Methanocaldococcus bathoardescens sp nov., a hyperthermophilic methanogen isolated from a volcanically active deep-sea hydrothermal vent

A hyperthermophilic methanogen, strain HG146(T), was isolated from 26 degrees 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 H-2 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 JH146(T) represents a novel species. For each analysis, strain JH146(T) was most similar to Methanocaldococcus sp. FS406-22, which can fix N-2 and also comes from Marker 113 vent. However, strain JH146(T) differs from strain FS406-22 in that it cannot fix N-2. The average nucleotide identity score for strain JH146(T) was 87%, the genome-to-genome direct comparison score was 33-55% and the species identification score was 93%. For each analysis, strain JH146(T) was below the species delineation cut-off. Full-genome gene synteny analysis showed that strain JH146(T) and strain FS406-22 have 97% genome synteny, but strain JH146(T) was missing the operons necessary for N-2 fixation and assimilatory nitrate reduction that are present in strain FS406-22. Based on its whole genome sequence, strain JH146(T) is suggested to represent a novel species of the genus Methanocaldococcus for which the name Methanocaldococcus bathoardescens is proposed. The type strain is JH146(T) (=DSM 27223(T)=KACC 18232(T)).