DNA from low-biodiversity fracture water collected at 2.8-kilometer depth in a South African gold mine was sequenced and assembled into a single, complete genome. This bacterium, Candidatus Desulforudis audaxviator , composes >99.9% of the microorganisms inhabiting the fluid phase of this particular fracture. Its genome indicates a motile, sporulating, sulfate-reducing, chemoautotrophic thermophile that can fix its own nitrogen and carbon by using machinery shared with archaea. Candidatus Desulforudis audaxviator is capable of an independent life-style well suited to long-term isolation from the photosphere deep within Earth's crust and offers an example of a natural ecosystem that appears to have its biological component entirely encoded within a single genome.

Bacterial nanowires are extracellular appendages that have been suggested as pathways for electron transport in phylogenetically diverse microorganisms, including dissimilatory metal-reducing bacteria and photosynthetic cyanobacteria. However, there has been no evidence presented to demonstrate electron transport along the length of bacterial nanowires. Here we report electron transport measurements along individually addressed bacterial nanowires derived from electron-acceptor–limited cultures of the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1. Transport along the bacterial nanowires was independently evaluated by two techniques: ( i ) nanofabricated electrodes patterned on top of individual nanowires, and ( ii ) conducting probe atomic force microscopy at various points along a single nanowire bridging a metallic electrode and the conductive atomic force microscopy tip. The S. oneidensis MR-1 nanowires were found to be electrically conductive along micrometer-length scales with electron transport rates up to 10 9 /s at 100 mV of applied bias and a measured resistivity on the order of 1 Ω·cm. Mutants deficient in genes for c -type decaheme cytochromes MtrC and OmcA produce appendages that are morphologically consistent with bacterial nanowires, but were found to be nonconductive. The measurements reported here allow for bacterial nanowires to serve as a viable microbial strategy for extracellular electron transport.

Banded iron formations (BIFs) are prominent sedimentary deposits of the Precambrian, but despite a century of endeavor, the mechanisms of their deposition are still unresolved. Interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation, in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy. On the basis of chemical analyses from BIF units of the 2.5 Ga Hamersley Group, Western Australia, we show here that even during periods of maximum iron precipitation, most, if not all, of the iron in BIFs could be precipitated by iron-oxidizing bacteria in cell densities considerably less than those found in modern Fe-rich aqueous environments. Those ancient microorganisms would also have been easily supported by the concentrations of nutrients (P) and trace metals (V, Mn, Co, Zn, and Mo) found within the same iron-rich bands. These calculations highlight the potential importance of early microbial activity on ancient metal cycling.