A thermophilic and hydrogenotrophic methanogen, strain RMAST, was isolated from gasassociated
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
(0.53.5–10.5 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 RMAST 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 DHT 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 (5DSM 23052T5JCM 16532T5NBRC 106236T).