Aging behaviors intensify the impacts of microplastics on nitrate bioreduction-driven nitrogen cycling in freshwater sediments

Microplastics (MPs) inevitably undergo aging processes in natural environments; however, how aging behaviors influence the interactions between MPs exposures and nitrate bioreduction in freshwater sediments remains poorly understood. Here, we explored the distinct impacts of virgin and aged MPs (polystyrene (PS) and polylactic acid (PLA)) on nitrate bioreduction processes in lake sediments through a long-term microcosm experiment utilizing the N isotope tracing technique and molecular analysis. Compared to virgin MPs, aged PLA significantly increased the rates of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) (p < 0.05), facilitating sediment nitrogen loss, while aged PS only significantly improved the rates of DNRA by 272-297 % and contributed to nitrogen retention in sediments. Metagenomic sequencing demonstrated that a more significant enrichment of functional genes responsible for nitrate bioreduction pathways occurred with aged MPs exposures than with virgin MPs. By combining analyses of MPs aging traits and the key drivers of nitrate bioreduction, we revealed that aging behaviors directly regulated sediment nutrient status (e.g., DOC/NO ratio) and microbiological properties (from genes to bacteria), thereby further determining the activity of nitrate bioreduction. This work provides new insights into the impacts of aged MPs on sediment nitrate reduction and highlights the role of MPs aging in future assessments of long-term MPs pollution in freshwater ecosystems.