Methanoculleus horonobensis sp nov., a methanogenic archaeon isolated from a deep diatomaceous shale formation

A methanogenic organism from the domain Archaea, designated strain T10T, was isolated from groundwater sampled from a deep diatomaceous shale formation located in Horonobe, Hokkaido, Japan. The strain utilized H-2/CO2 and formate as substrates for methanogenesis. Cells were strictly anaerobic, Gram-negative-staining, flagellated, irregular coccoids, 0.7-1.6 mu rn in diameter, and occurred singly. The strain grew at 25-45 degrees C (optimum 37-42 degrees C), at pH 5.8-8.2 (optimum pH 6.7-6.8) and in the presence of 0-1.3 M NaCl (optimum 0.1-0.2 M NaCl). The G+C content of the genomic DNA was 62.9 mol%. 16S rRNA gene sequencing revealed that, although the strain is a member of the genus Methanoculleus, it clearly differed from all described species of this genus (95.5-98.3% sequence similarity). Values for DNA DNA hybridization with type strains of closely related Methanoculleus species were less than 50%. Phenotypic and phylogenetic features of strain T10(T) clearly indicate that it represents a novel species of the genus Methanocufleus, for which the name Methanoculleus horonobensis sp. nov. is proposed. The type strain is T10(T) (=DSM 21626(T)=JCM 15517(T)).