Molecular fingerprints of dissolved organic matter leached from microplastics over prolonged photochemical aging: Implications for aquatic carbon cycling

Microplastics are well-known emerging pollutants in aquatic environments, increasingly shown to release significant amounts of non-natural dissolved organic matter (DOM) into water systems. However, the molecular composition of DOM leached from different types of microplastics after long-term photochemical aging, and their impact on the aquatic carbon cycle, remain poorly understood. The potential presence and contribution of microplastic-derived DOM to the river-to-ocean continuum on a large scale have yet to be well-established. Using ultra-high-resolution Fourier-transform ion cyclotron resonance mass spectrometry, this study identified 155, 152, and 465 DOM molecules released from microplastics of polypropylene, polyethylene, and polystyrene, respectively, showing high potential for accumulation after 180 days of ultraviolet exposure. These molecules were subsequently detected in an extensive DOM dataset comprising 947 natural water samples, with an average detection frequency of 64.6 %. Polystyrene-derived DOM molecules exhibit a strong resemblance to natural refractory DOM at the molecular level, suggesting its potential contribution to the long-term carbon pool. In contrast, polypropylene- and polyethylene-derived DOM molecules indicate more biologically labile structures that favor short-term carbon cycling. These findings emphasize the varying impacts of different microplastic types on carbon cycling, with polystyrene-derived DOM potentially contributing to refractory carbon pools and broader climate implications.