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-6T (T = type strain) and Methanosarcina frisia C 16T 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 16T) and Methanosarcina acetivorans C2A(T), were slightly halotolerant (rather than halophillic, as originally described). Methanosarcina sp. strain FR-1, which has ps vesicles, was more similar to Methanosarcina barkeri MS(T) than to Methanosarcina vacuolata Z-761T in both its protein patterns and its DNA sequence (80% similarity to M. barkeri MS(T) and 38% similarity to M. vacuolata Z-761T). Thus, the presence of gas vesicles is not an adequate taxonomic characteristic for assigning Methanosarcina strains to M. vacuolata.