Biodegradable versus persistent nanoplastics reshape nitrogen metabolism and biofilm architecture in denitrifying...

The presence of nanoplastics (NPs) in biological wastewater treatment systems is an emerging concern. Nevertheless, their differential influence on critical biofilm-mediated processes has yet to be fully elucidated. In this study, denitrifying biofilters were exposed to biodegradable polylactic acid nanoplastics (PLA-NPs) and non-biodegradable polystyrene nanoplastics (PS-NPs) to simulate both typical and cumulative high-exposure scenarios. Results showed that long-term NP stress significantly reduced the denitrification performance, with a maximum inhibition of 35% in total nitrogen (TN) removal. Mechanistically, PLA and PS induced distinct biofilm remodeling strategies. PLA exposure enhanced nitrate assimilation pathways, promoting nitrogen sequestration into microbial biomass. In contrast, PS-NPs elicited concentration-dependent stress responses. Low PS exposure was associated with reduced extracellular polymeric substances (EPS) and enhanced carbohydrate degradation potential, whereas high PS concentrations were linked to altered EPS composition, decreased microbial diversity, and directional succession toward stress-tolerant genera. Metagenomic analysis revealed shifts in central carbon metabolic strategies, including enhanced gluconeogenesis and EPS precursor synthesis under NP exposure. Differences in substrate bioavailability between PLA and PS treatments further contributed to distinct carbon utilization patterns within the biofilms. Overall, this study demonstrates that NP biodegradability governs biofilm functional stability, nitrogen transformation, and denitrification performance, providing mechanistic insight into NP-biofilm interactions in engineered systems.