Influence of redox potential on product distribution in Clostridium thermosuccinogenes

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. In this study, organic acid production in 2-L fermentations having an initially low (-300 to -330 mV) or high (-220 to -250 mV) redox potential was compared for two strains of C. thermosuccinogenes (DSM 5808 and DSM 5809). Although DSM 5809 consistently provided higher succinate yield, high variability in results was attributed to the absence of redox control during the fermentations, and, therefore, fermentations at three controlled redox potentials (-240, -275, and -310 mV) were conducted. At an intermediate redox potential (-275 mV), the succinate yield was the greatest (0.36 g of succinate/g of hexose unit), whereas ethanol yield was the least (0.02 g/g). Redox potential did not significantly affect acetate or lactate formation. At a controlled redox potential of -275 mV, the growth of DSM 5809 on three substrates was also compared: inulin, fructose, and glucose. DSM 5809 had similar growth rates when inulin (0.20/h) or glucose (0.21/h) was the carbon source but grew more slowly when fructose (0.16/h) was the carbon source. Also, the specific rate of utilization of fructose by DSM 5809 was higher (0.89 g of fructose/[g of biomass.h]) compared to glucose (0.53 g/[g.h]) or inulin (0.55 g/[g.h]). Succinate was the major product formed by DSM 5809 fermenting inulin (0.50 g/[g.h]) or glucose (0.36 g/[g.h]), and ethanol was the principal product when DSM 5809 fermented fructose (0.54 g/[g.h]).