Dissolved organic matter (DOM) is a ubiquitous and highly reactive component in aquatic environments, and its oxidative degradation has long been a central topic in organic geochemistry. While microbial Fenton-like reactions (Biogenic Fenton) have gained attention for their role in degrading synthetic organic pollutants, their potential to degrade natural DOM under dark conditions remains poorly understood. Recent studies suggest that heterotrophic microorganisms can mediate extracellular redox reactions via cell-surface enzymes (e.g., NAD⁺/NADH), producing superoxide (O₂^●⁻) as a precursor to hydrogen peroxide (H₂O₂), a key reactant in Fenton chemistry. However, the fate and reactivity of O₂^●⁻ in natural aquatic systems have not been fully elucidated. In this study, we demonstrate that in the presence of DOM–Fe(III) complexes, O₂^●⁻ can induce the formation of hydroxyl radicals (^●OH), with yields 2 to 24 times higher than those predicted by classical Fenton stoichiometry. The generated ^●OH significantly alters the optical properties of DOM, suggesting structural transformations. Furthermore, dark incubation experiments reveal that Fe(III) complexed with DOM enhances microbial utilization of DOM and reshapes microbial community composition. These findings indicate that microbial activity, in concert with Fe(III)-DOM complexation, can drive extracellular biogenic Fenton-like reactions and facilitate DOM degradation under dark conditions. This biogenic-Fenton pathway may represent an important but previously overlooked mechanism in the microbial processing of DOM and the broader iron-mediated aquatic carbon cycle.

Iron, dissolved organic matter, microbe, degradation, Biogenic Fenton