Acetate-assisted carbon monoxide fermentation of Clostridium sp. AWRP

Abstract

Acetogens are of great interest on conversion of synthesis gas in that they can efficiently utilize CO-containing gases to produce C2 and higher metabolic products. However, high concentration of CO is known to inhibit the reduction of CO2 into formate, thereby diminishing an overall carbon fixation rate. We report that acetate supplementation can be a simple, effective approach to mitigating CO inhibition in Clostridium sp. AWRP (hereinafter called AWRP). The added acetate rapidly reduced into ethanol by the AWRP cells, which coincided with CO oxidation. From stoichiometric calculations, the acetate reduction was observed to allow carbon fixation, indicating that the CO oxidation might reduce the intracellular CO levels below inhibitory levels. Furthermore, AWRP exhibited an 83% higher maximum specific growth rate (μmax) in the acetate-supplemented bottle cultures than in the control. In the bioreactor experiments, the AWRP culture with exogenous acetate displayed 2.9- and 2.3-fold higher titers of ethanol (232 mM) and 2,3-butanediol (23 mM), respectively, as well as μmax (2.6-fold) and maximum cell density (1.9-fold), than without acetate supplementation. These results indicate that this approach can be applied to various fermentative experiments using CO-containing gases to shorten the overall cultivation time.