacturing of building products.", CONSTRUCT BUILD MATER, vol. 22, 2008, pages 1857 - 1862, XP022670943, DOI: doi:10.1016/j.conbuildmat.2007.04.024
EL ZRELLI R; COURJAULT-RADE P; RABAOUI L; CASTET S; MICHEL S; BEJAOUI N: "Heavy metal contamination and ecological risk assessment in the surface sediments of the coastal area surrounding the industrial complex of Gabes city, Gulf of Gabes, SE Tunisia", MAR POLL BULL, vol. 101, 2015, pages 922 - 929, XP029344940, DOI: doi:10.1016/j.marpolbul.2015.10.047
JUSZCZAK A; WALIORSKA M; SEIFERT K; MELLER A; HABRYCH M; DOMKA F: "The Effect of phosphogypsum on the activity of Desulfotomaculum ruminis in lactate medium.", POLISH J OF ENVIRON STUD, vol. 11, 2002, pages 361 - 366
KACIMI L; SIMON-MASSERON A; GHOMARI A; DERRICHE Z: "Reduction of clinkerization temperature by using phosphogypsum", J HAZARD MATER, vol. 137, 2006, pages 129 - 137, XP025022327, DOI: doi:10.1016/j.jhazmat.2005.12.053
LUTHER SM; DUDAS MJ; RUTHERFORD PM: "Radioactivity and chemical characteristics of Alberta phosphogypsum", WATER AIR SOIL POLLU, vol. 69, 1993, pages 277 - 290
MAY A; SWEENEY JW: "The Chemistry and Technology of Gypsum. ASTM Special Technical", vol. 861, 1984, article "Assessment of environmental impacts associated with phosphogypsum in Florida", pages: 116 - 139
NAYAK S; MISHRA CS; GURU BC; RATH M: "Effect of phosphogypsum amendment on soil physico-chemical properties, microbial load and enzyme activities", J ENVIRON BIOL, vol. 32, 2011, pages 613 - 607
NESBO CL; DLUTEK M; ZHAXYBAYEVA O; DOOLITTLE WF.: "Evidence for existence of ''mesotogas'', members of the order Thermotogales adapted to low-temperature environments", APPL ENVIRON MICROBIOL, vol. 72, 2006, pages 5061 - 5068
NESBØ CL; BRADNAN D; ADEBUSUYI A; DLUTEK M; PETRUS A; FOGHT J ET AL.: "Mesotoga prima gen. nov., sp. nov., the first described mesophilic species of the Thermotogales", EXTREMOPHILES, vol. 16, 2012, pages 387 - 393
PAPASTEFANOU C; STOULOS S; IOANNIDOU A; MANOLOPOULOU M: "The application of phosphogypsum in agriculture and the radiological impact.", JENVIRON RADIOACT, vol. 89, 2006, pages 188 - 98, XP025128286, DOI: doi:10.1016/j.jenvrad.2006.05.005
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TAHER MA: "Influence of thermally treated phosphogypsum on the properties of Portland slag cement.", RES CONSERV RECYCL, vol. 52, 2007, pages 28 - 38, XP022240694, DOI: doi:10.1016/j.resconrec.2007.01.008
TAYIBI H; CHOURA M; LOPEZ FA; ALGUACIL FJ; LOPEZ-DELGADO A: "Environ impact and management of phosphogypsum", J ENVIRON MANAGE, vol. 90, 2009, pages 2377 - 2386, XP026160821, DOI: doi:10.1016/j.jenvman.2009.03.007
WOLICKA D.; BORKOWSKI A: "Phosphogypsum biotransformation in cultures of sulphate reducing bacteria in whey", INTERNATIONAL. BIODETER BIODEGRAD, vol. 63, 2009, pages 322 - 327, XP025951226, DOI: doi:10.1016/j.ibiod.2008.09.011
Attorney, Agent or Firm:
BOURGOUIN, André et al. (Grosset-Fournier & Demachy, 54 rue Saint-Lazare, Paris, 75009, FR)
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Claims:
REVENDICATIONS

1. Process for the co -culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium.

2. Process for the co-culture according to claim 1, wherein the said co-culture is performed in a culture medium comprising at least one carbon source and at least one sulfur source.

3. Process for the co-culture according to claim 2, wherem the said at least one sulfur source is a sulfate or elemental sulfur and wherein the said at least one carbon source is a sugar.

4. Process for the co-culture according to any one of claims 2 to 3, wherein the said at least one sulfur source is phosphogypsum containing sulfate.

5. Process for the co-culture according to any one of claims 1 to 4, wherein the said anaerobic mesophilic bacterium pertaining to the genus Mesotoga is the bacterial strain deposited at Collection Nationale de Cultures de Microorganismes (CNCM) under n°I-4238, or mutant thereof.

6. Process for the co-culture according to any one of claims 1 to 5, wherein the said hydrogenotrophic sulfate-reducing bacterium is chosen among the Deltaproteobacteria class, particularly among the Desulfovibrionales order, particularly among the Desulfovibrio genus, the Desulfomicrobium genus and the Desulfocurvus genus, and more particularly the said hydrogenotrophic sulfate- reducing bacterium is the species Desulfovibrio vulgaris of the Desulfovibrio genus, particularly the subspecies Desulfovibrio vulgaris vulgaris.

