Nutritional constraints at the cyanobacteria—Daphnia magna interface: The role of sterols

In past decades, a considerable amount of research has been conducted to elucidate the factors that affect the carbon transfer across the cyanobacteria‐Daphnia interface. It is well accepted that cyanobacteria are a nutritionally inadequate food source for cladocerans, but the underlying mechanisms are still controversial. Morphological properties, toxicity, and the absence of essential lipids, i.e., polyunsaturated fatty acids (PUFAs) and sterols, are discussed as the most important factors that account for this nutritional inadequacy. Here, we conducted standardized growth experiments with the herbivore Daphnia magna feeding on coccal, filamentous, and putatively toxic cyanobacterial strains comprising the genera Synechococcus, Anabaena, Aphanizomenon, and Microcystis and on a cyanobacterial mixture containing strains of these genera. The relative importance of PUFAs and sterols in determining the food quality of cyanobacteria for Daphnia was assessed by supplementation of eicosapentaenoic acid‐ and/or cholesterol‐containing liposomes to the cyanobacterial carbon. We provide evidence that somatic growth of daphnids on coccal as well as on filamentous cyanobacteria is primarily constrained by the absence of sterols, provided that the cyanobacterial carbon is readily ingested and nontoxic. The absence of PUFAs in cyanobacteria appears to be of minor importance for somatic growth but potentially affects egg production in Daphnia. Thus, the absence of sterols has to be considered a major food‐quality constraint that potentially affects the efficiency of carbon transfer across the cyanobacteria‐Daphnia interface.