Microplastic contamination enhances benthic dissolved organic carbon fluxes and molecular transformations in...

Microplastics (MPs) are pervasive in aquatic environments and ultimately accumulate in sediments, yet their role in regulating benthic carbon cycling remains poorly constrained, particularly with respect to dissolved organic matter (DOM) exchange across the sediment-water interface. In this study, we conducted 28-day laboratory sediment-column incubations under controlled oxic and hypoxic conditions to quantify MP-associated changes in DOM quantity and composition in porewater and overlying water. Polyethylene (PE; petroleum-based) and polylactic acid (PLA; bio-based) MPs were added at 5% (w/w) as a high-end (hotspot) loading scenario. MP contamination increased porewater dissolved organic carbon (DOC) concentrations by up to 27-fold relative to controls, resulting in 1.6-18-fold higher cumulative benthic DOC fluxes across the sediment-water interface. Optical and size-fraction analyses revealed polymer-specific shifts in exported DOM quality. PLA treatments were characterized by strong enrichment of low-molecular-weight neutral fractions, consistent with MP-associated DOM inputs, whereas PE treatments were associated with enhanced mobilization and redistribution of sediment-derived DOM. Molecular-level analyses further showed that MP-associated formulas accounted for up to one-third of all detected molecular features, despite MPs comprising only ∼5% of sediment mass. This disproportionate molecular representation suggests that MPs may influence DOM composition beyond their direct mass contribution. Oxygen availability further modulated these molecular patterns, with oxic conditions characterized by higher proportions of formulas classified as metabolically active within the operational reactivity-activity framework, whereas hypoxic conditions favored the accumulation of relatively inactive and more persistent molecular fractions. Overall, these results suggest that sediment-associated MPs may alter both the quantity and molecular characteristics of DOM exchange across the sediment-water interface, highlighting their potential role in modifying benthic carbon exchange processes in MP-impacted aquatic systems.