WO2019086894A1 - Exoelectrogenic reactors, methods of cultivating microorganisms and of metabolising c1 carbon sources - Google Patents

Abstract

The present invention provides an exoelectrogenic reactor comprising: an anode chamber, comprising an anode, an exoelectrogen; and a fluid comprising a source of C-1 carbon and an electron donor; at least one cathode chamber, comprising a cathode and an electron acceptor; an electrical connector between the anode and cathode; and at least one proton-permeable barrier between the anode chamber and cathode chamber. The invention further provides methods of cultivating exoelectrogens using C-1 carbon sources, and methods of fixing carbon using exoelectrogens.

Description

EXOELECTROGENIC REACTORS, METHODS OF CULTIVATING MICROORGANISMS AND OF METABOLISING CI CARBON SOURCES

Technical Field of the Invention

The present invention relates to exoelectrogenic reactors, methods of fermenting C- 1 carbon sources and methods of cultivating microorganisms. In particular, the present invention is directed towards cultivation of microorganisms for carbon fixation in exoelectrogenic reactors using C-l carbon sources.

Background to the Invention

Renewable carbon feedstocks represent an important opportunity in transforming the current chemical industry from a dependency on fossil reserves and energy intensive processing to a more sustainable, low carbon bio-economy in the production of bio- derived chemicals and fuels. Whereas monosaccharides represent first generation feedstocks and lignocellulosic feedstocks represent second generation feedstocks, third generation C-l carbon feedstocks such as C0₂, CO and methane offer opportunities to exploit a wider array of renewable and waste carbon feedstocks.

It is known to culture microorganisms which can metabolise C-l carbon sources, such as CO2, CO and methane. Such cultures can provide some carbon fixation and enable the synthesis of larger organic molecules for subsequent use. On the other hand, C-l carbon source fermentations in suspended culture generally require H2/O2 and CH4/O2 mixtures as a feedstock and so mass culturing and conversion is limited by flammability limits associated with the ¾ / O2 and CH4 / O2 mixtures, which adversely impacts on exploitation of C-l molecules as a raw material for fermentation applications. Known reactors (such as fermentation tanks) utilising microorganisms metabolising C-l carbon feedstocks therefore suffer from flammability safety concerns, as the flammable components (for example, H₂ and CH4) are mixed with 0₂. Furthermore, the vast volumetric flow rates associated with attaining sufficient mass transfer rates in such fermentations, impacts negatively on operating costs. Accordingly, against this background, it is clear that there is a need to improve the techno-economics of C-l carbon source fermentations using alternate reactor configurations.

Microbial fuel cells (MFC) are a bio-electrochemical device that harnesses the power of respiring microbes to convert organic substrates directly into electrical energy. At its core, the MFC is a fuel cell, which transforms chemical energy into electricity using oxidation reduction reactions. MFCs comprise an anode and a cathode, separated by a proton exchange membrane, connected through an electric circuit for electricity generation. Carbon-based fuel sources for microorganisms within known microbial fuel cells are limited to complex carbon compounds (C2 and above carbon sources) which include sugars; soil and sediments derived from plant and animal detritus, dead bacteria and plankton; fecal matter; and anthropogenic organic materials, but a particularly important source is organic acids in wastewater and chemical waste, such as acetic acid (CH3COOH). Organic acid waste streams are fed to a typically mixed culture immobilised on an anode, where the organic acid is converted to C0₂, biomass and electrons. The electrons pass through the electric circuit with the aim of producing renewable electricity, after which the electrons are donated to an electron acceptor such as O2 at the cathode. There are two main types of microbial fuel cells, mediator-based and mediator- less. Historically most microbial fuel cells required a mediator chemical to transfer electrons from the bacterial cells to the electrode, but mediators are often toxic and expensive, making commercialisation difficult. Around 1999, a new type of microbial fuel cell, so-called mediator-less microbial fuel cells, were developed. Shewanella putrefaciens, unlike most fuel cell bacteria at the time, was found to be electrochemically active. This bacterium (and others found and developed subsequently) can respire directly into the electrode under certain conditions by using the anode as an electron acceptor. Bacteria that can transfer electrons extracellularly, are called exoelectrogens and are all generally electrochemically active bacteria. While aerobic bacteria use oxygen as their final electron acceptor and anaerobic bacteria use other soluble compounds as their final electron acceptor, exoelectrogens can use a strong oxidizing agent or solid conductor as a final electron acceptor.

