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^Tutilizes H₂/CO₂ 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).