CO₂ uptake in ethanol-driven chain elongation system: Microbial metabolic mechanisms

CO₂ as a byproduct of organic waste/wastewater fermentation has an important impact on the carboxylate chain elongation. In this study, a semi-continuous flow reactor was used to investigate the effects of CO₂ loading rates (Low = 0.5 L_CO2·L^-1·d^-1, Medium = 1.0 L_CO2·L^-1·d^-1, High = 2.0 L_CO2·L^-1·d^-1) on chain elongation system Ethanol and acetate were utilized as the electron donor and electron acceptor, respectively. The results demonstrate that low loading rate of CO₂ has a positive effect on chain elongation. The maximum production of caproate and CH₄ were observed at a low CO₂ loading rate. Caproate production reached 1.88 g COD·L^-1·d^-1 with a selectivity of 62.55 %, while CH₄ production reached 129.7 ml/d, representing 47.4 % of the total. Metagenomic analysis showed that low loading rate of CO₂ favored the enrichment of Clostridium kluyveri, with its abundance being 3.8 times higher than at of high CO₂ loading rate. Metatranscriptomic analysis revealed that high CO₂ loading rate induced oxidative stress in microorganisms, as evidenced by increased expression of heat shock proteins and superoxide dismutase genes. Further investigation suggested that genes associated with the reverse β-oxidation pathway, CO₂ uptake pathway and hydrogenotrophic methanogenesis pathway were reduced at high CO₂ loading rate. These findings provide insight into the underlying mechanisms of how CO₂ affects chain elongation, and it could be a crucial reason for the poor performance of chain elongation systems with high endogenous CO₂ production.