PHYLOGENY AND TAXONOMY OF MESOPHILIC METHANOCOCCUS SPP AND COMPARISONOF RIBOSOMAL-RNA, DNA HYBRIDIZATION, AND PHENOTYPIC METHODS

The phylogeny and taxonomy of the mesophilic methane-producing archaea of the order Methanococcales were examined by DNA relatedness, 16S rR NA sequence analysis, cellular protein pattern, and phenotypic methods . The mesophilic species Methanococcus maripaludis, Methanococcus vann ielii, Methanococcus voltaei, and ''Methanococcus aeolicus'' formed a deep group with 5 to 30% DNA relatedness and 92 to 96% 16S rRNA sequen ce similarity. Twenty-two additional isolates and Methanococcus deltae were similar to the type strain of either M. voltaei or M. maripaludi s. Two isolates, strains A2 and A3, exhibited 37% DNA relatedness and 99.2% 16S rRNA sequence similarity to M. voltaei PST (T = type strain) . In the absence of phenotypic differences, these organisms were assig ned to M. voltaei. Similarly, four autotrophic isolates, strains C5, C 6, C7, and C8, exhibited 54 to 69% DNA relatedness and 99.2% 168 rRNA sequence similarity to M. maripaludis JJ(T) and were assigned to M. ma ripaludis. While these isolates were sufficiently genetically diverse to justify classification in novel species, few differences were appar ent in the phenotypic properties available for measurement. Thus, the phenotypic properties of these lithotrophic archaea were highly conser ved and poor indicators of genetic diversity. Partial sequencing of ab out 200 bases of both the 16S and 23S rRNAs of the isolates demonstrat ed allelic diversity within methanococcal species. This allelic divers ity did not correlate with diversity measured by DNA relatedness, cell ular protein pattern, and other methods. Similarly, antisera to whole cells of the type strains did not cross-react strongly to whale cells of strains that were genetically similar, and serological cross-reacti vity was not a useful taxonomic method for methanococci. Lastly, on th e basis of the results of 16S rRNA sequence analyses and biochemical d ata, the ancestor of the mesophilic methanococci may have been an auto trophic thermophile.