Humin is a fraction of humic substances, not soluble in any pH and functions as extracellular electron mediator. Various microorganisms receive extracellular electrons from humin for their respiring reactions, such as anaerobic dehalogenation, iron reduction, and nitrate reduction. The redox potentials of these reactions are in more oxidative than that of humin, estimated in the range from +0.04 V to -0.30V by cyclic voltammetry, suggesting thermodynamically feasible electron flow. In this study, we have examined the effect of humin on the CO2-reducing acetogenesis, whose redox potential is -0.29V (vs. standard hydrogen electrode, at pH=7), being much lower than other microbial reactions. A humin-dependent pentachlorophenol-dechlorinating microbial consortium was used as microbial source, which had the activity of CO2-reducing acetogenesis. The consortium was cultured with humin in an anaerobic medium containing bicarbonate as CO2 source. In one condition, H2 was externally added as an electron donor. In another condition, H2 was not added where humin was expected to function as sole electron donor in the culture. In the presence of H2, acetate was produced, and acetate was utilized for humin reduction then the electron was transferred to pentachlorophenol-dechlorinating bacteria. Acetate was used as secondary electron donor to reduce humin. While, in the absence of H2, CO2 was microbially reduced to acetate. These results suggested at the first time the alternate functionality of humin as extracellular electron mediator in the CO2-reducing acetogenesis, depending on the availability of other electron donors. It should be also noted that humin played active role for anaerobic CO2-fixation. The spectroscopic analyses suggested the multiple redox reaction centers in humin such as partial structures containing quinone and peptidoglycan. Further study should be conducted to elucidate the redox reaction centers in humin reacting with the CO2-reducing acetogenic microorganisms.

Alternate functionality of humin, a solid-phase humic substance, as extracellular electron mediator in carbon dioxide-reducing acetogenesis

ACS Spring Meeting 2021

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