Size- and charge-dependent phytotoxicity of polystyrene nanoplastics to Vallisneria natans

The pervasive contamination of nanoplastics (NPs) in aquatic ecosystems has raised significant ecological concerns, yet their size- and charge-dependent effects on submerged macrophytes remain poorly understood. This study systematically investigated the phytotoxicity of polystyrene nanoplastics (PS-NPs) with different sizes (30 nm and 300 nm) and surface functional groups (-NH₂ and -COOH) on Vallisneria natans (V. natans) for a 30-day exposure period. Results showed that all PS-NPs treatments inhibited plant growth and reduced photosynthetic pigment content, with the 30 nm PS-NH₂ group exhibiting the most pronounced toxicity. Specifically, 30 nm PS-NH₂ exposure led to a 29.5% reduction in biomass compared to the control group and a significant decline in maximum photochemical efficiency (Fv/Fm), non-photochemical quenching (NPQ), and photochemical quenching (qP), indicating severe impairment of Photosystem II. Microscopic observations via SEM and TEM revealed that PS-NPs formed "particle-microbe complexes" on leaf surfaces and that 30 nm particles effectively translocated into root vascular bundles, inducing chloroplast deformation and starch granule accumulation. Furthermore, PS-NPs stress triggered an oxidative burst, evidenced by elevated H₂O₂ and MDA levels despite the activation of antioxidant enzymes (SOD, CAT, POD, and APX). 16S rDNA sequencing further demonstrated that PS-NPs markedly restructured the epiphytic bacterial community, altering the ecological balance of leaf-associated microorganisms. Collectively, this study highlights that smaller and positively charged PS-NPs pose the greatest risk to submerged macrophytes, driving physiological impairment and microecological shifts. These findings provide critical insights into the biological impacts and environmental risks of PS-NPs in freshwater ecosystems.