7. Process for the co-culture according to any one of claims 1 to 5, wherein the said hydrogenotrophic sulfate-reducing bacterium is chosen among the Clostridia class, particularly among the Clostridiales order, particularly among the Desulfotomaculum genus and the Desulfosporosinus genus and more particularly the said hydrogenotrophic sulfate-reducing bacterium is the species Desulfotomaculum ruminis of the Desulfotomaculum genus.

8. Process for the co-culture according to any one of claims 3 to 7, wherein the said sugar is a C6 sugar, in particular fructose or glucose.

9. Process for the co-culture according to any one of claims 3 to 7, wherein the said sugar is a C5 sugar, in particular xylose. 10. Process for the co-culture according to any one of claims 3 to 7, wherein the said sugar is sucrose or lactose.

11. Process for the co-culture according to any one of claims 3 to 10, wherein the said sugar has a concentration from 2 mM to 50 mM, particularly from 10 mM to 20 mM.

12. Process for the co-culture according to any one of claims 3 to 11, wherein the said co- culture is carried out at a temperature ranging from 20 °C to 50 °C, in particular at the temperature of 37°C. 13. Process for the co-culture according to any one of claims 2 to 12, wherein the said sulfate is at a concentration ranging from 2 to 40 mM, particularly 20 mM.

14. Process for the co-culture according to any one of claims 1 to 13, wherein the anaerobic mesophilic bacterium of bacterial strain deposited at Collection Nationale de Cultures de Microorganismes (CNCM) under n°I-4238 is co-cultured with the hydrogenotrophic sulfate-reducing bacterium Desulfovibrio vulgaris or Desulfotomaculum ruminis, in a culture medium containing sulfate and glucose.

15. Method for the cleanup of sites or sediments contaminated by phosphogypsum or heavy metals and/or metalloids, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacteria pertaining to the genus Mesotoga, with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar.

Description:
PROCESS FOR THE CO-CULTURE OF A BACTERIUM OF THE MESOTOGA LINEAGE AND AT LEAST ONE HYDROGENOTROPHIC SULFATE-REDUCING

BACTERIUM

The present invention relates to a process for the co-culture of mesophilic bacteria pertaining to the genus Mesotoga, order Kosmotogales, phylum Thermotogae (Bhandari and Gupta, 2014) and at least one hydrogenotrophic sulfate-reducing bacterium.

For a long time, cultivated representatives within Thermotogae were known to be essentially thermophilic to hyperthermophilic. The situation has changed when 16S rRNA gene sequences have been reported in many mesothermic environments suggesting that a mesophilic lineage {Mesotoga) of Thermotogae exists (Nesb0 et al, 2006). The first mesophilic representative of Thermotogae {Mesotoga prima strain PhosAc3) was cultivated in 2011 (Ben Hania et al, 2011, Ben Hania et al, 2015), thereafter two Mesotoga species, M. prima MesGl.Ag.4,2 T (Nesb0 et al, 2012) and M. infera VNsl00 T (Ben Hania et al, 2013) were characterized. Mesotoga strain PhosAc3, from its side, was recently recognized as a M. prima strain (Ben Hania et al., 2015). In contrast to the other members of the phylum Thermotogae, recognized as being thermophilic to hyperthermophilic, the species of the genus Mesotoga, such as the three cultivated strains of Mesotoga previously quoted namely the Mesotoga prima PhosAc3, Mesotoga prima MesGl.Ag.4.2 and Mesotoga infera VNsl00 T , exhibit a mesothermic range of growth temperature with optimum occurring between 37 °C and 45 °C and no growth occuring over 50 °C, thus demonstrating the existence of mesophilic species in Thermotogae.

To date, beside the genus Mesotoga, Thermotogae comprise 12 other genera including Defluviitoga, Fervidobacterium, Geotoga, Oceanotoga, Petrotoga, Thermococcoides, Thermosipho, Thermotoga, Athalassotoga and Mesoaciditoga. Thermotogae are usually considered as heterotrophic fermentative microorganisms able to use sugars, polysaccharides or complex organic substrates such as peptone and yeast extract.

Phosphogypsum (PG) is an industrial waste resulting from phosphate fertilizer production by treating apatite and phosphorite with sulfuric acid according to the following reaction:

Around 5 tons of PG are generated per ton of phosphoric acid produced, depending on the phosphate rock source material (Azabou et at, 2005, Nayak et at, 2011, Tayibi et at, 2009). The main components of PG are gypsum (CaSO 4 x 2H20) and bassanite (CaSO 4 x 1/2H20); sulfate ions accounts for approximately 50%. PG also contain high concentration of hazardous impurities such as a heavy metals (Cd, Zn, Hg, Cu, Ni...) (May and Sweenly,1984; Luther et at 1993; Azabou et at 2005; El Zrelli et at 2015) and radioactive elements (U 238 ,Th 230 ,Po 210 ,Ra 226 ) (Borrego et at 2007; Burnett et at 1996; Rutherford et at 1994). Several ways of valorization of phosphogypsum were developed; it is used in agriculture as fertilizer for the cultures of peanuts (Papastefanou et at, 2006). The phosphogypsum was largely used in various applications of construction and building (Degirmenci 2008). Other researchers tested the effect of the use of phosphogypsum on Portland cement. They proved that it can improve mechanical properties (Kacimi et at, 2006 ; Taher, 2007). Unfortunately, these solutions of treatment of phosphogypsum are not of economical relevance ( Juszczak et at 2002).

