A thermophilic and hydrogenotrophic methanogen, strain RMAST, was isolated from gas-associated formation water of a gas-producing well in a natural gas field in Japan. Strain RMAST grew solely on H2/CO2 but required Casamino acids, tryptone, yeast extract or vitamins for growth. Growth of strain RMAST was stimulated by acetate. Cells were non-motile, straight rods (0.5×3.5–10.5 µm) and occurred singly or in pairs. Bundles of fimbriae occurred at both poles of cells and the cell wall was thick (approximately 21 nm, as revealed by ultrathin section electron microscopy). Strain RMAST grew at 45–80 °C (optimum, 70 °C), at pH 5.8–8.7 (optimum, pH 6.9–7.7) and with 0.001–20 g NaCl l−1 (optimum, 2.5 g NaCl l−1). Phylogenetic analysis revealed that Methanothermobacter thermautotrophicus ΔHT was most closely related to the isolate (95.7 % 16S rRNA gene sequence similarity). On the basis of morphological, phenotypic and phylogenetic characteristics, it is clear that strain RMAST represents a novel species of the genus Methanothermobacter , for which we propose the name Methanothermobacter tenebrarum sp. nov. The type strain is RMAST ( = DSM 23052T = JCM 16532T = NBRC 106236T).
The hyperthermophilic crenarchaeon Thermoproteus neutrophilus V24StaT was originally classified before sequence-based phylogenetic analysis became standard for bacterial taxonomy. Subsequent phylogenetic analyses by various groups have shown that strain V24StaT groups more closely with strains of the genus Pyrobaculum than with those in the genus Thermoproteus . Based on phylogenetic comparison of rRNA gene sequences and ribosomal proteins, we propose that strain V24StaT be reclassified as Pyrobaculum neutrophilum comb. nov., with the type strain V24StaT ( = DSM 2338T = JCM 9278T). An emended description of the genus Pyrobaculum is also presented.