In known exoelectrogen-based microbial fuel cells, the exoelectrogen is generally fixed to the surface of the anode in an anode chamber and uses an oxidoreductase pathway to directly transfer electrons to the surface of the anode. Electron transfer mechanism may involve conductive pili, biofilm formation, or shuttling via intermediate compounds. The cathode chamber of a microbial fuel cell typically contains a cathode in an electrolyte flow consisting of an oxidizing agent in solution. The oxidizing agent is reduced as it receives electrons that have been transported to the cathode through an electrical circuit bridging the anode and the cathode.

In most microbial fuel cells, the cathode and anode are separated by a cation exchange membrane and linked together via an electrical wire (or other circuit). When an organic food source enters the anode chamber, the bacteria catabolise the organic matter to generate metabolites as well as protons, electrons, and carbon dioxide, with the anode serving as the electron acceptor in the bacteria's electron transport chain.

The electrons produced by the exoelectrogens pass from the anode to the cathode whilst the protons flow into the cathode chamber through the cation exchange membrane separating the two chambers. Oxygen or another electron acceptor at the cathode recombines with protons and the electrons from the cathode to produce water, completing the circuit. The electric circuit can include any manner of electrical components to harness the flow of electricity.

Whilst it is therefore known to utilise exoelectrogens in microbial fuel cells, to convert acetic acid, and other complex organic compounds to C0₂, and at the same time produce electrons for the generation of electricity, it would be advantageous to be able to provide a microbial fuel cell system which can utilise alternate feedstocks, especially those which before now have not been useful in such microbial fuel cells. Such alternate feedstocks could then be converted to (bio)chemicals and electricity. In most large-scale microbial fuel cells, the distinguishing features are: (1) the conversion of liquid organic (C-2 + carbon chains) waste streams to renewable electricity as product and CO2 as by-product, (2) the utilisation of a mixed culture population immobilised on the anode, and (3) avoiding the proliferation lithotrophic microorganisms in the immobilised culture. It would be advantageous to overcome at least one of the aforesaid limitations of such known large-scale microbial fuel cells.

It would be advantageous to make use of large-scale microbial fuel cell systems but utilising alternate feedstocks to organic acid waste streams and other complex carbon sources, producing biochemicals and electricity. It would furthermore be advantageous to provide a reactor, which is able to utilise C- 1 carbon sources to produce commercially useful products, or which fixes carbon from the carbon feedstock. It would further be advantageous If such reactors and methods could generate electricity as well as provide cultivation of exoelectrogens and conversion of C-l carbon sources into useful products.

It would also be advantageous to utilise C-l carbon source metabolising exoelectrogens in a reactor which reduces or eliminates problems caused by inherent flammability of the exoelectrogen reactants and feedstock materials.

It is therefore an aim of embodiments of the invention to overcome or mitigate at least one problem of the prior art.

Summary of the Invention

According to a first aspect of the invention there is provided an exoelectrogenic reactor comprising: At least one anode chamber, comprising: an anode; an exoelectrogen; a source of

C-l carbon; and an electron donor; at least one cathode chamber, comprising: a cathode: and an electron acceptor; an electrical connector between the anode and cathode; and optionally at least one proton-permeable barrier between the anode chamber and cathode chamber.

Surprisingly, the inventors have found that that an exoelectrogenic reactor configuration of the first aspect of the invention can be utilised for the valorisation of C-l carbon feedstocks, thereby providing a commercially useful apparatus and process for not only cultivating exoelectrogens, but also fixing C-1 carbon and producing valuable products of C-1 carbon metabolization by exoelectrogens, with reduced risk of flammable reactions. By "source of C- 1 carbon" we mean a molecule comprising a single carbon atom.

Suitable C-1 carbon sources include, carbon monoxide (CO), carbon dioxide (C02) and methane, or any mixture thereof (e.g. CO and C0₂; CO2 and methane; CO and methane; and CO, CO2 and methane). Formic acid may also be used as a source of C-1 carbon.

The source of C-1 carbon and/or the electron donor may be present in an electrolyte composition.

Suitable electron donors include hydrogen, carbon monoxide, methane, or any mixture thereof (e.g. hydrogen and CO; methane and CO, methane and hydrogen; or hydrogen, methane and CO).

