Co-fermentation of syngas and plant biomass

Production of carboxylates via anaerobic fermentation is highly dependent on local and degradable biomass feedstocks. Syngas and different mixtures of H2, CO, and CO2 can be co-fed to a fermenter to alleviate this dependence. To understand the effects of adding these gases during anaerobic fermentation of plant biomass, we realized a series of batch experiments with different syngas compositions using corn silage (pH 6.0, 32°C). Co-fermentation of syngas with corn silage could increase the overall carboxylate yield despite slowing down biomass degradation. Ethylene, an impurity of syngas, synergized with CO to inhibit methanogens and achieve net carbon fixation. Less than 10% of electrons were misrouted to CH4 when either 15 kPa CO or 5 kPa CO + 1.5 kPa ethylene was used. CO increased the selectivity to acetate and propionate by favoring lactic acid bacteria and actinobacteria over n-butyrate and n-caproate producers. This happened even when an inoculum pre-acclimatized to syngas and lactate was used. Curiously, the effect of CO on n-butyrate and n-caproate production was reversed when formate was present in the broth. So far, the concept of co-fermenting syngas and plant biomass shows promise in two fronts: by making anaerobic fermentation more sustainable and by increasing production of propionate and acetate.