Analysis of metabolic activities in Clostridium carboxidivorans under mixotrophic and anaerobic condition

Abstract

Clostridium carboxidivorans is capable of mixotrophic fermentation using sugar (e.g. glucose, xylose, and fructose) and synthesis gas (H2, CO2, and CO) to produce various acids and solvents (acetate, butyrate, ethanol, butanol, etc.) under anaerobic condition. This study describes metabolic profiles in four different gas compositions (H2/Ar [60%/40%], CO/Ar [40%/60%], CO/H2 [40%/60%], and CO2/H2 [20%/80%]) under glucose fermentation. The cultures were operated at 37 ℃ in 1 L bottle with 300 mL working volume containing 10 g/L glucose, and the pH was automatically controlled at 6.0 by 1.5 N NaOH. A low reducing power in the Embden-Meyerhof-Parnas glycolysis pathway can be covered by additional gas. The enough electron donor/reducing equivalents such as H2 and CO contributed to be re-assimilated via Wood Ljungdahl Pathway. Notably, supplying H2 significantly impacts carbon distribution and proportional increment in flux to 26.91% more ethanol as reduced product. The presence of H2 provided more reducing power, resulting in an 69% increase in the NADH/NAD+ ratio than heterotrophic growth. Furthermore, cyclic glucose fed-batch culture under high pressure H2 gas was operated in 3 L reactor at 3.0 bar and 500 rpm due to the low solubility of H2 gas. High ethanol productivity from glucose was achieved 0.339 g/h/g DCW in 3 L reactor. Mixotrophic fermentation showed metabolic shift leading to acetate with CO gas and ethanol with H2 gas. Interestingly, the compositions of feed gas can be consideration for a key factor in developing glucose based fermentation.