It is to be appreciated that some sources of C-1 carbon may also act as an electron donor, such as carbon monoxide and methane, for example, and therefore in some embodiments there may not be separate C- 1 carbon sources and electron donors; however in other embodiments there is a both a C-1 carbon source and a separate electron donor. In some embodiments the anode chamber may comprise both a C-1 carbon source, and also hydrogen as an electron donor. Suitable electron acceptors include oxygen and/or nitrate, for example.

The exoelectrogenic reactor may comprise a plurality of anodes and/or a plurality of cathodes. The exoelectrogenic reactor may comprise a plurality of anode chambers and/or cathode chambers. For example, there may be at least 2, 3, 4, 5, 10, 15 or 20 anode chambers and/or at least 2, 3, 4, 5, 10, 15 or 20 cathode chambers. The plurality of anode chambers may be interconnected (fluidly interconnected) and may be either in series or in parallel. The plurality of cathode chambers may be interconnected (fluidly interconnected) and may be either in series or in parallel.

Each anode chamber may comprise one or more anodes, such as 2, 3, 4, 5, 10, 15 or 20 anodes. Each cathode chamber may comprise one or more cathodes, such as 2, 3, 4, 5, 10, 15 or 20 cathodes. The proton-permeable barrier may comprise a proton-exchange membrane or cation-exchange membrane, for example. Suitable materials for proton-permeable barriers include polymeric materials, such as fluoropolymers, graphene, and ceramic membranes. Suitable fluoropolymers include those sold under the trade name NAFION (RTM) by DuPont. The exoelectrogen may be selected from a lithotrophic and/or methanotrophic microorganism. The exoelectrogen may be an exoelectrogen from a genus selected from the group consisting of genus Acetobacterium, Clostridium, Cupriavidus, Eubacterium, Methylococcus, Methylomicrobium, Ralstonia and Rhodococcus. The aforesaid genera are particularly advantageous, as they encompass species which may convert C-l carbon sources, whereas many other genera of exoelectrogens are limited to C2+ sources for metabolism. Preferably, the exoelectrogen comprises a majority of a single genus selected from the aforesaid group and is preferably substantially a monoculture of one of the aforesaid genera. In some embodiments the exoelectrogen is Cupriavidus metallidurans and may be substantially a monoculture of the same. Cupriavidus metallidurans is particularly useful as it provides conversion of C-1 carbon sources to complex bio(chemicals) at relatively effective rate and concentration.

In some embodiments the exoelectrogen is one which produces an electron transfer mediator.

The exoelectrogen may be present as an immobilised culture, which may comprise a culture immobilised on the or each anode. In some embodiments the exoelectrogen may be present as a suspended culture. In yet other embodiments the exoelectrogen may be present in both a suspended culture and immobilised (preferably on the or each anode). When immobilised, the exoelectrogen is preferably immobilised on the anode as a biofilm.

The exoelectrogen may be present in a concentration which confers an OD₆oo (optical density at 600nm) of at least 2, 3, 5, 10, 15 or 20, and preferably at least 5 or at least 10. The anode chamber may further comprise one or more aqueous phase media, which media may include one or more nutrients, pH modulators (such as aqueous ammonia/ammonium, for example) and/or electron transport mediators (such as flavin compounds, for example). In some embodiments the aqueous phase medium comprises one or more nutrients, a pH modulator (which may be aqueous ammonia/ammonium) and an electron transport mediator (which may be a flavin).

The exoelectrogenic reactor may comprise a fluid circuit through which the aqueous phase media is circulated into, through and out of the or each anode chamber. Nutrients, electron donors, exoelectrogen and other materials may circulate through the fluid circuit, in use. The exoelectrogenic reactor may further comprise an effluent circuit through which electron acceptor may be introduced, and spent electron acceptor may be removed, from the or each cathode chamber

According to a second aspect of the invention there is provided a method of cultivating microorganisms and/or of metabolising a C-l carbon source, the method comprising the steps of: a) providing an exoelectrogenic reactor comprising at least one anode chamber comprising at least one anode and at least one exoelectrogen within the anode chamber; at least one cathode chamber comprising at least one cathode and at least one electron acceptor; at least one proton-permeable barrier between the anode and cathode; and wherein the anode and cathode are electrically connected; b) feeding a C-l carbon source and electron donor to the anodic chamber such that the exoelectrogen metabolises the C-l carbon source, to produce: at least one carbon- based product of metabolism; and electrons and protons which pass into the cathode chamber; and c) reducing the electron acceptor in the cathode in the presence of electrons and protons

Step a) may comprise cultivating the exoelectrogen in the anode chamber or on the anode, which may comprise cultivating the microorganism to an OD₆oo of at least 5 or at least 10. Step a) may comprise immobilising the exoelectrogen onto the anode, and the exoelectrogen may comprise a biofilm on the anode.

