The utilization of extracellular electron transfer by microorganism is highly engaging for remediation of toxic pollutants under "energy-starved" conditions. Humin, an organo-mineral complex of soil, has been instrumental as an external electron mediator for suitable electron donors in the remediative works of reductive dehalogenation, denitrification, and so forth. Here, we report, for the first time, that humin assists microbial acetogenesis as the extracellular electron donor using the electron acceptor CO 2 . Humin was obtained from Kamajima paddy soil, Japan. The anaerobic acetogenic consortium in mineral medium containing CO 2 / HCO 3 - as the inorganic carbon source used suspended humin as the energy source under mesophilic dark conditions. Retardation of acetogenesis under the CO 2 -deficient conditions demonstrated that humin did not function as the organic carbon source but as electron donor in the CO 2 -reducing acetogenesis. The consortium with humin also achieved anaerobic dechlorination with limited methanogenic activity. Total electron-donating capacity of humin was estimated at about 87 µeeq/g-humin. The metagenomic sequencing of 16S rRNA genes showed the predominance of Firmicutes (71.8 ± 2.5%) in the consortium, and Lachnospiraceae and Ruminococcaceae were considered as the CO 2 -reducing acetogens in the consortium. Thus, microbial fixation of CO 2 using humin introduces new insight to the holistic approach for sustainable treatment of contaminants in environment.
Keywords: CO2 reduction; autotrophic; dechlorination; humin; methanogenesis; reductive acetogenesis.