The anaerobic bioremediation process of PG by sulfate reducing bacteria has been of peculiar interest in recent years (Azabou et at 2007; Wolika and Borkowski 2009).

Successful experiments have been undertaken to cultivate sulfate- reducing bacteria on PG in the presence of hydrogen, acetate, lactate, formate, propionate and butyrate as electron donors (Azabou et at 2007). Despite significant results have been obtained with regard to heavy metals removal in these conditions, the use of expensive organic compounds (e.g. lactate) or dangerous gas (e.g. H2) as compared to cheap easily available substrates (e.g. sugars) is still an obstacle to the use of this biological process.

Such a process makes the cleanup of soil contaminated by phosphogypsum, expensive and dangerous.

The Inventors have surprisingly discovered that co-culturing Mesotoga species with hydrogenotrophic sulfate-reducing partners in the presence of a sulfur source, in particular in presence of sulfate or elemental sulfur, as terminal electron acceptors, significantly improved carbon source oxidation by the former. Thus, they demonstrate two major points regarding Mesotoga species: (i) tlieir dependence on the presence of a sulfur source as terminal electron acceptor to use glucose, thus indicating that in contrast to all Thermotogae known so far, sugars are rather oxidized than fermented and (ii) the efficient syntrophic association with hydro genotrophic sulfate-reducing partners as biological terminal electrons facilitating carbon source oxidation. The Inventors have thus demonstrated an unusual prokaryotic metabolism within the Thermotogae, e.g. the obligate oxidation of carbohydrates (known as easily fermented by other members within the Thermotogae in the absence of any chemical or biological electron acceptor) thanks to a syntrophic association with a hydrogenotrophic sulfate-reducing partner and its possible ecological significance in nature. The Inventors have furthermore surprisingly discovered that the syntrophic association between an anaerobic mesophilic bacterium pertaining to the genus Mesotoga and a hydrogenotrophic sulfate-reducing bacterium was able to use sulfate originating from phosphogypsum, in the presence of a carbon source, such as sugars (hexoses and pentoses), producing acetate and sulfide. Thus, they have discovered a process for the cleanup of soils contaminated by heavy metals, metalloids and radionucleids originating from phosphogypsum, using a cheap easily available substratum.

The present invention concerns a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate- reducing bacterium.

The term "hydrogenotrophic" refers to a microorganism which oxidizes hydrogen as energy source in the presence of a teiminal electron acceptor (e.g. sulfate, carbon dioxide etc.) .

According to the present invention, the expressions " genus Mesotoga" and "Mesotoga lineage" can be used indifferently.

The term "sulfate-reducing" refers to a microorganism which is able to obtain energy by oxidizing organic compounds or molecular hydrogen (H 2 ) while reducing sulfate (SO 4 2~~ ) to hydrogen sulfide (H 2 S). In a sense, these organisms perform a sulfate respiration under anaerobic conditions. In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said co-culture is performed in a culture medium comprising at least one energy source and at least one sulfur source. In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said energy source is a carbon source.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said co-culture is performed in a culture medium comprising at least one carbon source and at least one sulfur source.

The term "energy source" refers to any mineral (e.g. hydrogen) or organic compound (e.g. carbohydrates, amino acids, fatty acids, etc.) that is used to deliver energy (ATP) during the process of catabolism.

The term "carbon source" refers to any carbon containing molecules (carbohydrates, amino acids, fatty acids, carbon dioxide) used by an organism for its growth and subsequent synthesis of organic molecules during anabolism.

