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Chapter 17 : Microbial Respiration of Anodes and Cathodes in Electrochemical Cells
Category: Applied and Industrial Microbiology; Environmental Microbiology
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The microbial fuel cell for bioenergy production from renewable fuels is one of many potential applications of microbial catalysis in electrochemical cells. Numerous investigators have proposed analogous bioelectrochemical cells for engineering applications in bioremediation and biohydrogen production. This chapter summarizes and discusses the interactions between microbes and electrodes for energy generation and environmental applications. The electrode of an electrochemical cell may serve as either an electron acceptor or an electron donor, depending on the needs of the application. The chapter emphasizes the microbial communities that develop on both anodes and cathodes of electrochemical cells, the known bacteria which conserve energy and grow on electrodes, and the current state of understanding for the molecular basis of electron transfer between bacteria and electrodes of electrochemical cells. Reduction products from microbial respiration accumulate in the sediment and result in the development of a depth-dependent potential gradient as measured by a reference electrode. Researchers have recently demonstrated ammonium-dependent electricity generation in a microbial fuel cell, where ammonium was amended to the medium as an electron donor, rather than produced as a primary metabolite.
(Top) The electrolytic cell. An external electrical potential is applied between the two electrodes and a chemical reaction at the electrodes is driven against its ΔG by the external potential. Current flowing between the electrodes drives the chemical reaction, consuming electrical energy. (Bottom) The galvanic cell. Chemical energy is converted to electrical energy via an electrochemical cell. The chemical reaction at each electrode occurs spontaneously with its ΔG, releasing energy which passes in the form of electrical current between the two electrodes. 10.1128/9781555817190.ch17.f1
A conventional (abiotic) hydrogen fuel cell. Hydrogen is spontaneously oxidized at the anode with reduction of oxygen at the cathode. Electrons from hydrogen flow through the external circuit and reduce oxygen to water. The oxidation of hydrogen generates protons that travel across the separator, commonly an ion-selective membrane, to maintain charge balance in the overall redox reaction. 10.1128/9781555817190.ch17.f2
A model of a microbial fuel cell featuring direct electron transfer mechanism. Glucose is oxidized to CO2 by the bacterium. Electrons liberated from glucose are transferred to the anode via direct contact of outer membrane and transmembrane electron carriers and transport structures. 10.1128/9781555817190.ch17.f3
A model of a benthic microbial fuel cell (BMFC). A complex community of sediment organisms participates in hydrolysis and fermentation of detritus material to produce simple organic compounds such as acetate that are used by electrode-respiring bacteria enriched from the sediment. The cells transfer electrons to the anode of the BMFC via direct respiration of anode or through an electron mediator, such as a primary metabolite (HS–, Fe2+, or reduced humic material) or secondary metabolite electron shuttles such as flavinoid compounds. 10.1128/9781555817190.ch17.f4
The indirect electron transfer mechanism for respiration of the anode in a microbial fuel cell. Electron transfer to the anode is enabled by a primary metabolite such as an electron shuttle (e.g., HS–, H2) or secondary metabolites electron shuttles such as flavins or phenazines. 10.1128/9781555817190.ch17.f5
Models of the molecular basis for electron transfer to anodes by Shewanella oneidensis and Geobacter metallireducens. Oxidation of organic matter in central metabolism of both organisms produces reduced, intracellular electron carriers such as NADH. Electrons are transferred through the membranes via a series of intermediates to outer membrane cytochromes. 10.1128/9781555817190.ch17.f6
Bacteria may respire at the cathode of an electrolytic cell. Analogous to electron transfer from cells to the anode of a galvanic cell, the mechanisms of electron transfer from the cathode to bacteria in an electrolytic cell may occur through direct and indirect mechanisms. 10.1128/9781555817190.ch17.f7