Synergistic effects of dual parasitism in Daphnia magna under nutrient limitation

Abstract

Human-induced increases in the bioavailability of carbon (C), nitrogen (N) and phosphorus (P) have 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 dual parasite exposure in the water flea Daphnia magna (Crustacea, Branchiopoda). For this, we fed specimens of D. magna with algae differing in C:N:P ratios and exposed them to two virulent parasites, Pasteuria ramosa (bacteria) and the agent causing White Fat Cell (WFCD, unknown classification), both in single and dual parasite exposure treatments. Exposure to the two parasites synergistically reduced host survival, mainly driven by WFCD exposure, especially under severe nutrient limitation. Under these conditions individuals of D. magna began reproducing earlier, which resulted in a higher reproductive output upon dual parasite exposure. We here discuss these results within the framework of host stress responses, nutrient allocation and energy budgets, and conclude that the way food quality interferes with host-parasite interactions varies, depending on the parasite species involved, the nutrient requirements of all actors and the trait investigated.