Electromicrobiology: Implications for Biogeochemistry and Bioenergy.

The ability of microorganisms to make electrical connections with insoluble, extracellular electron acceptors or other microbial species has a significant impact on the cycling of carbon, metals, and nutrients in anaerobic soils and sediments.  For example, Geobacter species are abundant in many environments in which organic matter is oxidized with the reduction of Fe(III) and in methanogenic environments.  Fe(III) reduction is one of the most profound biogeochemical changes in anaerobic soils because of its impact on iron solubility, soil redox status, and the associated release of phosphate and trace metals.  Methane release form soils and sediments is an important source of this potent greenhouse gas.  Studies with Geobacter sulfurreducens have demonstrated that electrons are transferred to Fe(III) oxides via electrically conductive pili.  Surprisingly, the pili have a metallic-like conductivity similar to that previously observed in some organic conducting polymers, but not previously documented in biological material.  The conductive pili also enable Geobacter species to make electrical connections with other microbial species.  This direct interspecies electron transfer (DIET) is a potentially more effective mechanism of interspecies electron exchange than the more well-known interspecies hydrogen transfer.  DIET has been documented in a diversity of methanogenic environments and studied in defined cocultures. The natural ability of Geobacter species to wire themselves to minerals and other cells has been exploited for practical applications in which electrodes are inserted into anaerobic environments to stimulate the degradation of organic contaminants or harvest electricity for powering monitoring devices.  The finding that microorganism can directly exchange electrons with electrodes has led to the development of microbial electrosynthesis, an artificial form of photosynthesis in which solar power is used to feed microorganisms electrons for the conversion of carbon dioxide to transportation fuels and other organic commodities in a potentially more sustainable strategy than biomass-based approaches.