Good food versus bad food: the role of sterols and polyunsaturated fatty acids in determining growth and reproduction of Daphnia magna

The absence of dietary sterols and polyunsaturated fatty acids (PUFAs) has been shown to affect the performance of the freshwater herbivore Daphnia. Here, we compared somatic growth rates and clutch sizes of Daphnia magna reared on a diet of low food quality (Synechococcus elongatus) and of high food quality (Cryptomonas sp.) and investigated if and to what extent the absence of sterols or PUFAs in the cyanobacterium S. elongatus accounts for the observed differences in food quality. The supplementation of S. elongatus with cell-free lipid extracts (fatty acids, sterols, total lipids) obtained from the flagellate Cryptomonas sp. suggested that the superior food quality of Cryptomonas sp. is predominantly, but not completely, a combined effect of its sterol and PUFA composition. Our laboratory study suggests that somatic growth of D. magna feeding on S. elongatus is primarily constrained by the absence of sterols, whereas egg production is primarily limited by the absence of long chain PUFAs.