C1 Gas Fermentation by the Acetogenic Bacterium Clostridium sp. AWRP

Abstract

Clostridium sp. AWRP is an acetogen that grows autotrophically with either CO or CO2/H2 to form acetate and ethanol via the Wood-Ljungdahl pathway (WLP). Here, we investigate the physiological properties of AWRP in C1 gas fermentation. In CO fermentation, the growth of AWRP was inhibited by high concentrations of CO. Acetate supplementation shortened the lag period and enhanced the maximum growth rate and maximum cell density by 2.6- and 1.9-fold, respectively. In CO2/H2 fermentation, AWRP produced acetate as the main product, and its growth was inhibited by high concentrations of acetate. To investigate the cellular response to acetate stress, serial transfers were performed in acetate-containing medium. The adapted strain displayed acetate tolerance in small-scale cultures and increased cell density but reduced gas consumption. Transcriptome analysis revealed that the expression of most genes involved in the WLP was down-regulated, while genes involved in nucleoside and carbohydrate degradation were up-regulated. Lastly, minimal requirements of vitamins were determined in a defined medium for fully-autotrophic growth. Replacement of pyridoxine with other forms of vitamin B6, supported the growth. To address vitamin B6, we attempted to express the pdxST genes involved in vitamin B6 biosynthesis. Engineered strains could be successfully grown without vitamin B6. Overall, these results provide practical implications for gas fermentation using acetogenic bacteria.