Deep subterranean granitic aquifers have not been explored regarding methanogens until now. Three autotrophic methane-producing were isolated from deep granitic groundwater at depths of 68, 409 and 420 m. These organisms were non-motile, small, thin rods, 0·1-0·15 μm 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 (20–40 °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 mol%, as determined by thermal denaturation. Phylogenetic studies based upon 16S rRNA gene sequence comparisons placed the isolate A8p in the genus . Phenotypic and phylogenetic characters indicate that the alkaliphilic, halotolerant strain A8p represents a new species. We propose the name for this species, and strain A8p (= DSM 11074) is the type strain.


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