Micro(nano)plastics reshape constructed wetlands: Linking biofilm succession’s role to key biogenic substance transformation

Constructed wetlands (CWs) are increasingly recognized as terminal sinks for micro- and nanoplastics (MNPs), yet how chronic MNPs accumulation reshapes biofilm-mediated biogenic substance transformation remains poorly understood. Here, using a 300-day CW experiment integrating process analysis, biofilm microstructure characterization, and metagenomics, we demonstrate that plastic particle size acts as a decisive ecological switch governing biofilm succession and multi-element cycling. Long-term microplastics (MPs) exposure unexpectedly enhanced denitrification and sulfate reduction, whereas nanoplastics (NPs) persistently suppressed carbon, nitrogen, phosphorus, and sulfur transformations. Mechanistic analyses reveal that these divergent outcomes arise not from direct metabolic toxicity but from size-dependent reorganization of biofilm architecture, regulatory gene networks, and microbial cooperation. MPs promoted extracellular polymeric substance synthesis, reinforced anaerobic redox stratification, and strengthened electron-transfer-driven microbial clustering, while NPs disrupted biofilm integrity, downregulated succession-related genes, and fragmented functional interactions. This study challenges the prevailing assumption that MNPs accumulation uniformly degrades treatment performance and establishes a mechanistic framework linking particle size, biofilm succession, and ecosystem functioning. Our findings provide new insights into the long-term ecological effects of emerging particulate pollutants and offer guidance for designing resilient biofilm-based treatment systems under increasing plastic pressure.