Three thermophilic strains, strains P (= DSM 6194) (T = type strain), CALS-1 (= DSM 3870), and Z-517 (= DSM 4774), were characterized chemotaxonomically and compared with five mesophilic strains, GP6 (= DSM 3671), Opfikon (= DSM 2139), FE (= DSM 3013), U, and P. These methanogens were exclusively acetotrophic and had a characteristic sheathed structure. The DNA base compositions of the strains which we studied ranged from 50.3 to 54.3 mol% guanine plus cytosine. The thermophilic strains often had phase-refractive gas vesicles inside their cells. Denaturing electrophoresis of proteins showed that the mesophilic and thermophilic strains formed two distinct groups and that there were differences in protein patterns between the groups. The difference between the thermophiles and mesophiles was also verified by comparing partial 16S rRNA sequences (ca. 30 base differences in ca. 540 bases). On the basis of our results, we propose the name for the three thermophilic strains. The type strain of is strain P (= DSM 6194). We also propose that the name , which was given to strain Z-517 (type strain), should be rejected because of its description, which was based on an enrichment culture, was inadequate.


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