Methanothermobacter tenebrarum sp. nov., a hydrogenotrophic, thermophilic methanogen isolated from gas-associated formation water of a natural gas field A thermophilic and hydrogenotrophic methanogen, strain RMAS T , was isolated from gasassociated formation water of a gas-producing well in a natural gas field in Japan. Strain RMAS T grew solely on H 2 /CO 2 but required Casamino acids, tryptone, yeast extract or vitamins for growth. Growth of strain RMAS T was stimulated by acetate. Cells were non-motile, straight rods (0.5¾3.5-10.5 mm) 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 RMAS T grew at 45-80 6C (optimum, 70 6C), 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 DH T 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 RMAS T represents a novel species of the genus Methanothermobacter, for which we propose the name Methanothermobacter tenebrarum sp. nov. The type strain is RMAS T (5DSM 23052 T 5JCM 16532 T 5NBRC 106236 T ).