New-generation pollutants: copper oxide nanoparticles disrupt freshwater ecosystem functioning, impacting host-parasite dynamics and organic matter decomposition.

To counter the inefficiency of conventional pesticides, nanopesticides have emerged as a promising alternative to enhance delivery and increase target specificity. However, the unique set of physicochemical properties that promise to enhance their efficacy also raises concerns about their environmental behaviour and toxicity. Among these, copper oxide nanoparticles (CuONPs) are widely used as an antimicrobial agent. While studies focus on their properties against pathogens, their effects on non-target fungal communities and broader ecological functions remain largely unknown. Our study aims to evaluate the impacts of CuONPs on freshwater fungal communities and the key ecosystem functions they support. Specifically, we assessed (1) host-parasite interactions and disease transmission using a Daphnia-microparasitic yeast experimental system (disease model), and (2) organic matter decomposition mediated by aquatic hyphomycetes (decomposition model). Results reveal contrasting and process-specific effects. CuONPs decreased disease prevalence (IC50 = 0.30 mg l-¹) but impaired host fitness through reduced survival and fecundity. In contrast, organic matter decomposition was functionally resistant, with stable litter mass loss and fungal biomass masking other effects, including the inhibition of fungal reproduction (IC50 = 2.53 mg l-¹) and a marked restructuring of the decomposer community (based on conidial morphology). Collectively, our findings demonstrate that CuO nanoparticles can subtly reshape ecological interactions and community composition. From a management perspective, this highlights the need of incorporating species interactions, community-level endpoints, and functional responses (disease dynamics, decomposition) into environmental risk assessments of contaminants of emerging concern.