The phylogeny and taxonomy of the mesophilic methane-producing archaea of the order were examined by DNA relatedness, 16S rRNA sequence analysis, cellular protein pattern, and phenotypic methods. The mesophilic species , and “” formed a deep group with 5 to 30% DNA relatedness and 92 to 96% 16S rRNA sequence similarity. Twenty-two additional isolates and were similar to the type strain of either or . Two isolates, strains A2 and A3, exhibited 37% DNA relatedness and 99.2% 16S rRNA sequence similarity to PS (T = type strain). In the absence of phenotypic differences, these organisms were assigned to . Similarly, four autotrophic isolates, strains C5, C6, C7, and C8, exhibited 54 to 69% DNA relatedness and 99.2% 16S rRNA sequence similarity to JJ and were assigned to . While these isolates were sufficiently genetically diverse to justify classification in novel species, few differences were apparent in the phenotypic properties available for measurement. Thus, the phenotypic properties of these lithotrophic archaea were highly conserved and poor indicators of genetic diversity. Partial sequencing of about 200 bases of both the 16S and 23S rRNAs of the isolates demonstrated allelic diversity within methanococcal species. This allelic diversity did not correlate with diversity measured by DNA relatedness, cellular protein pattern, and other methods. Similarly, antisera to whole cells of the type strains did not cross-react strongly to whole cells of strains that were genetically similar, and serological cross-reactivity was not a useful taxonomic method for methanococci. Lastly, on the basis of the results of 16S rRNA sequence analyses and biochemical data, the ancestor of the mesophilic methanococci may have been an autotrophic thermophile.


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