Methanocaldococcus abyssi sp. nov., a novel hyperthermophilic methanogen isolated from the recently discovered La Scala hydrothermal deep-sea vent field, Woodlark Ridge

Active hydrothermal chimneys were sampled from the newly discovered La Scala Vent Field, located on the Woodlark Ridge in the Pacific Ocean. Enrichment cultures yielded hyperthermophilic methanogens, from which several strains were isolated. Phylogenetic analysis based on the 16S rRNA gene and genome sequences revealed that strain 28A^T belongs to the genus <i>Methanocaldococcus</i>, within the family <i>Methanocaldococcaceae</i> and the order <i>Methanococcales</i>. Strain 28A^T shared 95.4-99.1% 16S rRNA gene sequence similarity with other species of <i>Methanocaldococcus</i> and was most closely related to <i>Methanocaldococcus bathoardescens</i>. Cells of strain 28A^T were coccoid in shape, with diameters ranging from 1.0 to 3.0 µm. The strain grew at temperatures between 50 and 85 °C (optimum: 75-80 °C), at pH values between 5.3 and 7.0 (optimum: pH 6.5) and in sodium chloride (NaCl) concentrations ranging from 5 to 50 g l⁻¹ (optimum: 20 g l⁻¹). Genome analysis revealed the presence of a 3.9 kbp plasmid. Based on 16S rRNA and genome phylogenetic analysis, average nucleotide identity and digital DNA-DNA hybridization values, we propose that strain 28A^T represents a novel species, for which the name <i>Methanocaldococcus abyssi</i> sp. nov. is proposed. The type strain is 28Aᵀ (=DSM 120179ᵀ=JCM 39640ᵀ).