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 .