The exoelectrogen may be suspended in culture media and/or immobilised in the or each anode chamber or on the or each anode. Step a) may comprise forming a biofilm of immobilised exoelectrogen on the or each anode. In preferred embodiments the exoelectrogen is substantially a monoculture, and each anode chamber and/or anode may comprise the same exoelectrogen.

Step a) may further comprise creating anaerobic conditions within the or each anode chamber, which may comprise feeding C0₂ and H₂, optionally with N₂ into the anode chamber until substantially all oxygen (such as dissolved oxygen in aqueous media, or atmospheric oxygen) is removed. This step may be performed during or after cultivation of the exoelectrogen within the anode chamber or on the anode.

Step b) may comprise feeding a carbon source comprising a molecule having 2 or more carbon atoms (C2+) to the or each anode chamber, in addition to the C-1 carbon source. Suitable C2+ carbon sources include ethanoic acid, propanoic acid, butanoic acid, levulinic acid, or any mixture thereof, for example. In such embodiments the exoelectrogen acts to convert both C-1 carbon sources and C2+ carbon sources to useful products and/or electricity. The exoelectrogen may be an exoelectrogen from a genus selected from the group consisting oiAcetobacterium, Clostridium, Cupriavidus, Eubacterium, Methylococcus, Methylomicrobium, Ralstonia and Rhodococcus.

Step b) may comprise feeding an aqueous medium containing one or more nutrients and the C-1 carbon source to the anode chamber. The nutrient medium may further comprise the electron donor and/or further exoelectrogen, in some embodiments.

Step b) may further comprise fixing the carbon from the C-1 carbon source, which may involve the exoelectrogen metabolising the C-1 carbon source to a complex carbon compound (i.e. a C2 or greater carbon compound). Suitable carbon fixation, dependent on the exoelectrogen used, may be via one of the following cycles: (1) the Reductive Tricarboxylic acid Cycle, (2) the 3-hydroxyproprionate - 4-hydroxybutyrate Cycle, (3) Dicarboxylate - 4-hydroxybutyrate Cycle, (4) 3-hydroxyproprionate Bicycle, (5) the Reductive Pentose Phosphate Cycle, (6) the Reductive Acetyl-CoA Pathway (acetogens) and/or (7) the Reductive Acetyl-CoA Pathway (methanogens).

Steps a) and b) may be performed sequentially or, in some embodiments, substantially simultaneously.

The method may further comprise feeding an electron transfer mediator to the exoelectrogenic reactor. The electron transport mediator may comprise a flavin, such as riboflavin.

The exoelectrogenic reactor, electron acceptor, electron donor, exoelectrogen, anode, cathode, anode chamber, cathode chamber, C-l carbon source and media are all preferably as described for the first aspect of the invention.

In some embodiments the nutrient media may be DSMZ fermentation medium 81. The pH of any media in the anode and cathode may be adjusted to desired levels using any suitable base (such as aqueous ammonia/ammonium, for example) or acid.

The exoelectrogenic reactor may be operated continuously, semi-continuously, or batch-wise, but is preferably operated continuously or semi-continuously.

According to a third aspect of the invention there is provided use of an exoelectrogen to generate electricity from a C-l carbon source in a microbial fuel cell.

According to a fourth aspect of the invention there is provided use of an exoelectrogen to fix carbon from a C-l carbon source, in an exoelectrogenic reactor. According to a fifth aspect of the invention there is provided use of a microbial fuel cell in the cultivation of a C-1 carbon source-metabolising exoelectrogen.

The present invention therefore provides an apparatus and method for the cultivation of chemolithotrophic or methanotrophic microorganisms using an exoelectrogenic reactor configuration, thereby providing separation between the electron donor and the final electron acceptor associated with cellular metabolism.

