Methylothermus thermalis gen. nov., sp. nov., a novel moderately thermophilic obligate methanotroph from a hot spring in Japan

Abstract

A novel moderately thermophilic methanotroph, strain MYHT^T, was isolated from a hot spring in Japan. The isolate grew on methane or methanol at 37–67 °C, and optimally at 57–59 °C. It was found to be a Gram-negative aerobe, with colourless colonies of non-motile coccoid cells, possessing type I intracytoplasmic membranes and regularly arranged surface layers of linear (p2) symmetry. Strain MYHT^T expressed only the particulate methane monooxygenase and employed the ribulose monophosphate pathway for formaldehyde assimilation. It is a neutrophilic and halotolerant organism capable of growth at pH 6·5–7·5 (optimum pH 6·8) and in up to 3 % NaCl (optimum 0·5–1 % NaCl). Phylogenetic analysis based on 16S rRNA gene sequence analysis indicated that strain MYHT^T is most closely related to the thermophilic undescribed methanotroph ‘Methylothermus’ HB (91 % identity) and the novel halophilic methanotroph Methylohalobius crimeensis 10Ki^T (90 % identity). Comparative sequence analysis of particulate methane monooxygenase (pmoA) genes also confirmed the clustering of strain MYHT^T with ‘Methylothermus’ HB and Methylohalobius crimeensis 10Ki^T (98 and 92 % derived amino acid sequence identity, respectively). The DNA G+C content was 62·5 mol%. The major cellular fatty acids were C_16 : 0 (37·2 %) and C_18 : 1ω9c (35·2 %) and the major polar lipids were phosphatidylethanolamine and phosphatidylglycerol. The major ubiquinone was Q-8. On the basis of comparative phenotypic and genotypic characteristics, a new genus and species, Methylothermus thermalis gen. nov., sp. nov., is proposed, with MYHT^T as the type strain (=VKM B-2345^T=IPOD FERM P-19714^T).