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

Abstract
Nanoplastic (NP) pollution is growing environmental concern due to its widespread occurrence and toxicity. While most studies focus on direct NP effects on individual organisms, NPs may 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 <em>Daphnia magna-Metschnikowia bicuspidata &lt;/em&gt;host-parasite system. First, transmission-stage fungal spores were exposed to two NP sizes (50 &amp; 100 nm) at two concentrations (1 &amp; 5 mg/L) in a full-factorial design to assess the effects on spore viability and infectivity. Second, &lt;em&gt;Daphnia&lt;/em&gt; were exposed to NPs at five time points linked to distinct stages of parasite development within the host. We found no evidence that NP exposure affected parasite transmission or fitness at any tested stage. However, &lt;em&gt;Daphnia&lt;/em&gt; exposed to NPs for 48 hours at an early age 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.</em>