Effects of benzyl viologen on increasing NADH availability, acetate assimilation, and butyric acid production by Clostridium tyrobutyricum

Clostridium tyrobutyricum produces butyric and acetic acids from glucose. The butyric acid yield and selectivity in the fermentation depend on NADH available for acetate reassimilation to butyric acid. In this study, benzyl viologen (BV), an artificial electron carrier that inhibits hydrogen production, was used to increase NADH availability and butyric acid production while eliminating acetic acid accumulation by facilitating its reassimilation. To better understand the mechanism of and find the optimum condition for BV effect on enhancing acetate assimilation and butyric acid production, BV at various concentrations and addition times during the fermentation were studied. Compared with the control without BV, the addition of 1 mu M BV increased butyric acid production from glucose by similar to 50% in yield and similar to 29% in productivity while acetate production was completely inhibited. Furthermore, BV also increased the coutilization of glucose and exogenous acetate for butyric acid production. At a concentration ratio of acetate (g/L) to BV (mM) of 4, both acetate assimilation and butyrate biosynthesis increased with increasing the concentrations of BV (0-6.25 mu M) and exogenous acetate (0-25 g/L). In a fed-batch fermentation with glucose and similar to 15 g/L acetate and 3.75 mu M BV, butyrate production reached 55.9 g/L with productivity 0.93 g/L/h, yield 0.48 g/g, and 97.4% purity, which would facilitate product purification and reduce production cost. Manipulating metabolic flux and redox balance via BV and acetate addition provided a simple to implement metabolic process engineering approach for butyric acid production from sugars and biomass hydrolysates.

Automated summary

In this study, benzyl viologen (BV) was utilized to enhance butyric acid production by Clostridium tyrobutyricum from glucose, while eliminating acetic acid accumulation. The addition of BV increased butyric acid yield and productivity, inhibited acetate production, and promoted glucose and acetate co-utilization for butyric acid production. Manipulating metabolic flux and redox balance through BV and acetate addition provided a simple approach for efficient butyric acid production.