The Inerterrestrials: Subsurface Microbial Communities
Metabolism at low redox potential is a feature of anaerobic deep subsurface microbial communities on Earth and potentially on early and even present-day Mars. Anaerobic communities in both surface and subsurface communities show a diversity of redox couples which can yield energy and drive the carbon cycle. Methane fermentation by the Antarctic psychrotolerant archaebacterium Methanococcoides burtonii from hypersaline Ace Lake and hydrogen oxidation by the extreme halophilic thermophilic archebacterium Pyrolobus fumari in geothermal deep-ocean vents are contrasting examples.
In deep subsurface geothermally-warmed aquatic environments on Mars, there may be chemolithotrophic microbes which depend on minerals for energy and nutrients. These use the transfer of electrons down redox gradients as an alternative energy source to solar radiation. Likely electron donors would include inorganic sulfides, and acceptors might include hydrogen.
Analogous deep subsurface microbial communities, occurring at least 750 m beneath the sea floor, are now well documented on Earth. With a viable population of up to 107 cells cm-3 at 500 meter beneath the sea bed, they may comprise as much as 50% of the Earth's biomass.
Sulfate-reducing and Fe(III)-reducing bacterial communities are reported at 2,800 meter depth in a natural gas-bearing formation in Taylorsville Basin, in Virginia comprising up to 104 cells g-1 at 76°C at a salinity of 0.8 wt-% NaCl equivalent and under 32 MPa pressure.
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