Evidence for biochemical limitation of population growth and reproduction of the rotifer Keratella quadrata fed with freshwater protists

The biochemical factors that determine the food quality of protists for rotifers are poorly understood. We evaluated population growth rates and egg production of the rotifer Keratella quadrata fed with four protist species growing on either an algal or a bacterial diet. The cryptomonad Cryptomonas phaseolus, considered as a good quality prey, and assays without prey served as controls. Population growth rates and egg numbers of K. quadrata were correlated with single biochemical compounds (fatty acids, amino acids and sterols) of the protists. Feeding on the alga C. phaseolus or the algivorous ciliates resulted in enhanced population growth rates and high egg production by K. quadrata, whereas feeding on bacterivores supported only moderate egg production but no population growth. The rotifers' egg production was correlated with the protist biochemical composition, including polyunsaturated fatty acids (PUFAs), the sterol desmosterol, ergosterol, stigmastanol and the amino acid leucine. No significant relationships were observed between population growth rates of the rotifers and the protists' biochemistry, suggesting that population growth and reproduction of K. quadrata may have different nutritional requirements. To our knowledge, this is one of the first studies to analyze a large variety of biochemical compounds to determine the food quality of protists for a zooplankton predator and the first study to analyze the biochemical quality of protists to a rotifer species.