The term « sulfur source » refers to any molecule under an oxidized form containing at least one sulfur atom, including elemental sulfur and sulfate.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said at least one sulfur source is sulfate and wherein the said at least one carbon source is a sugar.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said at least one sulfur source is elemental sulfur and wherein the said at least one carbon source is a sugar. In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said at least one sulfur source is phosphogypsum containing sulfate. In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said anaerobic mesophilic bacterium pertaining to the genus Mesotoga is the bacterial strain PhosAc3, corresponding to the bacterial strain deposited at Collection Nationale de Cultures de Microorganismes (CNCM) under n°I-4238, or mutant thereof.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said hydrogenotrophic sulfate- reducing bacterium is chosen among the Deltaproteobacteria class.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said hydrogenotrophic sulfate- reducing bacterium is chosen among the Deltaproteobacteria class, particularly among the Desulfovibrionales order and particularly among the Desulfovibrio genus, the Desulfomicrobium genus and the Desulfocurvus genus.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said hydrogenotrophic sulfate- reducing bacterium is the species Desulfovibrio vulgaris of the Desulfovibrio genus, particularly the subspecies Desulfovibrio vulgaris subsp. vulgaris.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said hydrogenotrophic sulfate- reducing bacterium is chosen among the Clostridia class.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said hydrogenotrophic sulfate- reducing bacterium is chosen among the Clostridia class, particularly among the Clostridiales order and particularly among the Desulfotomaculum genus and the Desulfosporosinus genus.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said hydrogenotrophic sulfate- reducing bacterium is the species Desulfotomaculum niminis of the Desulfotomaculum genus.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said at least one sugar is a C6 sugar.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said C6 sugar is fructose.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said C6 sugar is glucose. In a particulai- embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said at least one sugar is a C5 sugar. In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said C5 sugar is xylose.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said sugar is sucrose.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said sugar is lactose. In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said at least one sugar has a concentration from 2 mM to 50 mM, particularly from 10 mM to 20 mM.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said co-culture is carried out at a temperature ranging from 20 °C to 50 °C.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said co-culture is carried out at the temperature of 37°C.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said at least one sulfur source is sulfate at a concentration ranging from 2 to 40 mM, particularly 20 mM.

In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said anaerobic mesophilic bacterium is the bacterial strain PhosAC3 deposited at Collection Nationale de Cultures de Microorganismes (CNCM) CNCM under n°I-4238 and the said one hydrogenotrophic sulfate- reducing bacterium is Desulfovibrio vulgaris, said co-culture being performed in a culture medium containing sulfate and glucose. In a particular embodiment, the present invention relates to a process for the co-culture of an anaerobic mesophilic bacterium pertaining to the genus Mesotoga with at least one hydrogenotrophic sulfate-reducing bacterium, wherein the said anaerobic mesophilic bacterium is the bacterial strain PhosAC3 deposited at Collection Nationale de Cultures de Microorganismes (CNCM) CNCM under n°I-4238 and the said one hydrogenotrophic sulfate- reducing bacterium is Desitlfotomaculum ruminis, said co-culture being performed in a culture medium containing sulfate and glucose.

The present invention also concerns a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar.

The expression "sites or sediments" means marine, freshwater, and terrestrial environments in the form of soil or water; or sediments thereof. The present invention also concerns a method for the cleanup of sites or sediments contaminated by heavy metals and/or metalloids, particularly contained in phosphogypsum., comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate- reducing bacterium, in presence of at least one sugar. Phosphogypsum is a waste product of the fertilizing industry containing high amounts of heavy metals and metalloids.

According to the invention, heavy metals essentially include Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sn, Tl and Zn.

Metalloids are chemical elements which have properties in between those of metals and nonmetals act very well with sulphides like other metals. Metalloids include in particular the elements As and Se.

Thus, according to the invention, heavy metals and metalloids essentially include Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sn, ΤΙ,Ζη, As and Se.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by Zn contained in phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga with a hydrogenotrophic sulfate-reducing bacterium, preferably cultured according to the process described above, in presence of at least one sugar.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said anaerobic mesophilic bacterium of the genus Mesotoga is the bacterial strain PhosAC3 deposited at Collection Nationale de Cultures de Microorganismes (CNCM) CNCM under n°I-4238.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said hydrogenotrophic sulfate-reducing bacterium is chosen among the Deltaproteobacteria class. In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydro genotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said hydro genotrophic sulfate-reducing bacterium is chosen among the Deltaproteobacteria class, particularly among the Desulfovibrionales order and particularly among the Desulfovibrio genus, the Desulfomicrobium genus and the Desulfocurvus genus.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contammated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said hydrogenotrophic sulfate-reducing bacterium is the species Desulfovibrio vulgaris of the Desulfovibrio genus, particularly the subspecies Desulfovibrio vulgaris subsp. vulgaris. In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contammated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said hydrogenotrophic sulfate-reducing bacterium is chosen among the Clostridia class, particularly among the Clostridiales order and particularly among the Desulfotomaculum genus and the Desulfosporosinus genus.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treating the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said hydrogenotrophic sulfate-reducing bacterium is the species Desulfotomaculum ruminis of the Desulfotomaculum genus.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said at least one sugar is a C6 sugar.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said C6 sugar is glucose or fructose.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said at least one sugar is a C5 sugar.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treating the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said C5 sugar is xylose.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said at least one sugar is a sucrose or lactose.

In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said at least one sugar is glucose and wherein the said anaerobic mesophilic bacterium pertaining to the genus Mesotoga is the bacterial strain PhosAC3 deposited at Collection Nationale de Cultures de Micro organismes (CNCM) under n°I-4238 and the said hydrogenotrophic sulfate-reducing bacterium is Desulfovibrio vulgaris. In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said sugar has a concentration from 2 mM to 50 mM, particularly from 10 niM to 20 mM.

