Nanoplastics reshape nitrogen cycling in submerged macrophyte systems: A metagenomic perspective

Nanoplastics (NPs) pose a potential risk to aquatic ecosystems. Submerged macrophytes are critical for nitrogen removal, but how nitrogen cycling responds to NP-induced stress remains unclear. This study used Myriophyllum aquaticum to evaluate nitrogen cycling in submerged macrophyte-sediment systems exposed to 100 nm polystyrene (PS) NPs at 10, 100, and 1000 μg/L, integrating stable isotope tracing and metagenomic profiling to explore microbial community and nitrogen-cycling gene responses across rhizosphere and non-rhizosphere compartments. Low PS-NP exposure (10 μg/L) slightly increased the NH₄⁺-N removal efficiency to 81.5%, whereas medium and high PS-NP exposures (100 and 1000 μg/L) reduced the NH₄⁺-N removal efficiency, with values around 70.9%. Low doses stimulated nitrification (NO₃^--N accumulation) and high doses inhibited N₂O emissions; δ¹⁵N tracing showed disrupted NH₄⁺-N to N₂ reduction. Plant-only microcosms had the highest N₂O release (1.37 mg, 1.5% of total N). Metagenomics revealed concentration-dependent, spatially distinct microbial community shifts: low PS-NPs increased rhizosphere α-diversity, while high concentrations depleted Proteobacteria, enriched Acidobacteria/Bacteroidetes, and reduced key nitrogen-cycling genera (e.g., Dechloromonas, Accumulibacter). In the rhizosphere, denitrification genes (nirK/S,nosZ) were upregulated by 2.5- and 3-fold, respectively, while DNRA (nrfA) and nitrogen fixation (nifH) genes were downregulated by 1.7- and 2.3-fold. Network and canonical correspondence analyses indicated stronger environmental filtering in bulk sediments (explaining 52.0% of variance) and spatially structured nitrogen metabolic pathway reorganization. These findings show concentration-dependent PS-NP exposure differentially shapes microbial community composition and nitrogen-cycling functions in rhizosphere and bulk sediments.