Taxonomy and Halotolerance of Mesophilic Methanosarcina Strains, Assignment of Strains to Species, and Synonymy of Methanosarcina mazei and Methanosarcina frisia

We examined 22 previously described and newly isolated Methanosarcina strains by performing denaturing gel electrophoresis of whole-cell proteins and assigned these strains to previously described species. Methanosarcina mazei S-6^T (T = type strain) and Methanosarcina frisia C 16^T were very similar in terms of the electrophoresis patterns of their proteins and in their DNA sequences (the results of reassociation experiments indicated that there was 77% sequence similarity). Thus, M. frisia is a junior subjective synonym of M. mazei, and strain C 16 is a reference strain of M. mazei. M. mazei C 16 was similar to M. mazei in other characteristics that have not been reported previously, including the ability to catabolize acetate and a lack of halophily. All of the Methanosarcina strains examined, including the marine strains M. mazei C 16 (= M. frisia C 16^T) and Methanosarcina acetivorans C2A^T, were slightly halotolerant (rather than halophilic, as originally described). Methanosarcina sp. strain FR-1, which has gas vesicles, was more similar to Methanosarcina barkeri MS^T than to Methanosarcina vacuolata Z-761^T in both its protein patterns and its DNA sequence (80% similarity to M. barkeri MS^T and 38% similarity to M. vacuolata Z-761^T). Thus, the presence of gas vesicles is not an adequate taxonomic characteristic for assigning Methanosarcina strains to M. vacuolata.