Effect of ethanol and butanol on autotrophic growth of model homoacetogens

Research efforts aimed at increasing ethanol and butanol productivity from syngas are currently gaining attention. For most model carboxydotrophic bacteria, production rates, yields and maximum product titres have been studied in detail, but little is known on alcohol toxicity in these bacteria. The aim of this work was to investigate the inhibitory effects of ethanol and butanol on the growth of Clostridium ljungdahlii PETC, C. carboxidiwrans P7, and 'Butyribacterium methylotrophicum DSM3468'. Experiments to determine inhibitory effects due to product accumulation were carried out using a synthetic mixture of CO:CO2:H-2 as a substrate. These conditions were chosen to mimic gaseous effluents of biomass and waste gasification plants. Inhibition effects were recorded as changes in growth parameters. No significant inhibition was observed for ethanol at concentrations below 15 g/L. The three species exhibited higher sensitivity to butanol. Half inhibition constants for butanol could be estimated for P7 (IC50 = 4.12 g/L), DSM3468 (IC50 = 1.79 g/L), and PETC IC50 = 9.75 g/L). In conclusion, at least for the tested strains, alcohol toxicity is not an immediate handicap for increasing alcohol production of the tested homoacetogenic strains.