A novel mesophilic, hydrogenotrophic methanogen, strain CYW5T, was isolated from a sediment sample of a piston core collected from submarine mud volcano MV5 located in the offshore area of southwestern Taiwan. Cells of strain CYW5T were irregular coccids, 0.5–1.0 µm in diameter and lysed easily by 0.01 % sodium dodecyl sulphate (SDS) treatment. Strain CYW5Tutilized formate or hydrogen plus carbon dioxide as catabolic substrates for methanogenesis. The optimal growth conditions were 37 °C, 0.043–0.085 M NaCl and pH 6.02–7.32. The genomic DNA G+C content calculated from the genome sequence of strain CYW5T was 56.2 mol%. The results of phylogenetic analysis of 16S rRNA gene sequences indicated that strain CYW5T represented a member of the family Methanomicrobiaceae in the order Methanomicrobiales , and was closely related to the members of the genus Methanogenium . The most closely related species was Methanogenium cariaci JR1T (94.9 % of 16S rRNA gene sequence identity). The average nucleotide identity and average amino acid identity values between strain CYW5T and members of the family Methanomicrobiaceae were 74.7–78.5 % and 49.1–64.9%, respectively. Although many of the morphological and physiological characteristics of strain CYW5T and the species of the genus Methanogenium were similar, they were distinguishable by the differences in genomic G+C content and temperature, NaCl and pH ranges for growth. Based on these phenotypic, phylogenetic and genomic results, we propose that strain CYW5T represents a novel species, of a novel genus, named Methanovulcanius yangii gen. nov., sp. nov. The type strain is CYW5T (=BCRC AR10048T=DSM 100756T=NBRC 111404T).

Taiwan is situated in the subtropical region and its geographical location and topographical features contribute to a rich ecological diversity and scenic landscapes. We investigated the diversity of methanogens in different environments of Taiwan using a culture-dependent method. This report presents the characterization and taxonomy of six hydrogenotrophic methanogens obtained from cold seep sediments (strain FWC-SCC1^T and FWC-SCC3^T), marine sediments (strain CWC-02^T and YWC-01^T), estuarine sediments (strain Afa-1^T), and a hot spring well (strain Wushi-C6^T) in Taiwan. The proposed names of the six novel species are Methanoculleus frigidifontis (type strain FWC-SCC1^T=BCRC AR10056^T=NBRC 113993^T), Methanoculleus oceani (CWC-02^T=BCRC AR10055^T=NBRC 113992^T), Methanoculleus methanifontis (FWC-SCC3^T=BCRC AR10057^T=NBRC 113994^T), Methanoculleus nereidis (YWC-01^T=BCRC AR10060^T=NBRC 114597^T), Methanoculleus formosensis (Afa-1^T=BCRC AR10054^T=NBRC 113995^T), and Methanoculleus caldifontis (Wushi-06^T=BCRC AR10059^T= NBRC 114596^T).

To date, only a few microbial community studies of cold seeps at the South China Sea (SCS) have been reported. The cold seep dominated by tubeworms was discovered at South Yungan East Ridge (SYER) offshore southwestern Taiwan by miniROV. The tubeworms were identified and proposed as Paraescarpia formosa sp. nov. through morphological and phylogenetic analyses. The endosymbionts in the trunk of P. formosa analyzed by a 16S rRNA gene clone library represented only one phylotype, which belonged to the family Sedimenticolaceae in Gammaproteobacteria. In addition, the archaeal and bacterial communities in the habitat of tubeworm P. formosa were investigated by using high-phylogenetic-resolution full-length 16S rRNA gene amplicon sequencing. The results showed that anerobic methane-oxidizing archaea (ANME)-1b was most abundant and ANME-2ab was minor in a consortia of the anerobic oxidation of methane (AOM). The known sulfate-reducing bacteria (SRB) partners in AOM consortia, such as SEEP-SRB1, -SRB2, and -SRB4, Desulfococcus and Desulfobulbus, occurred in a small population (0–5.7%) at the SYER cold seep, and it was suggested that ANME-1b and ANME-2ab might be coupled with multiple SRB in AOM consortia. Besides AOM consortia, various methanogenic archaea, including Bathyarchaeota (Subgroup-8), Methanocellales, Methanomicrobiales, Methanosarcinales, Methanofastidiosales and Methanomassiliicoccales, were identified, and sulfur-oxidizing bacteria Sulfurovum and Sulfurimonas in phylum Epsilonbacteraeota were dominant. This study revealed the first investigation of microbiota in and around tubeworm P. formosa discovered at the SYER cold seep offshore southwestern Taiwan. We could gain insights into the chemosynthetic communities in the deep sea, especially regarding the cold seep ecosystems at the SCS.

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).