_In a particular embodiment, the present invention relates to a method for the cleanup of sites or sediments contaminated by phosphogypsum, comprising treatment of the contaminated sites or sediments with a co-culture of anaerobic mesophilic bacterium pertaining to the genus Mesotoga , with a hydrogenotrophic sulfate-reducing bacterium, in presence of at least one sugar, wherein the said co-culture is carried out at a temperature ranging from 20°C to 50°C, in particular at the temperature of 37°C.

Material and Methods

Media and culture conditions

Mesotoga prima strain PhosAc3 corresponds to the bacterial strain CNCM 1-4238 deposited at Collection Nationale de Cultures de Microorganismes on November 3, 2009. M. prima strain PhosAc3 was grown with glucose (20mM) at 37°C in a basal medium (BM) containing per liter of distilled water 0.3 g KH 2 PO 4 ; 0.3 g K 2 HPO 4 ; 1.0 g NH 4 C1; 2.0 g NaC1 ; 0.1 g KC1; 0.1 g CaC1 2 .2H 2 O; 0.5 g MgC1 2 .6H 2 O; 1 g yeast extract; 0.5 g cysteine-HC1; 0,16 g sodium acetate ; 1 mL Widdel trace element solution (Widdel and Pfennig, 1984) and 1 mL resazurin 0.1 %. When specified, 10 grams of elemental sulfur (S°) were added per liter of medium.

Before culture inoculation, 0.2 mL of 10% (w/v) NaHC0 3 , 0.1 mL of 2% (w/v) Na 2 S-9H 2 O and glucose, were injected from sterile stock solutions in the culture medium.

The sulfate-reducing bacteria Desulfovibrio vulgaris subsp. vulgaris and Desulfotomaculum gibsoniae were grown in the same culture medium as described for M. prima strain PhosAc3 with glucose being replaced by hydrogen (H 2 -CO 2 ; 80:20 v/v) as electron donor and elemental sulfur by sodium sulfate (Na 2 SO 4 ) 4 g/L, as electron acceptor. Co-culture were performed in the same medium as for culturing M. prima PhosAc3, except that elemental sulfur was replaced by sodium sulfate (4 g/L).

Control culture of M. prima strain PhosAc3 was grown in the same culture medium without elemental sulfur and without hydrogenotrophic sulfate-reducing bacterium.

Cultures and co-cultures (obtained by mixing exponential growth cultures of Mesotoga strain PhosAC3 and sulfate-reducing bacteria 10% v/v each) were performed under anaerobic conditions in Hungate tubes or in large volume flasks inoculated with 10 % of growth exponential phase culture or co-culture and incubated at 37°C. Growth was monitored by measurement of the OD at 600 nm of the culture directly in the Hungate tube.

For co-culture in the presence of phosphogypsum as electron acceptor (sulfate source), cultures were performed under anaerobic conditions at 37°C in penicillin bottles.

The studied sample of PG was collected from a phosphogypsum stock at Sfax, Tunisia. The phosphogypsum samples were well homogenized by means of shaking.

Analytical methods

Soluble sulfides were quantified according to the Cord-Ruwish method (1985) by using a Shimadzu UV-160A spectrophotometer (Shimadzu Co., Kyoto, Japan). In an acid medium, sulfides were converted into colloidal solution of copper sulfide CuS by reaction with copper sulfate. The copper sulfide was determined spectrophotometrically at 480 nm. One hundred μL of the sample was added to 4 mL of an assay solution (50 mM HC1, 5 mM copper sulfate). The reaction mixtui'e was vortexed before reading the optical density. The determination of the sulfide concentration was done by using a calibration curve made with Na 2 S-9H 2 0. To determine the total sulfides, partitioned between the gas phase and liquid of the culture, the equilibrium was shifted to the liquid phase by raising the pH (up to 12) by addition of aliquot of 10 MNaOH. Sugars (lactose, glucose, fructose, sucrose), organic acids (acetate, lactate, propionate, butyrate ...) and ethanol concentrations are carried out by high performance liquid chromatography (HPLC) as described in Fardeau et al, (1997) by using a differential refractometer detector (Shimadzu RID 6 A, Japan) connected to a computer lmming WINILAB III software (Perichrom, France). Briefly, 1 mL samples were withdrawn from the culture at regular time interval. Samples were then centrifuged for 5 minutes at 14,500 rpm and 20 μΙ_, of the supernatant was loaded onto an Animex HPX-87H column (Biorad) set at 35°C and eluted at 0.5mL min -1 with H 2 SO 4 solution (0,75 mM). Product concentrations were determined by using a differential refractometer detector (Shimadzu RID 6 A, Japan) connected to a computer running WINILAB III software (Perichrom, France).

For hydrogen quantification, one mL of culture headspace sample was injected into a TCD- GC system (Shimadzu 8A, Japan) equipped with a concentric CTR1 column (Alltech, USA), connected to a computer running WINILAB III software (Perichrom, France) (Fardeau et al, 1997). Unless otherwise indicated, analytical measures were performed on duplicate culture tubes or bottles.

Soluble sulfides were quantified according to the Cord-Ruwish method (1985) by using a Shimadzu UV-160A spectrophotometer (Shimadzu Co., Kyoto, Japan).

