Mutant strains were derived fromClostridium thermoaceticumATCC 39 289 by treatment with chemical mutagenic agents and selective enrichment procedures. Some mutant strains exhibited growth when cultured in media containing 20 mabetm(1.75 g l−1) pyruvate of high-magnesium lime (dolime) above pH 6.0. One strain (G-20) grew and produced acetate when 80 mabetm(7 gl−1) pyruvate or 50 mabetm(2.3 g l−1) formate at pH 5.6 was the sole energy source. In a fed-batch process controlled at pH 6.2, this mutant produced 52.5 g l−1acetate (equivalent to 72.5 g l−1Na acetate) and 67 g l−1calcium-magnesium acetate (CMA) in 140 h when dolime was the neutralizing agent, with 93 substrate utilization. This mutant strain holds promise for CMA production due to its better tolerance of dolime and its ability to synthesize high levels of
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