Sterol limitation of Daphnia on eukaryotic phytoplankton: a combined supplementation and compound-specific stable isotope labeling approach

Essential biomolecules can critically influence the performance of consumers. A deficiency in dietary sterols has been shown to constrain the food quality of prokaryotic food sources for aquatic consumers. Here, we assessed the importance of dietary cholesterol for life history traits (survival, growth and egg production) of the freshwater herbivore Daphnia magna in supplementation experiments with various sterol-containing eukaryotic phytoplankton diets (dinoflagellates, diatoms, and golden algae). We combined cholesterol supplementation via liposomes with C-13-labelling of the phytoplankton diets and traced the origin of cholesterol in Daphnia using compound-specific stable isotope analysis. All phytoplankton strains used here were rich in polyunsaturated fatty acids (PUFA) but differed in their phytosterol composition. We show that growth and reproduction of D. magna can be limited by sterols even when feeding on sterol-containing eukaryotic phytoplankton diets. The impact of cholesterol supplementation on growth and reproduction of D. magna differed among phytoplankton diets (strains). The positive effect of cholesterol supplementation was most pronounced on diatom diets. Estimation of source proportions using stable isotopes revealed that D. magna preferentially assimilated the supplemented cholesterol rather than synthesizing it from dietary phytosterols. Our experiments suggest that the different responses to cholesterol supplementation on the various phytoplankton diets were unrelated to the suitability of dietary phytosterols to serve as cholesterol precursors but were caused by other biochemical or morphological food quality constraints. The combination of methods applied here could be very useful for uncovering nutritional constraints and thus for assessing the importance of essential biomolecules for the performance of herbivorous consumers.