Unraveling water depth dependent microplastic aging driven by functional microbial community interaction in a real...

Microplastic (MPs) biofilms are dynamic microhabitats that regulate substance transformation processes. However, the influence of natural urban water depth gradients on the biofilm functions and subsequent aging of MPs remains poorly understood. Herein, we characterized the aging process of MPs in different depths of a real urban river, and the biofilm driven aging mechanism was illustrated. Surface characterization of the MPs showed an increase in oxygen-containing functional groups (CO, C-O) and O/C in polylactic acid (PLA) during aging, which indicated oxidation and hydrolysis reactions, especially at 2.0 m deep water depth. In polyvinyl chloride (PVC) MPs, carbonyl index (CI) was 2 times higher at 2.0 m as compared to 0.1 m water depth and lower chlorine content was found, indicating higher oxidative degradation and dechlorination processes in deeper water. Moreover, biofilms may regulate the synergism between oxygenase and hydrolases in PLA biofilms and oxygenase and dehalogenase in PVC biofilms. Microorganisms with both denitrification and MPs degradation functions, such as Acidovorax, Comamonas, Dechloromonas, were enriched in MPs biofilm. In addition, a positive correlation was found between MPs degradation genes (TGL2, katG, ncd2) and denitrification genes (napA, nirS, norB) in PLA biofilms at deeper water depth, suggesting a potential effect of denitrification functions on MPs aging (45 d incubation). This research challenges the conventional thoughts of higher MPs aging in shallow water, emphasizing the significant role of moderate depth gradients water in regulating the ecological function of microplastic biofilm, which is essential for evaluating the fate of MPs in real urban rivers.