Daphnia growth on microcystin-producing and microcystin-free Microcystis aeruginosa in different mixtures with the green alga Scenedesmus obliquus

The hypothesis that negative effects of Microcystis on Daphnia growth and reproduction can be explained from the presence of microcystin in the Microcystis cells was tested by comparing the effects on Daphnia life history characteristics of a microcystin-free mutant strain and microcystin-containing strain of the cyanobacterium Microcystis aeruginosa PCC7806. To avoid nutritional deficiency, Microcystis was offered to Daphnia alone and in various mixtures with the high-quality green alga Scenedesmus obliquus. In contrast to expectation, growth of Daphnia on microcystin-free cells was not much better than growth on microcystin-containing cells. Because nutritional insufficiency, morphology, and feeding inhibition could not explain the observed effects, the results show that Microcystis must contain substances other than microcystins that are poisonous to Daphnia.