Methanosarcina mangrovi sp. nov. and Methanosarcina sediminis sp. nov., two novel methanogenic archaea isolated from mangrove sediments

Mangrove sediments are recognized as hotspots for methane emissions and harbour diverse methanogenic archaea. In this study, we describe two novel methanogens, strains T3^T and Mfa9^T, isolated from mangrove sediment samples in the Futian Mangrove Nature Reserve in Shenzhen, People's Republic of China. Cells of strains T3^T and Mfa9^T are irregularly coccoid in shape, with diameters of 0.7-1.4 µm and 0.6-1.7 µm, respectively. Both strains utilize methanol, methylamine, dimethylamine, trimethylamine, dimethyl sulphide and acetate as the sole carbon source for growth and methanogenesis, but not H₂/CO₂, formate, choline, betaine or pyruvate. Strain T3^T grew at 15-42 °C (optimum 37 °C), pH 6.0-8.0 (optimum 6.5) and 0-0.72 M NaCl (optimum 0.10 M NaCl). Strain Mfa9^T grew at 20-33 °C (optimum 25 °C), pH 5.5-7.5 (optimum 7.0) and 0-0.72 M NaCl (optimum 0.51 M NaCl). The genomic G+C contents of strains T3^T and Mfa9^T are 43.3 mol% and 46.2 mol%, respectively. Phylogenetic analyses revealed that both strains are classified within the genus <i>Methanosarcina</i>. Strain T3^T was mostly related to <i>Methanosarcina siciliae</i> T4/M^T, with 16S rRNA gene similarity of 99.7%, an average nucleotide identity of 88.8% and a digital DNA-DNA hybridization of 35.7%. By contrast, strain Mfa9^T shared 98.8% 16S rRNA gene similarity with <i>Methanosarcina semesiae</i> MD1^T. As <i>M. semesiae</i> MD1^T is unavailable and lacks genomic data, comparative analyses with its closer relative <i>Methanosarcina</i> sp. MTP4 (99.8% 16S rRNA gene identity), together with genomic and phenotypic differences from <i>M. semesiae</i>, support the designation of strain Mfa9^T as a novel species. In conclusion, two novel species of the genus <i>Methanosarcina</i>, <i>Methanosarcina mangrovi</i> sp. nov. and <i>Methanosarcina sediminis</i> sp. nov., are proposed. The type strain of <i>M. mangrovi</i> sp. nov. is T3^T (=CCAM 2189^T=JCM 39553^T), and the type strain of <i>M. sediminis</i> sp. nov. is Mfa9^T (=CCAM 2190^T=JCM 39554^T).