The terms, "substantially", "about" or "approximately when used in connection with a specific value, means that acceptable deviations from that value are also encompassed but still provide substantially the same function as the specific value. According to a sixth aspect of the invention there is provided a method for lithotrophic cultivation of microorganisms using an exoelectrogenic reactor configuration, providing a C-1 carbon source and electron donor to an anodic chamber and an electron acceptor to a cathodic chamber; producing an electric current between the anode and cathode; and metabolising the C-1 carbon in the anodic chamber as primary carbon source.

The method of the sixth aspect may be performed with an exoelectrogenic reactor of the first aspect of the invention. The exoelectrogen, reactor, electron donor, electron acceptor, C-1 carbon source and other components, may be as described hereinabove for the other aspects of the invention. Detailed Description of the Invention In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 Figure 1 is a schematic of an embodiment of exoelectrogenic reactor of the invention in which a C-l carbon source, e.g. C0₂, and an electron donor, e.g. H₂, are in the anodic chamber. The final electron acceptor, e.g. O2, is in the cathodic chamber.

Figure 2 is a graph showing the power curve for C. metallidurans (using an apparatus and method of the invention) and Geobacter sulfurreducens (using a control apparatus and method with no C- 1 carbon source) against the produced current for a series of resistor loads in the extracellular circuit.

Figure 3 is a graph showing the specific C0₂ uptake rate per planar area of the anodic electrode of the C. metallidurans example of Figure 2, as determined via gas chromatography, for an exoelectrogenic reactor configuration as per Figure 1, between 215 - 290 hours of continuous operation, demonstrating C0₂ uptake under anaerobic anodic operation.

Figure 4 is a scanning electron microscope image, demonstrating efficient biofilm formation by Cupriavidus metallidurans on the anodic surface.

Figure 1 illustrates a schematic of an exoelectrogenic reactor 2 of the invention. The reactor 2 comprises a reactor chamber 4, comprising an anode chamber 6 and a cathode chamber 8. The anode chamber 6 comprises an anode 10 which includes a layer of immobilised exoelectrogen (which may be a biofilm; in other embodiments the exoelectrogen may be suspended in culture within the anode chamber). The cathode chamber comprises a cathode 12. The anode 10 and cathode 12 are connected by an electrical wire 15 (connected to a voltmeter), thereby completing an electrical circuit. The anode 10 and cathode 12 are also separated by a proton-permeable membrane 14 (such as a NAFION (RTM) membrane). Within the anode chamber 6 there is also located a C-l carbon source 16 and an electron donor, which may be continuously or batch- wise added via an anode fluid inlet 17. Within the cathode chamber 8 there is located an electron acceptor, such as 02, which may be continuously or batch-wise added via a cathode fluid inlet 19. In some embodiments there may be multiple anode chamber and/or cathode chambers, and each anode and/or cathode chamber may comprise more than one anode and/or cathode respectively.

In use, a C-l carbon source, such as C0₂ (or CO or methane), and an electron donor, such as ¾, are sparged into the anodic chamber(s) 6 of the exoelectrogenic reactor 2 (FIGURE 1), facilitating mass transfer from the gas bubbles to the aqueous phase.

A gas disengagement hold-up vessel 18 is also present in the reactor 2, and provided with the following inlets: a nutrient feed inlet 20, a base feed inlet 22 and an electron transport mediator inlet 24. An aqueous phase containing nutrients and optionally electron transporting mediators can be sent to the anode chamber 18 from the gas disengagement hold-up vessel 18 via a recirculation pump 38, thereby ensuring a sufficient supply of nutrients to the exoelectrogen 16 in the anode chamber 6, supporting metabolic activity. Fresh nutrients and optionally electron transporting mediators are fed to the gas disengagement hold-up vessel 18 via the nutrient feed inlet 20, base feed inlet 22 and electron transport inlet 24. A bleed stream 32 is provided to remove depleted nutrients and liquid phase products and by-products from the reactor 2. An off-gas stream 40 is provided in the gas disengagement hold-up vessel 18 to remove depleted gases and vapour phase products from the reactor 2. The temperature of the gas disengagement hold-up vessel 18 and its contents is monitored by a temperature sensor and modulator 30.

