Isolation and characterization of Thermococcus sibiricus sp. nov. from a Western Siberia high-temperature oil reservoir
Anaerobic organotrophic hyperthermophilic Archaea were isolated from five of eight samples from oil wells of the Samotlor oil reservoir (depth, 1,799–2,287 m; temperature, 60°–84°C). Three strains were isolated in pure cultures and characterized phylogenetically on the basis of comparison of the 16S rRNA gene sequences. All strains belonged to a new species of the genus Thermococcus, with Thermococcus litoralis, Thermococcus aggregans, Thermococcus fumicolans, and Thermococcus alcaliphilus being the nearest relatives (range of sequence similarity, 97.2%–98.8%). Strain MM 739 was studied in detail. The new isolate grew on peptides but not on carbohydrates. Elemental sulfur had a stimulatory effect on growth. The temperature range for growth was between 40° and 88°C, with the optimum at 78°C; the pH range was 5.8 to 9.0, with the optimum around 7.3; and the salinity range was 0.5% to 7.0%, with the optimum at 1.8%–2.0%. The doubling time at optimal growth conditions was about 43 min. The G+C content of the DNA was 38.4 mol%. The DNA–DNA relatedness between strain MM 739 and T. litoralis was 27%; between strain MM 739 and T. aggregans, it was 22%. Based on the phenotypic and genomic differences with known Thermococcus species, the new species Thermococcus sibiricus is proposed. The isolation of a hyperthermophilic archaeum from a deep subsurface environment, significantly remote from shallow or abyssal marine hot vents, indicates the existence of a subterranean biosphere inhabited by indigenous hyperthermophilic biota.