Food quality of heterotrophic bacteria for Daphnia magna: evidence for a limitation by sterols

The quality of heterotrophic bacteria as food for metazoan grazers has been investigated poorly. We conducted growth experiments with juvenile Daphnia magna feeding on different strains of heterotrophic bacteria that represent typical pelagic bacteria of five phylogenetically distinct groups. The bacterial food suspensions were supplemented with cholesterol and/or the polyunsaturated fatty acid eicosapentaenoic acid (EPA), two essential nutrients that are either absent or scarcely represented in bacteria. Our data imply that the selected heterotrophic bacteria are of poor food quality for D. magna, which was indicated either by very low somatic growth rates or by high mortality. However, with four out of six bacterial strains tested, the somatic growth rates increased significantly upon supplementation with cholesterol, which shows that the lack of sterols in bacteria is a major food quality constraint. We did not find clear evidence for a limitation by EPA on bacterial diets within our growth experiments. High mortality was observed when D. magna was fed with Hydrogenophaga sp. or Pseudomonas sp., which indicates that these two bacterial strains are toxic to D. magna. Our findings highlight the limitations of bacteria as a carbon source for Daphnia and point to a so far underestimated diversity of interactions between grazers and its bacterial food.