Nanoplastics intensify metal-induced impacts in freshwater ecosystems

Nanoplastics (NPs) are an emerging concern in freshwater ecosystems due to their persistence and potential to interact with persistent pollutants, such as metals. These combined stressors threaten freshwater ecosystems functioning, where leaf litter decomposition, primarily driven by aquatic hyphomycetes, supports energy transfer to higher trophic levels. A microcosm experiment was conducted to assess the joint impacts of polystyrene NPs (bare and carboxylated) at environmentally relevant (0.25 and 2.5 µg L⁻¹) and elevated (25 and 250 µg L⁻¹) concentrations in reference and metal-polluted stream waters. Fungal biomass, aquatic hyphomycetes sporulation and community composition, microbial leaf litter decomposition, as well as invertebrate feeding behaviour were quantified. Carboxylated NPs showed greater aggregation and surface alterations than bare NPs. Fungal biomass and sporulation declined significantly at 2.5 µg L⁻¹, with carboxylated NPs exerting stronger effects. Non-metric multidimensional scaling (NMDS) revealed clear shifts in fungal community structure across two water types (reference and metal-polluted) and NP concentrations. Metal pollution alone reduced decomposition by 17 %, with reductions reaching up to 24 % when combined with NPs, particularly carboxylated NPs. Invertebrate feeding was reduced by 27 % under metal pollution alone, with maximum inhibition (47 %) observed at the highest NP concentration, although NP type did not significantly alter feeding rates. Together, these results demonstrate that NPs, especially carboxylated forms, exacerbate the ecological impacts of metal pollution, impairing microbial processes and detritivore feeding. These findings underscore the importance of considering NPs surface chemistry and multiple stressors interactions in ecological risk assessments of polluted freshwater systems.