Formate-induced CO tolerance and innovative methanogenesis inhibition in co-fermentation of syngas and plant biomass for carboxylate production
Production of monocarboxylates using microbial communities is highly dependent on local and degradable biomass feedstocks. Syngas or 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, a series of batch experiments was carried out with different syngas compositions and corn silage (pH 6.0, 32{degrees}C). Co-fermentation of syngas with corn silage increased the overall carboxylate yield per gram of volatile solids (VS) by up to 44% (0.36 {+/-} 0.07 g gVS-1; in comparison to 0.23 {+/-} 0.04 g gVS-1 with a N2/CO2 headspace), despite slowing down biomass degradation. Ethylene and CO exerted a synergistic effect in preventing methanogenesis, leading to net carbon fixation. Less than 12% of the 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, which accounted for 86% (electron equivalents) of all products at 49 kPa CO, 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. Intriguingly, the effect of CO on n-butyrate and n-caproate production was reversed when formate was present in the broth. The concept of co-fermenting syngas and plant biomass shows promise in two aspects: by making anaerobic fermentation a carbon-fixing process and by increasing the production of propionate and acetate. Testing the concept in a continuous process could improve selectivity to n-butyrate and n-caproate by enriching chain-elongating bacteria adapted to CO and complex biomass.