Microbial electrochemical technologies (METs) use microorganisms as electrochemical catalyst, merging the microbial metabolism with electrochemical processes for the production of bioelectricity, biofuels, H2 and other valuable chemicals.Logan, B.E., et al., Electroactive microorganisms in bioelectrochemical systems. Nature Reviews Microbiology, 2019. 17(5): p. 307-319. Microbial fuel cells (MFC) and microbial electrolysis cells (MEC) are prominent examples of METs. While MFC is used to generate electricity from organic matter typically associated with wastewater treatment, MEC use electricity to drive chemical reactions such as the production of H2 or methane. Recently, microbial electrosynthesis cells (MES) have also emerged as a promising MET, where valuable chemicals can be produced in the cathode compartment.Nevin, K.P., et al., Microbial Electrosynthesis: Feeding Microbes Electricity To Convert Carbon Dioxide and Water to Multicarbon Extracellular Organic Compounds. mBio, 2010. 1(2): p. e00103-10.Logan, B.E., Exoelectrogenic bacteria that power microbial fuel cells. Nature Reviews Microbiology, 2009. 7(5): p. 375-381.Rozendal, R.A., et al., Efficient hydrogen peroxide generation from organic matter in a bioelectrochemical system. Electrochemistry Communications, 2009.11(9): p. 1752-175 Other MET applications include microbial remediation cell, microbial desalination cell, microbial solar cell, microbial chemical cell, etc.Harnisch, F. and U. Schroder, From MFC to MXC: chemical and biological cathodes and their potential for microbial bioelectrochemical systems. Chem Soc Rev, 2010. 39(11): p. 4433-48.Kumar, P., et al., Electro-Fermentation in Aid of Bioenergy and Biopolymers. Energies, 2018. 11(2): p. 343-343.Bajracharya, S., et al., An overview on emerging bioelectrochemical systems (BESs): Technology for sustainable electricity, waste remediation, resource recovery, chemical production and beyond. Renewable Energy, 2016. 98: p. 153-170.

History The use of microbial cells to produce electricity was perceived by M.C. Potter in 1911 with the finding that "The disintegration of organic compounds by microorganisms is accompanied by the liberation of electrical energy".Potter, M.C. and A.D. Waller, Electrical effects accompanying the decomposition of organic compounds. Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character, 1911. 84(571): p. 260-276. A noteworthy addition in MFC research was made by B. Cohen in 1931,Cohen, B., The bacterial culture as an electrical half-cell. J. Bacteriol, 1931. 21(1): p. 18-19. when microbial half fuel cells stack connected in series was created, capable of producing over 35 V with a current of 0.2 mA. Two breakthroughs were made in the late 1980s when two of the first known bacteria capable of transporting electron from the cell interior to the extracellular metal oxides without artificial redox mediators: Shewanella (formerly Alteromonas) oneidensis MR-1 Myers, C.R. and K.H. Nealson, Bacterial Manganese Reduction and Growth with Manganese Oxide as the Sole Electron-Acceptor. Science, 1988. 240(4857): p. 1319-1321. and Geobacter sulfurreducens PCA were isolated. In late 90s, Kim et al.Kim, H.J., et al., A mediator-less microbial fuel cell using a metal reducing bacterium, Shewanella putrefaciens. Enzyme and Microbial Technology, 2002. 30(2): p. 145-152. showed that the Fe(III)-reducing bacterium, S. oneidensis MR-1 was electrochemically active and can generate electricity in a MFC without any added electron mediators. These findings set basis for the development of electromicrobiology, and the field of MFC started. However, due to low power generation, it was also doubtful whether the MFC can be practical application on wastewater organics reduction. This view was changed when it was established that domestic wastewater could be treated to practical limits while simultaneously producing power.Liu, H., R. Ramnarayanan, and B.E. Logan, Production of electricity during wastewater treatment using a single chamber microbial fuel cell. Environ Sci Technol, 2004. 38(7): p. 2281-5. Furthermore, power densities two orders of magnitude higher was demonstrated in an MFC using glucose, without the need for exogenous chemical mediators.Rabaey, K., et al., A microbial fuel cell capable of converting glucose to electricity at high rate and efficiency. Biotechnology Letters, 2003. 25(18): p. 1531-1535. Building upon these works, a race to develop practical applications of MFCs initiated to emerge at a very fast pace, with the major goals being development of a large scale technology for the treatment of domestic, industrial, and other types of wastewaters.Logan, B.E., et al., Microbial Fuel Cells:  Methodology and Technology. Environmental Science & Technology, 2006. 40(17): p. 5181-5192.

