Evidence for a Hydrogen-driven, Intra-terrestrial Biosphere in Deep Granitic Rock Aquifers

Transmission electron microscopy was used to detect bacterial microfossils buried in fracture surface calcites in a granitic rock aquifer from 207 m below ground. 16S rRNA sequencing demonstrated that bacteria in deep rock groundwater (400 m) were not contaminants from drilling operations. All groundwaters investigated contain hydrogen and methane, ranging from 0.1 to 70 µM and 0.1 to 1000 µM, respectively. Homoacetogens and acetoclastic and autotrophic methanogens have been cultured from many different deep sites down to 900 m at viable count numbers, constituting at an average 10- 15 % of the total number of microorganisms. A new methanogenic species from deep rock aquifers, Methanobacterium subterraneum, was recently reported. In vitro methanogenic and acetogenic activities were shown for many of the sites investigated. The hydrogen driven deep biosphere theory includes chemoautotrophic production of acetate, methane and biomass with hydrogen produced by geological processes as the primary energy source. This biosphere is, therefore, independent of photoautotrophic organic carbon. The produced chemoautotrophic organic carbon can be used by heterotrophic microbes. Sulphate and iron reducing bacteria frequently occur in deep granitic groundwater, e.g. Desulfovibrio aespoeensis and Shewanella putrefaciens. Yeast and other fungi have been obtained from depth as 18S sequences and also as isolates. The existence of a deep hydrogen-driven, intra-terrestrial biosphere seems very probable