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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. Metha- nosarcina mazei S-6T (T = type strain) and Methanosarcinafi.isia 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. fish is a junior subjective synonym of M. mazei, and strain C 16 is a reference strain of M. mazei. M. muzei 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. jiisia C 16T) and Methunosarcina acetivorans C2AT, were slightly halotolerant (rather than halophilic, as originally described). Methunosarcina sp. strain FR-1, which has gas vesicles, was more similar to Methanosarcina barken' MST than to Methanosarcina vacuolata Z-761T in both its protein patterns and its DNA sequence (80% similarity to M. barken' MST and 38% similarity to M. vacuolata Z-761T). Thus, the presence of gas vesicles is not an adequate taxonomic characteristic for assigning Methunosarcina strains to M. vacuolata. The five commonly recognized mesophilic Methanosar- cina species (Methanosarcina barkri, Methanosarcina mazei, Methanosarcina vacuolata , Methanosarcina fnsia , and Methanosarcina acetivorans) are found in marine and freshwater environments (24,33,34). The first three of these species are considered to be freshwater species, and their type strains were isolated from freshwater environments. However, early strains of M. barkri were isolated from marine or brackish mud (36,42), and the current type strains of M. barkeri and M. mazei thrive at marine salinities (40). It has been postulated that Methanosarcina species evolved in marine habitats and adapted to freshwater environments by developing a heteropolysaccharide matrix or capsule (41). When Methanosarcina strains are grown in the presence of marine concentrations of Na+ and Mg2+, they have a protein cell wall, are sensitive to lysis by detergents, and occur as individual cells (35,40,46). When these organisms are grown in the presence of low concentrations of these cations, a heteropolysaccharide matrix is often formed, and cells may occur as aggregates, "cysts," or lamina (27, 40, 46). The other two mesophilic Methanosarcina species, M. fnsia and M. acetivorans, are considered to be marine species because all of the known strains (M. fnsia C 16T (T = type strain) and M. acetivorans C2AT and C2E) were isolated from marine sediments (4, 39). The cells of these strains are sensitive to lysis by detergent, and their cell walls are protein (4, 39). In this paper we describe some of the physiological characteristics, especially halotolerance, of some Metha- nosarcina strains, the results of a phylogenetic analysis, and taxonomic assignments for strains and propose that M. mazei and M. fnsia are synonymous.

international journal of systematic bacteriology

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