Characterization of Methanosarcina barkeri MST and 227, Methanosarcina mazei S-6T, and Methanosarcina vacuolata Z-761T

Members of the genus Methanosarcina are recognized as major aceticlastic methanogens, and several species which thrive in low-salt, pH-neutral culture medium at mesophilic temperatures have been described. However, the environmental conditions which support the fastest growth of these species (Methanosarcina barkeri MST [T = type strain] and 227, Methanosarcina mazei S-6T, and Methanosarcina vacuolata Z-761T) have not been reported previously. Although the members of the genus Methanosarcina are widely assumed to grow best at pH values near neutrality, we found that some strains prefer acidic pH values. M. vacuolata and the two strains of M. barkeri which we tested were acidophilic when they were grown on H2 plus methanol, growing most rapidly at pH 5 and growing at pH values as low as 4.3. M. mazei grew best at pH values near neutrality. We found that all of the strains tested grew most rapidly at 37 to 42°C on all of the growth substrates which we tested. None of the strains was strongly halophilic, although the growth of some strains was slightly stimulated by small amounts of added NaCl. The catabolic substrates which supported most rapid growth were H2 plus methanol; this combination sometimes allowed growth of a strain under extreme environmental conditions which prevented growth on other substrates. The cell morphology of all strains was affected by growth conditions.