The pH of the aqueous phase is controlled via the addition of a base, such as H₃(aq), to the gas disengagement hold-up vessel 18, via a pH modulator port 28. The temperature of the aqueous phase is controlled via heat transfer area 33 provided within the recirculation loop and/or the anode and cathode chambers. The heat transfer area consists of a heat exchanger 35 with attached cooling fluid supply inlet 34 and cooling fluid return outlet 36. The dissolved oxygen concentration may be monitored to ensure that the anodic chamber is operated under anaerobic or approximately anaerobic operating conditions by a dissolved oxygen outlet 26 is located in the gas disengagement hold-up vessel 18.

In use, an electron acceptor, such as oxygen or nitrate, is fed or sparged into the cathode chamber(s) 8 of the exoelectrogenic reactor 2. An effluent stream 42 is provided to remove spent electron acceptor from the cathodic chamber(s).8

A barrier to gas transfer from the cathodic to the anodic chamber is provided, facilitating proton exchange between the anodic and the cathodic chambers. The barrier may be comprised of, but not limited to, a polymeric membrane or a ceramic membrane.

A modicum of renewable electrical power is harvested as by-product, such electrons flowing from the anode to the cathode. Example

An exoelectrogenic reactor 2 of the invention was set up as outlined in FIGURE 1 and used in a method of the invention as follows. The initial charge comprised DSMZ fermentation medium 81, adjusted and controlled to pH = 6.8 using approximately 6 % (w/w) H₃(aq) as base. The temperature in the off-gas disengagement vessel 18 was controlled to 30 °C. The anode chamber 6 of the reactor 2 was fed via inlet 17 with an initial charge inoculated with a 10 % (v/v) inoculum containing Cupriavidus metallidurans CH34 as the exoelectrogen 16 (or, as a control experiment Geobacter sulfurreducens as the exoelectrogen but using acetic acid as the sole carbon feedstock, and therefore no C-l carbon source). During the batch phase, H₂, 0₂ and C0₂ was sparged into the off-gas disengagement vessel 18 in a molar ratio of 21.5:2.5: 1, representing a non-flammable gas mixture. The inoculum was proliferated lithotrophically to an OD₆oo = 10 [-]. Initiating the continuous (cultivation) phase, the gas feed into the anodic chamber(s) 6 was changed to an anaerobic gas mixture with aN₂, H₂ and C0₂ molar ratio of 4:5: 1 (N2, and acetic acid for the control reactor). Once the dissolved oxygen was confirmed by the probe to be 0 % sat at 1 bar, the recirculation pump 38 was started, pumping the broth phase (culture medium) through the anode chamber(s) 6. Electron acceptor, either ferri cyanide or 100 % 0₂ sparged into H₂0, was continuously fed into the cathode chamber(s) 8 via the electron acceptor inlet 19. The nutrient feed pump was started, achieving an effective dilution rate of 0.02 1/h. Thereby, the suspended culture was afforded time to form a biofilm of exoelectrogen 16 on the anode 6 surfaces and the remaining suspended culture was bled from the reactor 2 via bleed 32 in the off gas disengagement vessel 18.

Achieving steady state using 0₂ as electron acceptor, current production from the anode to the cathode was observed via a voltmeter connected to the electrical wire 15, at various resistor loads in the external circuit as shown in the graph of FIGURE 2. Electroactive H₂ was ruled out as contributing to the current generation.

FIGURE 2 demonstrates a favourable power curve for the C-l exoelectrogenic reactor containing Cupriavidus metallidurans as host, compared to the control microbial fuel cell configuration, where Geobacter sulfurreducens was employed as host using acetic acid/acetate as carbon source.

FIGURE 3 demonstrates the continuous uptake of CO2 by the immobilised Cupriavidus metallidurans CH34 exoelectrogen 16 biofilm in the absence of an electron acceptor such as 0₂ in the anodic chamber 6. Given the obligate respiratory metabolism of Cupriavidus metallidurans CH34, the CO2 fixation is facilitated via the transfer of electrons from the biofilm to the O2 in the cathodic chamber 8.

FIGURE 4 demonstrates effective biofilm formation of Cupriavidus metallidurans CH34 post the continuous operation as determined by scanning electron microscopy, noting that the biofilm has been disturbed by the preparation for microscopy.

