Humic substances as fully regenerable electron acceptors in recurrently anoxic environments

Humic substances form through the degradation of microbial and plant precursors, and make up a significant fraction of natural organic matter in terrestrial and aquatic environments(1). Humic substances are redox-active(2-4) and can act as terminal electron acceptors in anaerobic microbial respiration(4). Reduced humic substances may become re-oxidized during aeration of temporarily anoxic systems, such as wetlands, sediments and many soils. If the transfer of electrons from anaerobic respiration through humic substances to oxygen is sustained over many redox cycles, it may competitively suppress electron transfer to carbon dioxide, and thereby lower the formation of methane in temporarily anoxic systems(5-8). Here, we monitor changes in the redox states of four chemically distinct dissolved humic substances over successive cycles of reduction by the bacterium Shewanella oneidensis MR-1 and oxidation by oxygen, in a series of laboratory experiments. We show that electron transfer to and from these substances is fully reversible and sustainable over successive redox cycles. We suggest that redox cycling of humic substances may largely suppress methane production in temporarily anoxic systems.