1. The accumulation of micro- and nanoplastics (MNPs) poses a significant threat to freshwater ecosystems. Nanoplastics (NPs; < 1000 nm) are particularly concerning due to their ability to penetrate cellular membranes and disturb intracellular functions. While current research has predominantly focused on the toxicological impacts of MNP on individual species, their broader ecological effects, particularly on species interactions, remain poorly understood. 2. Prior studies have indicated that smaller NPs within the nano-size range generally cause more severe effects on individual organisms. However, the impact of varying NP sizes on species interactions has not been thoroughly explored. This study addresses this gap by examining the effects of polystyrene NP beads of two sizes (50 nm and 100 nm) and two concentrations (1 mg/L and 5 mg/L) on the infection dynamics of the fungal parasite Metschnikowia bicuspidata in two genotypes of the freshwater crustacean Daphnia magna. 3. Our results indicated that lower NP concentrations (1 mg/L) had no significant effects on either host or parasite fitness. Exposure to 50 nm NPs at 5 mg/L significantly diminished both the parasite's transmission success and the host's lifespan. Conversely, 100 nm NPs at the same concentration enhanced parasite fitness. Given that M. bicuspidata is a widespread and virulent parasite affecting various Daphnia species globally, alterations in infection dynamics due to NP pollution could have broader implications for Daphnia populations and freshwater food webs. 4. These findings highlight the critical need to incorporate species interactions into plastic pollution research and emphasise the importance of evaluating the effects of different NP sizes on ecological relationships to fully comprehend the ecological impact of MNP pollution.
