Life-history characteristics of Daphnia exposed to dissolved microcystin-LR and to the cyanobacterium Microcystis aeruginosa with and without microcystins.

In the current study, the role of microcystin(MC)-LR in inhibiting Daphnia growth was examined. Somatic growth, time to first reproduction, number of newborns, mortality, and population growth were measured in Daphnia fed mixtures of the cyanobacterium Microcystis aeruginosa (with and without microcystin) with a high quality green algal food (Scenedesmus obliquus) to avoid nutritional deficiency. Microcystin-LR-containing cells caused a dramatic reduction in growth and resulted in death of the animals, but the addition of purified toxin (microcystin-LR) had no effect on Daphnia growth. In contrast with the nutritional inadequacy hypothesis, growth on a mixture of Scenedesmus and the microcystin-free food was significantly reduced. This observation could not be explained from morphological characteristics of the strains that were uni- and bicellular. Clearance rates of Daphnia fed mixtures of Microcystis and Scenedesmus were significantly lower than rates of animals fed solely Scenedesmus. The results of an additional life-history experiment with different quantities of Scenedesmus could not reject the hypothesis that reduced feeding may be the causal factor in explaining reduced Daphnia growth on the food mixture with microcystin-free Microcystis. Thus, feeding inhibition should be considered in explaining the food quality of Microcystis and other cyanobacteria, as it could affect the total food intake and consequently Daphnia growth.