Zinc concentrations present in the soluble fraction were determined by atomic absorption spectrometry (Spectra AA 220 FS, Varian) as described by Bouain et al. (2014) (Bouain N., Kisko M., Rouached A., Dauzat M, Lacombe B., Belgaroui N., Ghnaya T., Davidian J-C, Berthomieu P., Abdelly C, Rouached H., 2014. Phosphate/Zinc Interaction Analysis in Two Lettuce varieties Reveals Contrasting Effects on Biomass, Photosynthesis, and Dynamics of Pi Transport. BioMed Research International, Volume 2014, article ID 548254).

FIGURES Figure 1: (A) Glucose consumption in either pure culture of M. prima in the absence (black square) or presence (white circle) of elemental sulfur or co -culture with Desulfovibrio vulgaris subsp. vulgaris (white triangle) over the time.

(B) Acetate production in either pure culture of M. prima in the absence (black square) or presence (white circle) of elemental sulfur or co-culture with Desulfovibrio vulgaris subsp. vulgaris (white triangle) over time.

Figure 2: (A) evolution of the growth rate of the co-culture in function of the number of sub- culturing. (B): Growth curve of M. prima (■) and D. vulgaris (A) in pure culture or in co- culture (·) in glucose/sulfate medium.

Figure 3: Concentration of glucose (A), of acetate production (B), and of sulfide production (C) by pure culture of Mesotoga prima strain pure culture of Desulfovibrio vulgaris subsp. and co-culture in the presence of

different concentrations of phosphogypsum after three weeks of incubation.

Figure 4: Sugar consumption (A), acetate production (B), and sulfide production (C) after 3 weeks of incubationby pure culture of Mesotoga prima strain PhosAc3 (MESO), pure culture of Desulfovibrio vulgaris subsp. vulgaris (DVV) and co-culture Mesotoga-Desulfovibrio. Negative control (without carbon source), 11 glucose,■ fructose, sucrose.

Figure 5: Time evolution of sugar consumption (A), acetate (B) and total sulfide (C) production by co-culture {Mesotoga-Desulfovibrio) for negative control (without carbon

source), fructose and glucose as carbon source. Figure 6: Co-culture of Mesotoga prima PhosAc3 (clear cells) with Desulfotomaculum gibsonia (dark cells)

EXAMPLES I- Growth of Mesotoga prima PhosAC3 in pure culture and in co-culture with sulfate-reducing bacteria

When M. prima PhosAc3 was cultured with glucose, only a slight glucose consumption was observed even after 250 days of incubation at 37°C only in the presence of yeast extract (Figure 1A). When elemental sulfur was added in the medium, a slow linear degradation of glucose was measured with 6.62 ± (0.19) mM of the glucose consumed (around 33%) after 250 days of incubation (Figure 1A, table 1). Accordingly, only a slight acetate production was measured in the absence of elemental sulfur while 10 mM acetate was produced in its presence after 250 days. The end-products of glucose metabolism detected were only acetate (Figure IB), C0 2 and sulfide (Table 1). Surprisingly, only trace amounts of hydrogen (less than 1 μΜ) were detected in the gas phase during glucose consumption whatever the presence or the absence of elemental sulfur.

These data clearly showed that M. prima PhosAc3 was unable to ferment glucose while in the presence of an external electron acceptor (elemental sulfur), it was able to oxidize glucose although with a low efficiency as only around 33% of the initial glucose was consumed after 250 days of incubation. The slight glucose consumption in the absence of elemental silfur was probably due to the presence of an electron acceptor available in yeast extract since hydrogen was detected in the gas phase at concentration as low as 1μΜ (limit of detection by gas chromatography).

Table 1 : End-product quantification of the single and co-cultures of M, prima PhosAc3 after 250 days of incubation.

Desulfovibrio vulgaris subsp. vulgaris was used to test the syntrophic association with M. prima PhosAc3. Sulfate-reducing bacteria are anaerobic prokaryotes which gain energy for biosynthesis and growth by coupling oxidation of organic compounds or molecular hydrogen to reduction of sulfate to sulfide. When either M. prima PhosAC3 or Desulfovibrio vulgaris subsp. vulgaris alone was cultured in a glucose/sulfate medium, no growth was observed (Figure 2B). However, when the two bacteria were co-cultured, growth occurred as evidenced by a substantial increase in the optical density at 580 nm (Figure 2 A and 2B) and in sulphide and acetate produced (Table 1). 80% of the initial glucose was consumed (-12 mM) with the concomitant acetate production of - 25 mM after only 20 days of incubation (Figures 1A-1B). After 20 days, glucose consumption and acetate production only slighty progressed to give -13 mM glucose consumed and ~28 mM acetate produced after 250 days of incubation (Figures 1A-1B, Table 1). At this time, about 10 mM sulfide had been produced (Table 1).

Similar results were obtained when M prima PhosAc3 was co-cultured with another sulfate- reducing bacterium (SRB) Desulfotomaculum gibsonia as shown on Figure 6. In the same way, M. infer a VNs100 T , which was also not able to grow on glucose in pure culture in the absence of elemental sulfur as terminal electron acceptor, grew well when it was co-cultured with Desulfovibrio vulgaris subsp. vulgaris.

