A methanogenic archaeon, strain HC-2T, was isolated from a deep diatomaceous shale formation. The strain grew on methanol, monomethylamine, dimethylamine, trimethylamine and dimethylsulphide, but not on acetate, H2/CO2, formate, 2-propanol, 2-butanol or cyclopentanol. Cells were Gram-stain-negative, non-motile, and coccus-like, 0.9–1.4 µm in diameter, and occurred singly, in pairs, or as aggregates. The strain grew at 10–40 °C (optimum 35 °C), pH 5.9–7.4 (optimum pH 6.6–6.8) and in 0–0.6 M NaCl (optimum 0.1–0.2 M). The genomic DNA G+C content was 41.5 mol% and the 16S rRNA gene sequence was closely related to those of Methanosarcina lacustris DSM 13486T (99.1 %) and Methanosarcina siciliae DSM 3028T (98.3 %). Values for DNA–DNA hybridization with these strains were less than 30 %. The phenotypic and phylogenetic features of HC-2T indicate that it represents a novel species of the genus Methanosarcina , for which the name Methanosarcina subterranea sp. nov. is proposed. The type strain is HC-2T ( = DSM 22503T = JCM 15540T = NBRC 102578T).
A hyperthermophilic methanogen, strain JH146T, was isolated from 26 °C hydrothermal vent fluid emanating from a crack in basaltic rock at Marker 113 vent, Axial Seamount in the northeastern Pacific Ocean. It was identified as an obligate anaerobe that uses only H2 and CO2 for growth. Phylogenetic analysis based on 16S rRNA gene sequences showed that the strain is more than 97 % similar to other species of the genus Methanocaldococcus . Therefore, overall genome relatedness index analyses were performed to establish that strain JH146T represents a novel species. For each analysis, strain JH146T was most similar to Methanocaldococcus sp. FS406-22, which can fix N2 and also comes from Marker 113 vent. However, strain JH146T differs from strain FS406-22 in that it cannot fix N2. The average nucleotide identity score for strain JH146T was 87 %, the genome-to-genome direct comparison score was 33–55 % and the species identification score was 93 %. For each analysis, strain JH146T was below the species delineation cut-off. Full-genome gene synteny analysis showed that strain JH146T and strain FS406-22 have 97 % genome synteny, but strain JH146T was missing the operons necessary for N2 fixation and assimilatory nitrate reduction that are present in strain FS406-22. Based on its whole genome sequence, strain JH146T is suggested to represent a novel species of the genus Methanocaldococcus for which the name Methanocaldococcus bathoardescens is proposed. The type strain is JH146T ( = DSM 27223T = KACC 18232T).