Clostridium thermosuccinogenes are the only known anaerobic thermophilic bacteria that ferment inulin to succinate and acetate as major products and formate, lactate and ethanol as minor products. Fermentations having different initial redox potentials were compared for two strains of Clostridium thermosuccinogenes (DSM 5808 and DSM 5809). Although DSM 5809 consistently provided higher succinate yield, high variability in the results was attributed to the absence of redox control during the fermentations. Therefore, fermentations at three controlled redox potentials (-240 mV, -275 mV and -310 mV) were conducted with strain DSM 5809. At an intermediate redox potential (-275 mV) the succinate yield was the greatest (0.36 g succinate/g hexose unit), while ethanol yield was the least (0.02 g/g). DSM 5809 had similar growth rates while growing on inulin and glucose to form succinate as a major product, but grew more slowly on fructose forming ethanol as a principal product.; The biochemical pathways for the fermentation of inulin by C. thermosuccinogenes strain DSM 5809 are proposed. Activities of nine fermentative enzymes were measured at four temperatures (37°C, 47°C, 58°C and 70°C). Each of the enzymes increased 1.5--2.0-fold in activity between 37°C and 58°C, but only lactate dehydrogenase, fumarate reductase, malate dehydrogenase and fumarase increased by similar amounts between 58°C and 70°C. No acetate kinase activity was observed at 70°C. Maximum succinate yield (0.23 mol/mol hexose unit), acetate yield (0.79) and biomass yield (1.20) occurred at 58°C whereas the maximum yields for lactate (0.19) and hydrogen (0.25) and the lowest yields for acetate (0.03) and biomass (0.78) were observed at 70°C.; Metabolic carbon flux analysis was used to evaluate the effect of pH and culture redox potential on C. thermosuccinogenes. The flux of carbon towards succinate, formate and acetate increased significantly with a pH increase from 6.50 to 7.25, while the flux to lactate decreased significantly. Lower values of culture redox potential significantly increased succinate, acetate and formate fluxes and significantly decreased ethanol and hydrogen fluxes in C. thermosuccinogenes. The presence of hydrogen in the headspace or the addition of nicotinic acid to the growth media resulted in increased hydrogen and ethanol fluxes and decreased succinate, acetate, formate and lactate fluxes.
展开▼
