Effect of Organic Substrate on Promoting Solventogenesis in Ethanologenic Acetogene Clostridium ljungdahlii ATCC5538

Clostridium ljungdahlii is a strictly anaerobic acetogene known for its ability to ferment a wide variety of substrates to ethanol and acetate. This bacterium presents a complex anaerobic metabolism including the acetogenic and solventogenic phases. In this study, the effect of various carbon sources on triggering the metabolic shift toward solventogenesis was considered. The bacterium was grown on fructose, glucose, acetate and ethanol in batch cultures. The fermentation results demonstrated that fructose improved ethanol production (27.1 mM) over acetate (26.3 mM), but glucose was predominantly metabolized to acetate. The bacterial cells were able to either utilize or produce ethanol (25 mM) probably through different metabolic pathways. The presence of acetate as the carbon source in the culture shifted the metabolic pathway of the cells toward solventogenic phase, but ethanol formation was not considerable (3.5 mM). It was also attempted to improve ethanol production yield by varying the fructose concentration (1 to11 g/L) in the batch culture. Under the conditions of substrate depletion or high fructose concentrations, the cell growth declined and the metabolic pathway of C. ljungdahlii was unable to switch from acetogenesis to solventogenesis. The fructose concentration of 5 g/L was found as the suitable concentration to yield an ethanol to acetate molar ratio of 1:1.