Aging dynamics and subsequent dissolved organic matter evolution of microplastics regulated by UV–salinity conditions

The aging of marine microplastics (MPs) and their release of plastic-derived dissolved organic matter (MP-DOM) have raised growing concern, yet comparative assessments of conventional polyolefins versus biodegradable polyesters under synergistic UV-salinity stress remain limited. Here, we systematically compared the surface alteration and DOM release of polyethylene (PE), polypropylene (PP), and polylactic acid (PLA) across a salinity gradient (0, 15, and 30 PSU; Practical Salinity Unit) under UV-exposed and dark conditions over 60 days. UV irradiation markedly accelerated aging in all three MP types, as evidenced by progressive enrichment of oxygen-containing functional groups. For PE and PP, increased salinity further intensified photo-oxidative degradation, and released DOM exhibited optical shifts from protein-like toward humic-like fluorescence signatures characterized by elevated SUVA 254 and HIX values. In contrast, PLA degradation was consistent with ester bond hydrolysis, producing 2–6 times greater DOM yields than PE and PP, with released DOM dominated by protein-like fluorescence components. Partial least squares structural equation modeling (PLS-SEM) indicated that UV exposure exerted stronger effects than salinity on the surface oxidation of PE and PP ( β = 0.39–0.41 vs. 0.20–0.31), while salinity showed no significant effect on PLA aging. These findings reveal contrasting pathways by which conventional and biodegradable MPs contribute to marine dissolved organic carbon pools, providing a basis for assessing their differential environmental behaviors under variable salinity regimes.