Methanochimaera problematica gen. nov., sp. nov., a novel methanoarchaeon isolated from cold seep sediment and reclassification of Methanomicrobium antiquum as Methanoeremita antiquus gen. nov., comb. nov.

A hydrogenotrophic methanoarchaeon, designated strain FWC-SCC4 T , was isolated from cold seep sediment of Four-Way Closure Ridge, offshore southwestern Taiwan. Strain FWC-SCC4 T utilizes H 2 /CO 2 or formate, but not acetate, secondary alcohols, methylamines, methanol or ethanol for growth and methane production. Yeast extract is required for growth. The cell morphology is coccoid, with a diameter of 0.8–1.2 µm, and the cell envelope is composed of S-layer protein with M r about 137.00 kDa. Cells possess multiple flagella and usually occur singly. Strain FWC-SCC4 T grows at a temperature range of 20–40 °C (optimum 37 °C) and a pH range of 5.4–7.2 (optimum 7.0). The NaCl range for growth is 0–0.86 M (optimum 0.09 M). The result of phylogenetic analysis of 16S rRNA gene sequences indicates that the most closely related species are Methanoplanus limicola M3 T and Methanoplanus endosymbiosus MC1 T , with similarities of 95.95 and 95.63%, respectively. The G+C content of the genomic DNA is 40.3 mol%. The overall genome-relatedness indexes (OGRIs) and concatenated ribosomal protein (RBP) phylogenic analysis indicate that strain FWC-SCC4 T is a novel lineage of Methanomicrobiaceae . In addition to strain FWC-SCC4 T , the differences between Methanomicrobium antiquum MobH T and Methanomicrobium mobile BP T demonstrated by comparative analysis of genomic G+C content and phylogenetic analysis with non-type strain genomes are enough to support the establishment of a novel genus. In conclusion, strain FWC-SCC4 T (BCRC AR10058 T = NBRC 114595 T ) is proposed as the type strain of Methanochimaera problematica gen. nov., sp. nov., and Methanoeremita antiquus gen. nov., comb. nov. is proposed as the new name for Methanomicrobium antiquum MobH T (=DSM 21220 T = NBRC 104160 T ).