Synergistic effects of dual parasitism in Daphnia magna under nutrient limitation

Human-induced increases in the bioavailability of carbon (C), nitrogen (N) and phosphorus (P) has the potential to alter the context for host-parasite dynamics in aquatic ecosystems. Given that both eutrophication and infectious diseases are becoming more prominent, it is essential to disentangle the factors that determine virulence expression in keystone grazers. Here, we focus on the impact of nutrient limitation in single versus multiple parasite exposure in the water flea Daphnia magna (Crustacea, Branchiopoda). For this, we fed Daphnia with algae differing in C:N:P ratio and exposed them to two virulent parasites, Pasteuria ramosa (Bacteria) and White Fat Cell (WFCD, unknown classification), both in single and multiple parasite exposure treatments. Multiple parasite exposure synergistically interacted to reduce host survival, especially under severe nutrient limitation, which was mainly driven by WFCD exposure. At the same time, Daphnia shifted the time to start reproducing forward, which resulted in a higher reproductive output upon multiple parasite exposure. We here discuss these results within the framework of host stress responses, nutrient allocation and energy budgets and conclude that how food quality interferes with host-parasite interactions depends on the parasite species studied, the nutrient requirements of all actors and the trait investigated.status: publishe