Solid phases as important electron acceptors in freshwater organic sediments

Abstract

Ferric iron in particulate iron (oxyhydr–) oxides and quinone moieties in dissolved organic matter (DOM) are well-established terminal electron acceptors (TEA) in heterotrophic anaerobic microbial respiration. The importance of particulate organic matter (POM) as TEA is, however, much less studied and understood despite the fact that POM is more abundant than DOM in many soils and sediments. Here, we studied the microbial reduction of POM and Fe(III)-containing phases in freshwater sediments. We present an electrochemical approach that can be used to assess the combined contributions of POM and Fe(III) to the TEA pools of soils and sediments. Following oxidation and drying of sediments from two carbonate–buffered freshwater lakes in air, wetting re–initiated anaerobic microbial respiration in the sediment samples as evidenced from electron transfer to solid–phase electron acceptors over three weeks of anoxic incubations. The microbial reduction of POM and mineral-associated ferric iron was directly quantified by mediated electrochemical analysis. We estimate that the POM from the analyzed sediments provided a capacity to accept or donate electrons of about 650 µmol e− (g organic carbon)−1. Our work substantiates earlier studies that suggested that the reduction of redox–active moieties in POM is an important contributor to anaerobic microbial respiration and might be responsible for the competitive suppression of methanogenesis in organic matter rich wetland soils. Our results further indicate that microbial reduction of POM must be accounted for to close respiration balances in (temporary) anoxic freshwater systems and peatlands.