Long-term natural photodegradation of plastic films enhances the molecular complexity of plastic-derived dissolved organic matter.

Plastic debris contributes substantially to the marine carbon pool by continuously releasing dissolved organic matter (DOM) through photodegradation. However, the long-term influence of intrinsic plastic properties on the composition of plastic-derived DOM (PDOM) remains poorly understood. Here, we conducted a three-year field exposure experiment using five representative plastics (PBAT, PE, PO, PP, and PS) under natural sunlight to investigate the molecular characteristics of PDOM. Results showed that PBAT released the highest PDOM concentrations (1075.33 ± 14.67 mg L-1), over 14 times higher than those of the other plastics (< 73.86 ± 1.52 mg L-1). The release of PDOM was governed by both polymer composition and irradiation duration. Short-term dissolution of plastic fragments generated highly bioavailable compounds, whereas long-term photodegradation involved extensive oxidation and molecular recombination, leading to marked changes in chemical properties and structural complexity, and ultimately increasing PDOM heterogeneity. These findings highlight the decisive role of polymer type in regulating the quantity, composition, and photo-reactivity of PDOM, providing new insights into DOM dynamics in marine environments.