Methylothermus subterraneus sp nov., a moderately thermophilic methanotroph isolated from a terrestrial subsurface hot aquifer

A novel methane-oxidizing bacterium, strain HTM55(T), was isolated from subsurface hot aquifer water from a Japanese gold mine. Strain HTM55(T) was a Gram-negative, aerobic, motile, coccoid bacterium with a single polar flagellum and the distinctive intracytoplasmic membrane arrangement of a type I methanotroph. Strain HTM55(T) was a moderately thermophilic, obligate methanotroph that grew on methane and methanol at 37-65 degrees C (optimum 55-60 degrees C). The isolate grew at pH 5.2-7.5 (optimum 5.8-6.3) and with 0-1% NaCl (optimum 0-0.3%). The ribulose monophosphate pathway was operative for carbon assimilation. The DNA G+C content was 54.4 mol% and the major fatty acids were C-16:0, (52.0%) and C-18:1 omega 7c (34.8%). Phylogenetic analysis of the 16S rRNA gene sequence indicated that strain HTM55(T) was closely related to Methylothermus thermalis MYHTT (99.2% 16S rRNA gene sequence similarity), which is within the class Gammaproteobacteria. However, DNA DNA relatedness between strain HTM55(T) and Methylothermus thermalis MYHTT was On the basis of distinct phylogenetic, chemotaxonomic and physiological characteristics, strain HTM55(T) represents a novel species of the genus Methylothermus, for which the name Methylothermus subterraneus sp. nov. is proposed. The type strain is HTM55(T) (=JCM 13664(T)=DSM 19750(T)).