Anna-Louise Reysenbach

Three hyperthermophilic methanogens, designated strain SG7 T , 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 T , 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 T , 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 −1 NaCl (optimum 20–25 g l −1 ). Genome analysis revealed the presence of a 5.1 kbp plasmid in strain SG7 T . Based on the results of average nucleotide identity and digital DNA–DNA hybridization analyses, we propose that strains SG1 and SG7 T are representatives of a novel species, for which the name Methanocaldococcus lauensis sp. nov. is proposed; the type strain is SG7 T (=DSM 109608 T =JCM 39049 T ).