II - Co-culture in the presence of phosphogypsum as electrons acceptor

Experiments using basal medium and phosphogypsum (PG) as sulfate source have been performed with the aim to produce sulfide and also remove heavy metals from the waste of the industry producing phosphoric acid. The above experiments have demonstrated that a syntrophic association between Mesotoga prima strain PhosAC3 and D. vulgaris to oxidize glucose was successful. Thus, this co-culture was tested using economically advantageous substrates such as sugars, taking into account that the traditionally used substrates for treating PG are much more expensive (e.g lactate) and even dangerous (e.g. hydrogen) (Azabou et al. 2007) through industrial application. Neither Mesotoga prima strain PhosAC3 nor D. vulgaris were able to grow alone in the culture medium designed. In contrast, as demonstrated previously, when both bacteria Mesotoga prima strain PhosAC3 and Desulfovibrio vulgaris subsp. vulgaris were co-cultivated, growth was obtained in the basal medium containing on glucose (20 mM) in the presence of phosphogypsum (lOg/1) after three weeks of incubation at 37°C.

Acetate and sulfide were the main end-products of glucose oxidation by this co-culture as reported earlier. They result from a syntrophic association between Mesotoga prima strain PhosAc3 and D. vulgaris subsp. vulgaris where the former acts as the glucose oxidizing bacterium, while the latter acts as hydrogen and/or formate scavenger since none of these compounds accumulate in the gas (H 2 ) or the liquid (formate) phase.

The turbidity caused by the presence of phosphogypsum in the culture medium prevented the measurement of the bacterial growth by the optical density. Therefore the proportioning by HPLC of the consumption of glucose and production of acetate by Mesotoga prima strain PhosAc3, and sulfide production by D. vulgaris were used.

The syntrophic association between Mesotoga Prima strain phosAc3 and Desulfovibrio vulgaris subsp. vulgaris used the sulfate containing in the phosphogypsum as electron acceptor as confirmed by sulfide production (Figure 3C).

II- 1. Optimal concentration of phosphogypsum as sulfate source

The determination of the optimal concentration of phosphogypsum for the growth of pure culture of Mesotoga prima strain PhosAC3, Desulfovibrio vulgaris subsp. vulgaris and co- culture Mesotoga-Desulfovibrio was carried out in the presence of 20 mM of glucose in the culture medium. Concentration from 0 to 80 gram per liter of phosphogypsum in the culture medium was tested. The bacterial growth was esfimatedby measuring glucose consumption and acetate production on the one hand and the measuring ofsulfide production on the other hand. After three weeks of co-culture incubation, the optimum phosphogypsum concentration was obtained from 5 to 20 g/1 followed by 40 g/1 (Figure 3A). Starting from an initial concentration of glucose (~20 mM), Figure 3 A shows a consumption of 7,47 mM +/- 0,24 of glucose accompanied by a production of 12,44 mM +/- 0,77 of acetate by the co-culture in the presence of 5, 10 and 20 g/1 phosphogypsum, (Figure 3B). The consumption of glucose by the co-culture observed in the presence of 80 g/1 of phosphogypsum was only 2,5 mM +/- 0,17 with a production of 4,7 mM +/- 0,51 of acetate (Figure 3B) and 3 mM +/- 0,78 of sulfides (Figure 3C). No bacterial growth was observed in the absence of phosphogypsum by the co- culture. No bacterial growth was detected for two pure cultures tested {Mesotoga alone and Desulfovibrio alone) in the presence of different concentrations of phosphogypsum.

In this experiment, when grown on sugars, the tested co-culture tolerated up to 80 g/1 of phosphogypsum and optimal growth was observed at concentrations ranging from 5 to 20 g/1 of phosphogypsum. Maximum sulfide production was 15 mM. Co-culture growth decreased with concentration of phosphogypsum higher than 20 g/1 due possibly to inhibition caused by the toxic effect of sulfide or heavy metals/metalloids contained in phosphogypsum.

II- 2. Sugar utilization by the pure culture and the co-culture

20 mM of glucose, fructose and sucrose were tested in order to determine the preferred sugar to be used by the co-culture Mesotoga - Desulfovibrio in the presence of 15 g/1 of phosphogypsum.

Figure 4A shows that fructose was the preferred carbon/energy source to be used by the co- culture (l lmM +/- 0,77) which yielded the highest acetate and sulfide production 16 +/- 1,5 and 18,32 +/- 1,37 Mm respectively, followed by glucose with 7,12 +/- 0,76 mM of glucose consumed for a production of 11,84 +/- 1,03 mM of acetate (Figure 4B) and 11,42 +/- 1,31 mM of sulfides (Figure 4C), respectively.

Slight growth was obtained by the co -culture in the absence of sugar thus demonstrating that the co-culture can use yeast extract (1 g/1) present in the culture medium.

