LAUSR

Abstract Gas fermentation using C1 substrate (CO or CO2) is a novel and attractive technology for ethanol production. Clostridium autoethanogenum, a promising gas fermentation strain, can grow on 100 % CO and produce acetate and ethanol through the Wood-Ljungdahl pathway. Like other autotrophic acetogens, C. autoethanogenum produces ethanol primarily via the reduction of acetyl-CoA. In this study, we demonstrated that ethanol was also formed via acetate reduction. 13C-labeled acetate was employed to investigate the conversion of acetate in C. autoethanogenum during its autotrophic growth on CO. The addition of exogenous acetate can improve ethanol production in C. autoethanogenum, 13C abundance in ethanol increased significantly with the increase of exogenous 13C-labeled acetate, confirming that acetate can be transformed to ethanol. Moreover, transcriptional analysis of genes encoding phosphotransacetylase(pta), aldehyde:ferredoxin oxidoreductase(aor), carbon monoxide dehydrogenase(codh) and alcohol dehydrogenase(adh) in C. autoethanogenum revealed their differential expression under varied exogenous acetate levels. Among these genes, pta gene was down-regulated due to its involvement in acetate formation, whereas aor gene CAETHG_0102, codh gene CAETHG_3005, adh genes CAETHG_1841 and CAETHG_1813 were found to be highly up-regulated at higher levels of exogenous acetate, suggesting that they may play important roles in the conversion of acetate to ethanol.