Effect of Humin and Chemical Factors on CO(2)-Fixing Acetogenesis and Methanogenesis

Acetogenesis and methanogenesis have attracted attention as CO(2)-fixing reactions. Humin, a humic substance insoluble at any pH, has been found to assist CO(2)-fixing acetogenesis as the sole electron donor. Here, using two CO(2)-fixing consortia with acetogenic and methanogenic activities, the effect of various parameters on these activities was examined. One consortium utilized humin and hydrogen (H(2)) 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 MgSO(4) at concentration above 0.02 g/L and by NaCl at concentrations higher than 6 g/L. Another consortium did not utilize humin but H(2) 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 H(2) 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 MgSO(4). Comparative sequence analysis of partial 16S rRNA genes suggested that minor groups were the humin-utilizing acetogens in the consortium dominated by Clostridia, while Methanobacterium was the methanogen utilizing humin with H(2).