Exposure of a Daphnia-parasite system to polystyrene nanoparticles reveals interactive effects of plastic pollution and disease

• PS-NPs were observed in the gut of the host (Daphnia magna). • PS-NPs did not affect parasite prevalence, but reduced infection intensity. • PS-NPs decreased host reproduction only in co-exposure to the parasite. • PS-NPs potentiate the effect of the parasite via reduced host fitness. In recent years, a wealth of attention has been given to the effects of micro- and nanoplastic pollution. Most studies so far focused on the effects on single species; however, in real life scenarios, organisms must face competitors, predators and parasites along with plastic particles. Seminal studies with host-parasite experimental systems suggest context-dependent effects of micro and nanoplastics, which deserve further study. Here, we exposed the cladoceran Daphnia magna to 0.1, 0.3 and 0.9 mg/L of polystyrene nanoparticles (PS-NPs; 50 nm) in the presence and absence of the parasitic yeast Metschnikowia bicuspidata . While PS-NPs had no effect on infection prevalence nor host mortality, a significant reduction in the intensity of infection (measured as spore load per host) was observed at 0.3 and 0.9 mg/L. Moreover, a significant decrease in host reproduction was observed at 0.3 and 0.9 mg/L, but only in co-exposure to M. bicuspidata . These results show that complex scenarios can arise from the interaction of nanoplastic pollution and disease dynamics in natural populations and support the need for more attention to the effects of these widespread pollutants on inter-species relationships.