Phylogenetic Relationships Among the Methylotrophic Methane-Producing Bacteria and Emendation of the Family Methanosarcinaceae

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Abstract
The phylogenetic relationships among seven strains of methylotrophic methane-producing bacteria were determined by ribosomal ribonucleic acid hybridization and deoxyribonucleic acid homology techniques. The strains tested had deoxyribonucleic acid guanine-plus-cytosine contents of 39 to 43 mol% and represented a diversity of phenotypic characteristics. Our results indicate that these strains should be divided into six species within the genera Methanosarcina, Methanolobus, and Methanococcoides. The genus Methanosarcina includes Methanosarcina barkeri strains MS and 227, Methanosarcina sp. strain TM-1, Methanosarcina acetivorans strain C2A, and Methanosarcina mazei strain S-6. The genus Methanolobus is represented by Methanolobus tindarius strain Tindari 3, and the genus Methanococcoides is represented by Methanococcoides methylutens strain TMA-10. Despite phenotypic similarities between Methanolobus tindarius and Methanococcoides methylutens, we propose that these species remain in separate genera based on differences in ribosomal ribonucleic acid homology and fractional differences in midpoint temperatures. The divisions indicated by deoxyribonucleic acid homology experiments complemented the ribosomal ribonucleic acid hybridization results. Phenotypic characteristics were consistent with these phylogenetic divisions; an apparent exception was cell wall composition, which is a conserved trait. Methanosarcina acetivorans had only a thin protein cell wall, but all other strans of Methanosarcina previously studied have been reported to have heteropolysaccharide cell walls. We present evidence which indicates that a protein component may be associated with the heteropolysaccharide cell wall of Methanosarcina barkeri strain 227.