Methanocaldococcus lauensis sp. nov., a novel deep-sea hydrothermal vent hyperthermophilic methanogen

Three hyperthermophilic methanogens, designated strain SG7, strain SG1 and strain SLH, were isolated from the ABE and Tu’i Malila deep-sea hydrothermal vent fields along the Eastern Lau Spreading Center. Phylogenetic analysis based on 16S rRNA gene sequence indicated that strains SG7, SG1 and SLH were affiliated with the genus Methanocaldococcus within the family Methanocaldococcaceae, order Methanococcales. They shared 95.5–99.48 % 16S rRNA gene sequence similarity to other Methanocaldococcus species and were most closely related to Methanocaldococcus bathoardescens. Cells of strains SG7, SG1 and SLH were cocci, with a diameter of 1.0–2.2 µm. The three strains grew between 45 and 93 °C (optimum, 80–85 °C), at pH 5.0–7.1 (optimum pH 6.2) and with 10–50 g l NaCl (optimum 20–25 g l). Genome analysis revealed the presence of a 5.1 kbp plasmid in strain SG7. Based on the results of average nucleotide identity and digital DNA–DNA hybridization analyses, we propose that strains SG1 and SG7 are representatives of a novel species, for which the name Methanocaldococcus lauensis sp. nov. is proposed; the type strain is SG7 (=DSM 109608=JCM 39049).