1. The accumulation and degradation of plastic waste in freshwater bodies poses a threat to aquatic biota. Microplastics (<5 mm) can transfer upwards in food chains and have been shown to induce deleterious effects on important players of freshwater ecosystems, including zooplankton.
2. A smaller category of microplastic particles, the so‐called nanoplastics (≤100 nm) raise special concern due to their ability to act at sub‐cellular and molecular levels. Despite growing knowledge of their effects on physiological traits of individual species, the way they affect interactions between species remains largely unexplored.
3. We studied the effects of nanoplastics on host–parasite interactions by exposing the zooplankton host Daphnia galeata × longispina to the parasitic yeast Metschnikowia bicuspidata without plastic and at two different concentrations of polystyrene nanoplastic beads (100 nm): 5 and 20 mg/L.
4. Both concentrations of nanoplastics increased the proportion of infected hosts; at the higher concentration, however, elevated rates of host mortality and impaired spore production cancelled out the parasite's advantage. Consequently, parasite success was greatest at the lower level of nanoplastic exposure.
5. Infection by Metschnikowia greatly reduced host lifespan and total offspring production (regardless of nanoplastic exposure), but only decreased the proportion of successfully reproducing hosts when Daphnia were additionally exposed to nanoplastics. Nanoplastics alone did not cause such a reduction in host fitness parameters: instead, the lower concentration increased lifetime offspring production by about 50%, suggesting hormesis.
6. Given that parasitism is a ubiquitous lifestyle in nature and that parasites can play important roles in the shaping and functioning of ecosystems, these results highlight the importance of including interactions between host and parasite species as alternative ecotoxicological endpoints to better assess the ecological consequences of plastic pollution.
