The intermolecular differences in dissolved organic matter derived from microplastics during photo-aging and photo-bio-aging processes

Microplastics (MPs) at the ocean surface undergo synergistic aging through irradiation and biological processes, leading to substantial release of dissolved organic matter (DOM). While extensive characterizations have been conducted on MPs-DOM derived from single-photo-aging processes, the molecular properties derived from combined photo-bio-aging processes and their intermolecular differences remained poorly understood, despite the latter's heightened environmental relevance. This study systematically investigated the molecular characteristics and transformation pathways of MPs-DOM derived from aliphatic (polypropylene) and aromatic (polystyrene) polymers under single ultraviolet irradiation and continuous photo and enzymatic reactions. High-resolution mass spectrometry analysis revealed that MPs-DOM retained structural signatures of parent polymers, yet formed highly oxidized unsaturated compounds. Most MPs-DOM from aliphatic and aromatic polymers consisted of unsaturated oxy-functional aliphatic and polycyclic polyphenol compounds, respectively. The molecular composition and distribution of MPs-DOM were less influenced by the aging process, whereas the oxidation state was altered through distinct transformations. Ultraviolet irradiation initiated oxygen addition, producing highly oxidized intermediates, while enzymatic biodegradation via peroxidase facilitated decarboxylation and dealkylation, driving molecular simplification and oxidation-state reduction. Aromatic MPs-DOM exhibited enhanced oxidative reactivity, whereas aliphatic MPs-DOM accumulated persistent, low-oxidation-state hydrocarbons. These findings highlight the critical need for future research on interfacial photo-reactivity and biodegradation kinetics of MPs-DOM to better understand their roles in aquatic biogeochemical cycles and ecological impacts.