Some members of the Bacteria and the Archaea have the rare ability to transfer electrons beyond the surface of their cells. This extracellular electron transfer permits the reduction of otherwise inaccessible electron acceptors such as insoluble Fe(III) oxides. The mechanisms that enable this ability have direct implications for geochemical cycles today and for life on early Earth. The physical settings present on early Earth, hot and influenced primarily by hydrothermal activity, can be found in rare sites on modern Earth. These sites most often surround deep-sea hydrothermal vents. Organisms present at these sites thrive, in most cases, absent of the sun’s influence. Instead of primary production from photosynthetic microorganisms, these communities rest on the shoulders of chemoautolithotrophic bacteria and archaea. Two of these organisms, the thermophilic bacterium Geothermobacter ehrlichii and the hyperthermophilic archaeon Geoglobus ahangari were selected to undergo genome and physiological characterization to determine how they interact with the abundant insoluble Fe(III) oxides found at hydrothermal vents. Both genomes were sequenced and, while only the genome of G. ahangari was complete, this permitted identification of critical components for iron respiration. In addition, mechanistic studies were performed on G. ahangari to elucidate a direct-contact mechanism of iron reduction. Finally, the extracellular filaments from these microorganisms were characterized and the more abundant filaments, in both organisms, were found to be conductive. These are the first examples of nanowires discovered outside of the mesophilic bacteria. In addition, the phylogenetic and geographic diversity between these isolates suggests that microbial nanowires are more widespread than previously thought.
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