CO 2 uptake in ethanol-driven chain elongation system: Microbial metabolic mechanisms.

CO ₂ 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 CO ₂ 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 CO ₂ has a positive effect on chain elongation. The maximum production of caproate and CH ₄ were observed at a low CO ₂ loading rate. Caproate production reached 1.88 g COD·L ^-1 ·d ^-1 with a selectivity of 62.55 %, while CH ₄ production reached 129.7 ml/d, representing 47.4 % of the total. Metagenomic analysis showed that low loading rate of CO ₂ favored the enrichment of Clostridium kluyveri, with its abundance being 3.8 times higher than at of high CO ₂ loading rate. Metatranscriptomic analysis revealed that high CO ₂ 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, CO ₂ uptake pathway and hydrogenotrophic methanogenesis pathway were reduced at high CO ₂ loading rate. These findings provide insight into the underlying mechanisms of how CO ₂ affects chain elongation, and it could be a crucial reason for the poor performance of chain elongation systems with high endogenous CO ₂ production.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Copyright © 2023 Elsevier Ltd. All rights reserved.)