Timing of nanoplastic exposure during infection impacts viability of host in an aquatic host–parasite system

Abstract Nanoplastic (NP) pollution is a growing environmental concern. While most studies focus on direct NP effects on individual organisms, NPs also disrupt host–parasite dynamics. However, which specific host or parasite life stage may be key to this disruption is unknown. We conducted two experiments to investigate whether the timing of NP exposure during a parasite’s life cycle—inside or outside the host—modifies transmission and parasite fitness, using the Daphnia magna–Australozyma monospora host–parasite system. First, transmission-stage fungal spores were exposed to two NP sizes (50 and 100 nm) at two concentrations (1 and 5 mg/l) in a full-factorial design to assess the effects on spore viability and infectivity. Second, Daphnia were exposed to NPs (50 nm and 100 nm particles at 5 mg/l) at four timepoints linked to distinct stages of parasite development within the host (acute exposures), and over the entire parasite development (chronic exposure). We found no evidence that NP exposure affected the parasite. However, juvenile Daphnia exposed to NPs for 72 h suffered high mortality, suggesting that NPs may indirectly alter host–parasite dynamics by reducing host availability. Our findings highlight the need to consider temporal aspects of exposure and indirect effects when evaluating the ecological risks of NPs.