Members of the genus 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 ( MS [T = type strain] and 227, S-6, and Z-761) have not been reported previously. Although the members of the genus are widely assumed to grow best at pH values near neutrality, we found that some strains prefer acidic pH values. and the two strains of which we tested were acidophilic when they were grown on H plus methanol, growing most rapidly at pH 5 and growing at pH values as low as 4.3. 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 H 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.


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