Does the toxic effect of Microcystis aeruginosa on Daphnia galeata depend on microcystin ingestion rate?

The aim of this study was to test the hypothesis that toxicity of Microcyst is aeruginosa to Daphnia galeata can be explained by microcystins. For that purpose, the relationship between the strength of toxic effect and microcy stin ingestion rate (amount of microcystins ingested per time with the resp ective food) by the animals was analysed. Of the tested Microcystis strains , only those containing microcystins, were poisonous to daphnids. It was, f urthermore, shown that toxicity was closely related to the microcystin inge stion rate. Thus, the results support the hypothesis that microcystins are the most likely cause of daphnid poisoning, and they also emphasise that th e toxicity of a Microcystis strain depends on both, the cellular microcysti n content and the rate with which it is ingested by the animals. The data a lso demonstrate that the feeding activity of D. galeata is not correlated t o the microcystin content of the Microcystis culture fed. This may indicate that the strain-specific differences in feeding on M. aeruginosa are not d ue to the microcystins. As toxicity was also observed when colony-forming s trains with a mucilage sheath were offered, it seems likely that microcysti n release in the gut is not actually hindered by mucilage and that toxicity can occur even under natural conditions.