Distribution and activity of methanogens and homoacetogens in deep granitic aquifers at Äspö Hard Rock Laboratory, Sweden
This study reveals the existence of active chemoautolithotrophic microbial communities in deep granitic groundwater, containing hydrogen and methane. Homoacetogens and acetotrophic methanogens dominated in the groundwater down to a depth of 112 m, while autotrophic methanogens prevailed deeper, down to 446 m. The results from most probable number counts showed that viable methanogens and homoacetogens coexisted at all depths investigated, from 45 m to 446 m below sea level. In vitro radiotracer experiments demonstrated these microorganisms to form methane and acetate from CO2 with hydrogen, and methane from [14C]acetate at close to in situ temperature (17°C). The data suggest that autotrophic methanogens and homoacetogens initiate a deep subterranean food chain and that acetate-dependent methane formation is mediated by homoacetogenesis. The finding of an active, deep, hydrogen based autotrophic biosphere adds a significant but previously overlooked reducing activity to deep granitic rock.