Microbiology and mechanisms of exocellular electron transfer in microbial fuel cells

Following the principle of a fuel cell- where energy is converted to electricity, efficiently and free of harmful end-products- microbial fuel cells (MFC) have been studied for some time and were shown to produce considerable amounts of electricity in the laboratory as well as on location at remote marine environments.

The MFC concept is possible due to exocellular electron transfer where electrons are passed on from the organism via different mechanisms to insoluble extracellular electron acceptors. Electron transfer toinsoluble oxides in anaerobic environments is especially important for sustaining growth of Fe (III) - and Mn (IV) - respiring organisms. Soluble compounds, certain antibiotics, quinones, riboflavin, sulfur compound shuttles and even humic substances can function as exocellular electron mediators thus facilitating respiration via exocellular electron transfer. These mediators are either artificial or are produced by the microorganism itself. Certain microbes seem to also possess another means of transferring electrons directly without the use of mediator, instead special conductive pili are utilized, called nanowires.

The MFC generally consists of a closed system of two compartments with two graphite electrodes, separated by a cation-exchange membrane. Bacteria in the anaerobic compartment donate electrons to the anodic electrode upon degradation of organic substrates in the absence of alternative electron acceptors, thus creating a potential between the two electrodes. The protons released from respiration diffuse through the cation exchange membrane and enter the aerobic (cathodic) compartment whereas electrons enter the cathode by flowing up the anodic electrode. In turn, the electrons and protons in the aerobic thus creating a potential between the two electrodes. The protons released from respiration diffusethrough the cation exchange membrane and enter the aerobic (cathodic) compartment whereas electrons enter the cathode by flowing up the anodic electrode. In turn, the electrons and protons in the aerobic cathode compartment reduce oxygen to water.

Factors affecting the MFC operational effectiveness are, of course, the MFC design (e.g. proton exchange membrane, internal resistance of electrolytes) but the most critical factor is the bacterial metabolism and electron transfer aspect. The bacterial communities responsible of electrogenesis differ in different MFC setups and characterization of these populations is essential in understanding how the MFC can be controlled and optimized. Up to date, only some electrogenic consortia and single organisms have been studied and described in some detail as well as the mechanisms of which the function of an MFC is possible. The goal of this research is to study further the microbial ecology of running MFC, isolate and characterize new electrogenic bacterial strains and study different model MFC environments and mechanisms of electron transfer based on the characteristics of the bacterial isolates either in pure culture experiments or predetermined consortia.

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