In 2004, extracellular electron uptake (EEU) from cathodes to microbes (Geobacter spp.) was established with attached biofilm, where fumarate was reduced to succinate.Gregory, K.B., D.R. Bond, and D.R. Lovley, Graphite electrodes as electron donors for anaerobic respiration. Environ Microbiol, 2004. 6(6): p. 596-604. This reverse reaction for electron transport generated the research field of MES. In 2010, Nevin et al. discovered that the acetogenic microorganism Sporomusa ovata can convert CO2 to acetic acid in MES cells by uptaking electrons from the cathode electrode.Nevin, K.P., et al., Microbial Electrosynthesis: Feeding Microbes Electricity To Convert Carbon Dioxide and Water to Multicarbon Extracellular Organic Compounds. Mbio, 2010. 1(2). In the next years, also due to the growing concerns on greenhouse gas emissions, the field of CO2 bioelectroconversion in MES cell flourished. Several autotrophic microorganisms showed ability of capturing electrons from the cathode, either directly or through mediators.Logan, B.E., et al., Electroactive microorganisms in bioelectrochemical systems. Nat Rev Microbiol, 2019. 17(5): p. 307-319. Besides specific microbial species, it was shown that CO2 reducing communities can be enriched in MES cells from inoculum sources such as sewage sludge, digester sludge or marine/river sediments.Izadi, P., et al., Parameters influencing the development of highly conductive and efficient biofilm during microbial electrosynthesis: the importance of applied potential and inorganic carbon source. npj Biofilms Microbiomes, 2020. 6(1): p. 40.Bian, B., et al., Resistance assessment of microbial electrosynthesis for biochemical production to changes in delivery methods and CO2 flow rates. Bioresour Technol, 2021. 319: p. 124177.Alqahtani, M.F., et al., Enrichment of salt-tolerant CO2-fixing communities in microbial electrosynthesis systems using porous ceramic hollow tube wrapped with carbon cloth as cathode and for CO2 supply. Sci Total Environ, 2021. 766: p. 142668. In the following decade, technical improvements led to an increase of acetate production rate from few to hundreds g/m2cathode/d.Jourdin, L., et al., Bringing High-Rate, CO2-Based Microbial Electrosynthesis Closer to Practical Implementation through Improved Electrode Design and Operating Conditions. Environ Sci Technol, 2016. 50(4): p. 1982-9. MES cells demonstrated also a promising technology for converting CO2 into biomethane, with production rates up to 200 L CH4/m2cathode/d.Zhou, H., et al., Optimization of a newly developed electromethanogenesis for the highest record of methane production. J Hazard Mater, 2021. 407: p. 124363. Furthermore, the MES scope was expanded to target more valuable products, including ethanol and caproate.Srikanth, S., et al., Electro-biocatalytic conversion of carbon dioxide to alcohols using gas diffusion electrode. Bioresour Technol, 2018. 265: p. 45-51.

Principles

Microbial extracellular electron transfer There are various mechanisms for bacteria to electrons with an electrode. These include a "direct" process, where redox components located on the cell surface, that can be multiheme cytochromes or nanofilaments, contact directly with the solid surfaces (Figure 1A, C and D),Rosenbaum, M., et al., Cathodes as electron donors for microbial metabolism: which extracellular electron transfer mechanisms are involved? Bioresour Technol, 2011. 102(1): p. 324-33.Myers, C.R. and J.M. Myers, Localization of cytochromes to the outer membrane of anaerobically grown Shewanella putrefaciens MR-1. J Bacteriol, 1992. 174(11): p. 3429-38.Tang, H.Y., et al., Iron Corrosion via Direct Metal-Microbe Electron Transfer. mBio, 2019. 10(3).Okamoto, A., et al., Bound Flavin Model Suggests Similar Electron-Transfer Mechanisms in Shewanella and Geobacter. ChemElectroChem, 2014. 1(11): p. 1808-1812. and an "indirect" process that is mediated by soluble redox mediators that cyclically shuttle electrons between cells and electrodes [28-30]Newman, D.K. and R. Kolter, A role for excreted quinones in extracellular electron transfer. Nature, 2000. 405(6782): p. 94-7.Derek R. Lovley, J.D.C., Elizabeth L. Blunt-Harris, Elizabeth J. P. Phillips, Joan C. Woodward Humic substances as electron acceptors for microbial respiration. Nature, 1996(382): p. 445–448.You, L.X., et al., Flavins mediate extracellular electron transfer in Gram-positive Bacillus megaterium strain LLD-1. Bioelectrochemistry, 2018. 119: p. 196-202. (Figure 1B). Electron shuttles can be humic substances that are not produced by the cells,Holmes, D.E., D.R. Bond, and D.R. Lovley, Electron transfer by Desulfobulbus propionicus to Fe(III) and graphite electrodes. Applied and environmental microbiology, 2004. 70(2): p. 1234-1237. or secondary metabolites that are produced by the organisms including phenazines [32, 33]Rabaey, K., et al., Microbial Phenazine Production Enhances Electron Transfer in Biofuel Cells. Environmental Science & Technology, 2005. 39(9): p. 3401-3408.Rabaey, K., et al., Biofuel Cells Select for Microbial Consortia That Self-Mediate Electron Transfer. Applied and Environmental Microbiology, 2004. 70(9): p. 5373. and flavins [34, 35].Marsili, E., et al., Shewanella secretes flavins that mediate extracellular electron transfer. Proceedings of the National Academy of Sciences, 2008. 105(10): p. 3968.von Canstein, H., et al., Secretion of Flavins by Shewanella Species and Their Role in Extracellular Electron Transfer. Applied and Environmental Microbiology, 2008. 74(3): p. 615. In addition, some primary metabolites of bacteria, such as sulphur species and H2, can convey electrons towards extracellular electron acceptors. In addition to heme cofactors in multiheme cytochromes, flavin mononucleotide also were shown to enhance the rate of electron transfer in some outer membrane cytochrome as redox cofactors .Okamoto, A., et al., Bound Flavin Model Suggests Similar Electron-Transfer Mechanisms in Shewanella and Geobacter. ChemElectroChem, 2014. 1(11): p. 1808-1812 Because electrons are transferred from the interior to the exterior of microbial cells across the cellular membrane during EET, ions with positive charge need to simultaneously move in the same direction as the electron flow to maintain charge neutrality (Figure 1A).Okamoto, A., et al., Proton Transport in the Outer-Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport. Angewandte Chemie International Edition, 2017. 56(31): p. 9082-9086.

Bioelectrochemical systems (principles, components, configurations)