Neither Mesotoga strain PhosAC3 nor Desulfovibrio vulgaris subsp. vulgaris were shown to use sugars in pure cultures (Figure 4A).

This is the first time that a high sulfide production (15 and 19 mM of sulfide produced from glucose and fructose oxidation respectively) was obtained for phosphogypsum treatment using sugars as carbon source. II- 3. Kinetics of sugar consumption, acetate and sulfide production by the co- culture Mesotoga-Desulfovibrio

Figure 5A shows the evolution on time of glucose and fructose (20 mM) degradation by the co-culture, in the presence of 15 g/1 of phosphogypsum as sulfate source. 30 days are essential to obtain maximum sulfide production of 16,2 +/- 0,92 mM (Figure 5C) which is clearly correlated to sugar consumption and acetate production (18,5 +/- 0,12 mM) (Figure 5A and 5B), in the presence of fructose.

In term of incubation (between 20 and 30 days incubation), similar results with the production of 11,5 mM +/- 1,09 and 12,4 mM +/- 0,42 of acetate and sulfide respectively, were obtained with this co -culture grown on glucose in the presence of sulfate as terminal electron acceptor. This suggests that phosphogypsum at 15g/L has not an inhibitory effect despite containing significant amounts of heavy metals and possibly of other toxic products.

III- Bioprecipitation of Zn containing in the phosphogypsum by the co-culture

Zn is one of the major heavy metals which is contained in the Tunisian phosphogypsum. The production of sulfide by the co-culture growth was at the origin of the precipitation of Zn containing in the culture medium with 15 g/1 of phosphogypsum. This precipitation was studies after three weeks of incubation in the presence of glucose and fructose as carbon source.

Experiments were performed with the co-culture in the presence of 20 mM glucose or 20 mM fructose and 15g/l of phosphogypsum as sulfate source. They demonstrate a highly significant decrease in Zn concentrations after seven weeks of incubation at 37°C.

Table 2 shows the variation of Zn concentrations at initial stage and after treatment with the co-culture. In the presence of glucose, the Zn concentration decreases from 406 mg/1 +/- 86,3 to 20 mg/1 +/- 12,6. The reduction of Zn concentration was more important when the co- culture used fructose as carbon/energy source. Indeed in this case the Zn concentration decreases from 358 mg/1 +/- 74,3 to 8,66 mg/1 +/- 3,55. No significant Zn concentration decrease was observed under the same culture conditions by pure cultures of Mesotoga prima strain PhosAc3 or D. vulgaris.

The capacity of the sulfide-producing bacteria coupled to Mesotoga to catalyze the formation of insoluble precipitates mineral containing heavy metals, can represent an interesting process to immobilize and confine toxic metals. In this respect, the sulfate reductive activity of D. vulgaris linked to the oxidation of sugars by Mesotoga prima strain PhosAc3 is not only beneficial for sulfide production but also for Zn removal. This is in agreement with the sulfate-reducing bacteria which have the capacity to catalyze the formation of insoluble precipitates mineral containing heavy metals, since sulfide produced by sulfate-reducing bacteria may be recycled to be used in phosphoric acid production. Therefore, the proposed co-culture appears as a good candidate to immobilize and confine toxic metals originating from phosphogypsum. IV- Co-cultures for the bioremediation of heavy metals contaminated sites

While Mesotoga prima strain PhosAc3 is of terrestrial origin and does not require NaC1 for growth, Mesotoga prima strain MesGl.Ag.4.2 is slightly halophilic and thus requires NaCl. In this respect, Mesotoga prima strain PhosAc3 can be coupled to non-halophilic hydro genotrophic sulfate-reducing bacteria (e.g. Desulfovibrio vulgaris, etc.) and Mesotoga prima strain MesGl.Ag.4.2 to halophilic hydrogenophilic sulfate-reducing bacteria ( e.g. Desulfovibrio salexigens, etc.) for producing I¾S and removing metals and metalloids using freshwater and marine water, respectively, Mesotoga prima strain MesGl.Ag.4.2, Desulfotomaculum aquiferis, and Desulfotomaculum profundi have been isolated from the same deep aquifer-derived hydrocarbonoclastic (Benzene, Toluene, Ethylbenzene and Xylenes, also called BTEX) community and were found of ecological significance in this ecosystem (Berlendis et al, 2016). Thus, M infera can be coupled with D. aquiferis or D. profundi for treating BTEX.

For the bioremediation of heavy metal contaminated sites, the following association are effective:

(i) Mesotoga prima strain PhosAc3 + non halophilic hydrogenotrophic sulfate- reducers to be used with freshwater.

(ii) Mesotoga prima strain MesGl .Ag.4.2 + halophilic hydrogenotrophic sulfate- reducers to be used with marine water. For treating BTEX, the following association are effective:

(i) Mesotoga prima strain MesGl. Ag.4.2 + Desulfotomaculum aquiferis

(ii) Mesotoga prima strain MesGl. Ag.4.2 + Desulfotomaculum profundi

(iii) Mesotoga prima strain MesGl. Ag.4.2 + Desulfotomaculum aquiferis + Desulfotomaculum profundi

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