Summary auto-generated
This study examined the phylogeny and taxonomy of mesophilic Methanococcus species using multiple methods: 16S rRNA sequencing, DNA-DNA hybridization, cellular protein electrophoresis, and phenotypic analysis. Four known mesophilic species—M. maripaludis, M. vannielii, M. voltaei, and "M. aeolicus"—showed only 5-30% DNA relatedness and 92-96% 16S rRNA similarity, indicating they formed distinct groups. Twenty-two new isolates from anaerobic sediments clustered into two groups resembling either M. voltaei or M. maripaludis. Isolates A2 and A3 exhibited 37% DNA relatedness and 99.2% rRNA similarity to M. voltaei and were assigned to that species, while autotrophic isolates C5-C8 showed 54-69% DNA relatedness to M. maripaludis and were assigned there. Despite significant genetic diversity justifying separate species classification, these organisms displayed highly conserved phenotypic properties, making phenotypic characteristics unreliable taxonomic indicators for these archaea. Serological cross-reactivity and allelic diversity in rRNA sequences also proved unsuitable for methanococcal taxonomy. The authors suggest the mesophilic methanococci ancestor may have been an autotrophic thermophile.
Key findings
- Mesophilic Methanococcus species exhibit low DNA relatedness (5-30%) despite being closely related by 16S rRNA sequence analysis (92-96% similarity), indicating distinct species despite genetic conservation.
- Phenotypic properties of these methanogenic archaea are highly conserved and unreliable for species differentiation, unlike in other bacterial groups; genetic methods proved more discriminatory.
- Serological cross-reactivity and allelic diversity in rRNA sequences showed poor correlation with genetic diversity measured by DNA hybridization and other methods, making them unsuitable taxonomic markers.
- New isolates were assigned to existing species based on genetic similarity despite being genetically diverse enough to potentially represent novel species, due to lack of distinguishing phenotypic characters.
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Abstract
The phylogeny and taxonomy of the mesophilic methane-producing archaea of the order Methanococcales were examined by DNA relatedness, 16S rRNA sequence analysis, cellular protein pattern, and phenotypic methods. The mesophilic species Methanococcus maripaludis, Methanococcus vannielii, Methanococcus voltaei, and "Methanococcus aeolicus" formed a deep group with 5 to 30% DNA relatedness and 92 to 96% 16S rRNA sequence similarity. Twenty-two additional isolates and Methanococcus deltae were similar to the type strain of either M. voltaei or M. maripaludis. 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 assigned to M. voltaei. Similarly, four autotrophic isolates, strains C5, C6, C7, and C8, exhibited 54 to 69% DNA relatedness and 99.2% 16S rRNA sequence similarity to M. maripaludis JJT and were assigned to M. maripaludis. 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.