LITHOTROPHIC OXIDATION OF FE(II)-SILICATES AS THE BASIS FOR A DEEP GRANITIC BIOSPHERE

There is a long standing enigma regarding the energy source for microbial communities in circumneutral-pH deep granitic aquifers. Based on our recent results we propose that lithotrophic Fe(II)-silicate oxidizing microorganisms can serve as an energetic base of a granitic microbial ecosystem. In a model study, the lithotrophic Fe(II)-oxidizing, nitrate-reducing enrichment culture described by Straub et al. (Appl. Environ. Microbiol., 65:1458-1460, 1996) was able to grow via oxidation of structural Fe(II) in biotite (a Fe(II)-bearing trioctahedral silicate mineral found in granitic rocks) under nitrate-reducing conditions. Oxidation of silt/clay sized biotite particles was detected by a decrease in extractable Fe(II) content and simultaneous nitrate reduction. Mössbauer spectroscopy confirmed structural Fe(II) oxidation. Approximately 10⁷ cells were produced per μmol Fe(II) oxidized, in agreement with previous estimates of the growth yield of lithoautotrophic circumneutral-pH Fe(II)-oxidizing bacteria. Microbial oxidation of structural Fe(II) resulted in biotite alterations similar to those found in nature, including iron and potassium release and decrease in unit cell b-dimension toward dioctahedral levels. The ability of a lithotrophic culture to use structural Fe(II) in biotite as an electron donor suggests a mechanism by which lithotrophic microbial communities in the deep subsurface could sustain themselves in the absence of major photosynthetically-derived carbon input.