biostudies-literatureHa BNthe Carbon Recycle FundIwatani Naoji Foundationthe Institute of Fermentation, OsakaJapan Society for the Promotion of Science2546https://www.ebi.ac.uk/biostudies/studies/S-EPMC8909181biostudies-literatureUnknown19(5)Acetogenesis and methanogenesis have attracted attention as CO₂-fixing reactions. Humin, a humic substance insoluble at any pH, has been found to assist CO₂-fixing acetogenesis as the sole electron donor. Here, using two CO₂-fixing consortia with acetogenic and methanogenic activities, the effect of various parameters on these activities was examined. One consortium utilized humin and hydrogen (H₂) as electron donors for acetogenesis, either separately or simultaneously, but with a preference for the electron use from humin. The acetogenic activity was accelerated 14 times by FeS at 0.2 g/L as the optimal concentration, while being inhibited by MgSO₄ at concentration above 0.02 g/L and by NaCl at concentrations higher than 6 g/L. Another consortium did not utilize humin but H₂ as electron donor, suggesting that humin was not a universal electron donor for acetogenesis. For methanogenesis, both consortia did not utilize extracellular electrons from humin unless H₂ was present. The methanogenesis was promoted by FeS at 0.2 g/L or higher concentrations, especially without humin, and with NaCl at 2 g/L or higher concentrations regardless of the presence of humin, while no significant effect was observed with MgSO₄. Comparative sequence analysis of partial 16S rRNA genes suggested that minor groups were the humin-utilizing acetogens in the consortium dominated by <i>Clostridia</i>, while <i>Methanobacterium</i> was the methanogen utilizing humin with H₂.International journal of environmental research and public healthEffect of Humin and Chemical Factors on CO₂-Fixing Acetogenesis and Methanogenesis.PMC890918120H04363, 20K15431, 21K198622020 Research Project2020 research projectL-2019-003Ha BNKasai TPham DMKatayama AAwata TfalseEffect of Humin and Chemical Factors on CO₂-Fixing Acetogenesis and Methanogenesis.Acetogenesis and methanogenesis have attracted attention as CO₂-fixing reactions. Humin, a humic substance insoluble at any pH, has been found to assist CO₂-fixing acetogenesis as the sole electron donor. Here, using two CO₂-fixing consortia with acetogenic and methanogenic activities, the effect of various parameters on these activities was examined. One consortium utilized humin and hydrogen (H₂) as electron donors for acetogenesis, either separately or simultaneously, but with a preference for the electron use from humin. The acetogenic activity was accelerated 14 times by FeS at 0.2 g/L as the optimal concentration, while being inhibited by MgSO₄ at concentration above 0.02 g/L and by NaCl at concentrations higher than 6 g/L. Another consortium did not utilize humin but H₂ as electron donor, suggesting that humin was not a universal electron donor for acetogenesis. For methanogenesis, both consortia did not utilize extracellular electrons from humin unless H₂ was present. The methanogenesis was promoted by FeS at 0.2 g/L or higher concentrations, especially without humin, and with NaCl at 2 g/L or higher concentrations regardless of the presence of humin, while no significant effect was observed with MgSO₄. Comparative sequence analysis of partial 16S rRNA genes suggested that minor groups were the humin-utilizing acetogens in the consortium dominated by <i>Clostridia</i>, while <i>Methanobacterium</i> was the methanogen utilizing humin with H₂.2022-01-01T00:00:00Z2022 Feb2025-08-23T03:08:55.352Z2025-04-04T13:21:55.619ZS-EPMC89091813527023910.3390/ijerph19052546