Formate-induced CO tolerance and methanogenesis inhibition in fermentation of syngas and plant biomass for carboxylate production

Abstract

Background 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 sourced from biomass gasification, excess renewable electricity, industrial off-gases, and carbon capture plants and co-fed to a fermenter to alleviate dependence on local biomass. 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 °C). Results Co-fermentation of syngas with corn silage increased the overall carboxylate yield per gram of volatile solids (VS) by up to 29% (0.47 ± 0.07 g gVS−1; in comparison to 0.37 ± 0.02 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 85% (electron equivalents) of all products at 49 kPa CO, by favoring lactic acid bacteria and actinobacteria over n-butyrate and n-caproate producers. Inhibition of n-butyrate and n-caproate production by CO happened even when an inoculum preacclimatized 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. Conclusions The concept of co-fermenting syngas and plant biomass shows promise in three aspects: by making anaerobic fermentation a carbon-fixing process, by increasing the yields of short-chain carboxylates (propionate and acetate), and by minimizing electron losses to CH4. Moreover, a model was proposed for how formate can alleviate CO inhibition in certain acidogenic bacteria. Testing the fermentation of syngas and plant biomass in a continuous process could potentially improve selectivity to n-butyrate and n-caproate by enriching chain-elongating bacteria adapted to CO and complex biomass.

Carbonic anhydrase assisted acidogenic fermentation of forest residues for low carbon hydrogen and volatile fatty acid production: enhancedin situCO2reduction and microbiological analysis

“…Furthermore, the application of pressurization conditions promotes the utilization of H 2 and CO 2 by homoacetogens. 74 Similar to the pattern of lactic acid consumption, ethanol was found to be consumed at a rate ranging from 0.001 to 0.031 g COD h −1 until it was completely consumed. The production and consumption of ethanol was slightly higher with the reactors operated at pH 6 (0.023-0.031 g COD h −1 ) compared to pH 7 (0.022-0.023 g COD h −1 ) and pH 8 (0.013-0.014 g COD h −1 ).…”

Medium-chain carboxylates production from plant waste: kinetic study and effect of an enriched microbiome

“…Cell pellets of the fermentation broths from E1 and E2 were collected at different timepoints and washed with 12 mM phosphate-buffered saline pH of 7.4 [ 27 ]. The cells were then stored at -20 °C until further processing.…”

Upgrading dilute ethanol to odd-chain carboxylic acids by a synthetic co-culture of Anaerotignum neopropionicum and Clostridium kluyveri

“…In recent years, several studies have demonstrated the feasibility of coupling syngas fermentation to chain elongation for the production of MCCAs and higher alcohols [ 25 ]. One adopted strategy is the one-pot conversion of CO/syngas by open mixed cultures traditionally used in anaerobic digestion, which can be enriched for chain-elongating microorganisms [ 26 – 33 ]. Despite their robustness and suitability for waste-fed processes, these cultures require rather long acclimation and fermentation times when applied to syngas fermentation, and the abundance of competing pathways (e.g., methanogenesis) compromises product selectivity.…”