Unravelling the molecular fingerprint of plastic accumulation on blue carbon sediment

A disproportionately large accumulation of plastic waste has turned blue carbon ecosystems (mangrove forests, tidal marshes, and seagrass meadows) into hotspots of plastic pollution. Although our understanding of the effects of plastics on coastal ecosystems has advanced considerably, the underlying effects on blue carbon sediment biogeochemistry are yet to be assessed. Here, we examined the potential organic matter turnover and degradability of conventional plastics, i.e. polyethylene terephthalate (PET) and polypropylene (PP) and biodegradable plastic, i.e. polylactic acid (PLA) in a controlled microcosm experiment containing mangrove sediments. We measured dissolved organic carbon (DOC) leaching, changes in sediment dissolved organic matter (DOM), and the simultaneous greenhouse gas emissions caused by plastics. After 90 days of incubation, low molecular weight PP was visibly degraded on the surface, but high molecular weight PET and PLA were not. The degradation of PP and granular structure of PET led to higher DOC content and number of DOM molecules in sediment compared to that of PLA. Initially, PET contributed higher bio-labile compounds to the sediment, but after 90 days of incubation biologically recalcitrant compounds were more prominent. Despite contributing less DOC and DOM, sediment with PLA emitted higher CO₂, suggesting that PLA may accelerate the degradation of native organic matter, whereas PP reduced the cumulative emission. In short, both conventional and biodegradable plastics affect sediment biogeochemistry by altering DOC content, DOM composition and turnover. This study provides new insights into the signature of different plastics in blue carbon sediment dynamics.