Abstract

Resource quantity in the environment often changes over time and influences the nutritional status of hosts that may encounter parasites. If resource availability significantly alters both infection success and within-host growth of a parasite, fluctuations in resources may underlie the seasonal disease outbreaks that have been observed for some parasites. Moreover, resource quantity may affect how a parasite impacts host survivorship and traits, including feeding rate and assimilation of nutrients. Some parasites, such as intracellular microsporidia with highly reduced genomes, may be particularly sensitive to variation in host nutritional status and more likely to have resource-dependent impacts. To determine how resource quantity affects infection success and parasite impacts on hosts, we conducted laboratory experiments using the microsporidian parasite Ordospora pajunii and its zooplankton host, the dominant grazer Daphnia dentifera. We found that infection probability and spore burden were higher when resources were more abundant, suggesting O. pajunii benefits when host quality is higher. However, parasite virulence, which was measured in terms of host mortality, was greater when resources were more limited. Parasite exposure depressed host feeding rate, but the timing of this reduction differed across the resource gradient. Further investigation of host carbon assimilation efficiency and dissolved organic carbon release during infections when resources were more limited revealed no significant impact of infection. Overall, resource-dependent impacts of O. pajunii on hosts, including reduced host feeding rate and increased host mortality, may contribute to seasonal disease outbreaks and drive trophic cascades in lakes.