No abstract

Deep subterranean granitic aquifers have not been explored regarding methanogens until now. Three autotrophic methane-producing Archaea were isolated from deep granitic groundwater a t depths of 68,409 and 420 m. These organisms were non-motile, small, thin rods, 01-0-15 pm in diameter, and they could use hydrogen and carbon dioxide or formate as substrates for growth and methanogenesis. One of the isolates, denoted A8p, was studied in detail. It grew with a doubling time of 2.5 h under optimal conditions (2040 "C, pH 7.8-8.8 and 0.2-1.2 M NaCI). Strain A8p is eurythermic as it grew between 3-6 and 45 "C. It was resistant to up to 20 mg bacitracin I-'. The G+C content was 54.5 mol0/o, as determined by thermal denaturation. Phylogenetic studies based upon 16s rRNA gene sequence comparisons placed the isolate A8p in the genus Methanobacterium Phenotypic and phylogenetic characters indicate that the alkaliphilic, halotolerant strain A8p represents a new species. We propose the name Methanobacterium subterraneum for this species, and strain A8p (= DSM 110749 is the type strain.

No abstract

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.