Outer membranes of Geobacter sulfurreducens incorporate electron-conducting protein filaments that transfer electrons generated by the digestion of food inside the cell to iron atoms outside the cell.

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Waste-to-energy operations, where organic waste materials are burned to generate steam to produce electricity, are not uncommon in many cities around the world. Lowly, single-celled organisms are teaching us how to improve that process by leaps and bounds. Certain species of bacteria in the Geobacter genus are capable of "eating" organic material, but rather than using oxygen to soak up electrons generated in the process, like most creatures do, these bacteria pass these electrons on to iron atoms – the same type of iron atoms present in rust. More importantly, transferring these electrons from inside the cell, where the consumption of the organic material takes place, to insoluble electron receptors outside the cell requires overcoming the insulating oily cell membrane. Geobacter performs the task by incorporating conducting proteins (c-type cytochromes) within the cell membrane and periplasm to shuttle electrons to the exterior. Conductive filaments called pili extend from the outside of the cell and facilitate the transfer of electrons to the iron atoms.

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"[M]icroorganisms that power microbial fuel cells can oxidize a diverse range of 'dirty' fuels that are often of little perceived value, such as organic waste and the organic matter in soils and sediments...microbial fuel cells offer the possibility of extracting over 90% of the electrons from organic compounds, and can be self-sustaining and renewing when populated with microorganisms that conserve energy from electron transfer to electrodes." (Lovely 2006:497)

"Fe3+-reducing microorganisms (most often Geobacter species in temperate environments...) metabolize the fermentation products and the organic compounds that fermentative microorganisms do not readily metabolize, oxidizing them to carbon dioxide, with Fe3+ oxides serving as the electron acceptor." (Lovely 2006:499)

"Acetate is probably the most important electron donor because of its central role in the degradation of organic matter by anaerobic microbial consortia, but other organic acids, ethanol and aromatic compounds are also used, as is hydrogen. The organic compounds are oxidized to carbon dioxide, with nearly full recovery of the electrons derived from organic-matter oxidation as electricity...a soluble electron shuttle was not involved in electron transfer to the electrode." (Lovely 2006:503)

"[O]ne of the most formidable barriers to microorganisms transferring electrons onto Fe3+ or electrodes is the non-conducting lipid-membrane system that serves as an insulator, separating the cytoplasm, where electrons are extracted from organic matter during central metabolism, from the outside of the cell where the final electron transfer must take place...a series of c-type cytochromes associated with the inner membrane, the periplasm, and the outer membrane might interact to transfer electrons to the outer membrane surface. However, growth on Fe3+ oxides also requires the presence of specialized pili that are localized to one side of the cell. Fe3+ oxides specifically associate with these pili, which are electrically conductive. This suggests that pili are the electrical conduit between the cell and Fe3+ oxides." (Lovely 2006:505)

Journal article
Bug juice: harvesting electricity with microorganismsNature Reviews MicrobiologyJune 16, 2006
Derek R. Lovley

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