Size and Structure-Dependent Molecular Fingerprint Transformation of Microplastic-Derived Dissolved Organic Matter in Sunlit Seawater: Implication for Marine Carbon Cycles

Microplastics (MPs) widely pollute marine environments, where the release of MPs-derived dissolved organic matter (MPs-DOM) induced by UV irradiation has been widely documented. However, the effect of MPs' inherent properties on photochemical transformation of MPs-DOM and its implication for oceanic carbon cycles remain poorly understood. Herein, we conducted 180 day artificial weathering experiments under sunlit seawater using polyethylene (PE) and polypropylene (PP) with different sizes as models, investigating the temporally dynamic features of MPs-DOM. Results showed that PP can release more MPs-DOM than PE, and concentrations of MPs-DOM derived from small-sized MPs (∼250 μm) were 2-6 times higher than those of large-sized MPs (∼5 mm). Spectroscopy and ultrahigh-resolution mass spectrum further revealed that protein-like substances can be persistently produced from MPs with lower photochemical activity (i.e., PE and large-size PP), while DOM derived from MPs with higher photochemical activity (i.e., small-size PP) could be gradually transformed from biolabile components to biorecalcitrant molecules. Furthermore, the persistently accumulated molecules were matched and projected onto an aquatic DOM database, and their relative intensity exhibited a gradually increasing trend across the river-to-ocean continuum at the molecular level. This work reveals the structure-reactivity relationships for MPs-DOM transformation and highlights MPs' potential impact in marine organic carbon cycles.