Acetate augmentation boosts the ethanol production rate and specificity by Clostridium ljungdahlii during gas fermentation with pure carbon monoxide

Recycling waste gases from industry is vital for the transition toward a circular economy. The model microbe Clostridium ljungdahlii reduces carbon from syngas and primarily produces acetate and ethanol. Here, a gas fermentation experiment is presented in chemostats with C. ljungdahlii and pure carbon monoxide (CO) as feedstock while entirely omitting yeast extract. A maximum ethanol production rate of 0.07 +/- 0.01 g L-1 h-1 and a maximum average ethanol/acetate ratio of 1.41 +/- 0.39 was observed under steady-state conditions. This confirmed that CO as the sole feedstock pushes the metabolism toward more reduced fermentation products. This effect was even more pronounced when 15 mM sodium acetate was added to the feed medium. An ethanol production rate of 0.23 +/- 0.01 g L-1 h-1 was achieved, representing an increase of more than 240%. This increase was accompanied by an increase in cell density and selectivity toward ethanol, with a maximum average ethanol/acetate ratio of 92.96 +/- 28.39. Ox-ygen contaminations voided this effect, although the cultures were still able to maintain a stable biomass concen-tration and ethanol production rate. These findings highlight the potential of CO-fermentation with acetate augmentation and the importance of preventing oxygen contaminations.

Automated summary

Recycling waste gases from industry is crucial for transitioning towards a circular economy. A gas fermentation experiment using C. ljungdahlii and pure CO as feedstock was conducted, omitting yeast extract. The results showed that using only CO as the feedstock increased ethanol production rate and selectivity when supplemented with sodium acetate. Oxygen contaminations, however, reversed this effect. These findings emphasize the potential of CO-fermentation with acetate augmentation and the importance of preventing oxygen contaminations.