In other embodiments the reactor 2 may comprise a plurality of anode chambers 6, each with one or more anodes 10 and/or the reactor 2 may comprise a plurality of cathode chambers 8, each with one or more cathodes 12. In addition, each anode chamber may comprise the same or different exoelectrogen, but in preferred embodiments the exoelectrogen in each chamber is the same (thus forming a monoculture throughout the reactor 2).

In other embodiments the Cupriavidus metallidurans CH34 was replaced with a species from each of the genera: Acetobacterium, Clostridium, Eubacterium,

Methylococcus, Methylomicrobium, Ralstonia and Rhodococcus and Example 1 repeated, using C-1 carbon sources selected from CO, C0₂ and CH4. The results were similar to those shown in Figures 2 and 3 for Cupriavidus metallidurans CH34 and showed favourable power curves compared to equivalent reactors loaded with

Geobacter sulfurreducens and utilising acetate as a C-2 carbon source.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. An exoelectrogenic reactor comprising:

At least one anode chamber, comprising: an anode; an exoelectrogen; and a fluid comprising a source of C-l carbon and an electron donor; at least one cathode chamber, comprising a cathode and an electron acceptor; an electrical connector between the anode and cathode; and at least one proton-permeable barrier between the anode chamber and cathode chamber.

2. A microbial fuel cell as claimed in claim 1, wherein the C-l carbon is at least one compound selected from the group of: carbon monoxide, carbon dioxide and methane.

3. A microbial fuel cell as claimed in claim 1 or 2, wherein the electron donor is at least one selected from the group of: hydrogen, carbon monoxide and methane.

4. A microbial fuel cell as claimed in any one of claims 1 to 3, wherein the electron acceptor is oxygen and/or nitrate.

5. A microbial fuel cell as claimed in any preceding claim, wherein the exoelectrogen is in a genus selected from the group of: Acetobacterium, Clostridium, Cupriavidus, Eubacterium, Methylococcus, Methylomicrobium, Ralstonia and Rhodococcus.

6. A microbial fuel cell as claimed in any preceding claim, comprising a plurality of anode chambers and/or cathode chambers.

7. A microbial fuel cell as claimed in claim 6, wherein each anode chamber and/or cathode chamber comprises a plurality of anodes and/or cathodes, respectively.

8. A microbial fuel cell as claimed in any preceding claim, wherein the fluid in the anode comprise a carbon source having 2 or more carbon atoms, in addition to the C-1 carbon source.

9. A method of cultivating exoelectrogens and/or fixing carbon from a C-1 carbon source, the method comprising the steps of: a) providing an exoelectrogenic reactor comprising at least one anode chamber comprising at least one anode and at least one exoelectrogen in the anode chamber; at least one cathode chamber comprising at least one cathode and at least one electron acceptor; at least one proton-permeable barrier between the anode and cathode; and wherein the anode and cathode are electrically connected; b) feeding a C-1 carbon source and electron donor to the anodic chamber such that the exoelectrogen metabolises the C-1 carbon source, to produce at least one carbon- based product of metabolism, electrons and protons which pass into the cathode chamber; and c) reducing the electron acceptor in the cathode in the presence of electrons and protons

10. A method as claimed in claim 9, wherein step a) comprises cultivating the exoelectrogen in situ in the anode chamber.

11. A method as claimed in claim 9 or claim 10, wherein the exoelectrogen is substantially a monoculture.

12. A method as claimed in any one of claims 9, 10 or 11, wherein the exoelectrogen is in a genus selected from the group of: Acetobacterium, Clostridium, Cupriavidus, Eubacterium, Methylococcus, Methylomicrobium, Ralstonia and Rhodococcus

13. A method as claimed in any one of claims 9 to 12, wherein the microorganism produces an electron transfer mediator, during step b).

14. A method as claimed in any one of claims 9 to 13, wherein the method further comprises feeding an electron transfer mediator to the microbial fuel cell during step b).

15. A method as claimed in claim 13 or 14, wherein the electron transfer mediator is a flavin.

16 A method as claimed in any one of claims 9 to 15, wherein step b) comprises feeding a mixture of a C-l carbon source and a source of carbon having 2 or more carbon atoms, into the anode chamber.

17. Use of an exoelectrogen to generate electricity from a C-l carbon source in a microbial fuel cell.

18. Use of an exoelectrogen to fix carbon from a C-l carbon source in a microbial fuel cell.