Strains of Clostridium thermoaceticum were tested for H2- and CO-dependent growth in a defined medium containing metals, minerals, vitamins, cysteine-sulfide, CO2-bicarbonate, and H2 or CO. Ten of the thirteen strains tested grew at the expense of H2 and CO, and C. thermoaceticum ATCC 39073 was chosen for further study. The doubling times for H2- and CO-dependent growth under chemolithotrophic conditions (the defined medium with nicotinic acid as sole essential vitamin and sulfide as sole reducer) were 25 and 10 h, respectively. Product stiochiometries for chemolithotrophic cultures approximated: 4.1H2 + 2.4CO2----CH3COOH + 0.1 cell C + 0.3 unrecovered C and 6.8CO----CH3COOH + 3.5CO2 + 0.4 cell C + 0.9 unrecovered C. H2-dependent growth produced significantly higher acetate concentrations per unit of biomass synthesized than did CO- or glucose-dependent growth. In contrast, the doubling time for H2-dependent growth under chemolithotrophic conditions (the defined medium without vitamins and sulfide as sole reducer) by Acetogenium kivui ATCC 33488 was 2.7 h; as a sole energy source, CO was not growth supportive for A. kivui. The YH2 values for A. kivui and C. thermoaceticum were 0.91 and 0.46 g of cell dry weight per mol of H2 consumed, respectively; the YCO value for C. thermoaceticum was 1.28 g of cell dry weight per mol of CO consumed. The specific activities of hydrogenase and CO dehydrogenase in both acetogens were influenced by the energy source utilized for growth and were significantly lower in C. thermoaceticum than in A. kivui. With extracts of H2-cultivated cells and benzyl viologen as electron acceptor, the Vmax values for hydrogenase from C. thermoaceticum and A. kivui were 155.7 and 1,670 micromoles of H2 oxidized per min mg of protein, respectively; the Vmax values for CO dehydrogenase from C. thermoaceticum and A. kivui were 90.6 and 2,973 micromoles of CO oxidized per min per mg of protein, respectively.
