Microplastic-derived dissolved organic matter modulates microbial nitrate and arsenate reduction: Extracellular electron transfer and intracellular metabolic responses

Photoaged microplastics release dissolved organic matter (MPs-DOM) with redox activity that may influence coupled contaminant transformations under anoxic conditions. Here, we investigated DOM derived from polystyrene (PS) and polylactic acid (PLA) after accelerated UV/HO oxidation and examined its effects on concurrent nitrate and As(V) reduction by Shewanella sp. CN32. FT-ICR MS, EEM-PARAFAC, and redox measurements revealed clear polymer-dependent differences. PS-DOM contained higher proportions of formulas in polycyclic-aromatic and polyphenolic regions and exhibited a substantially higher electron-accepting capacity, whereas PLA-DOM contained more highly unsaturated formulas and showed a higher electron-donating capacity. Both DOM types accelerated nitrate reduction through the DNRA pathway, increasing reduction rates by 1.8-3.0-fold, and enhanced As(III) production, although the magnitude and kinetics varied with DOM type and loading. Whole-cell electrochemical measurements further showed more pronounced responses in PS-DOM-amended systems. Transcriptomic and metabolomic analyses revealed distinct cellular responses. PS-DOM was associated with stronger regulation of respiratory electron-transfer processes, the Nap/Nrf system, and arrA, whereas PLA-DOM induced more pronounced changes in central carbon metabolism, NAD-related pathways, and the Ars system. Together, these results indicate that polymer-dependent molecular composition and redox properties of MPs-DOM shape distinct kinetic, respiratory, and metabolic responses during coupled nitrate and arsenate reduction in anoxic environments.