Effects of microcystin-free and microcystin-containing strains of the cyanobacterium Microcystis aeruginosa on growth of the grazer Daphnia magna.

Harmful effects of the common bloom-forming cyanobacterium Microcystis aeruginosa on the grazer Daphnia have been explained from morphological features, nutritional insufficiency, and the production of toxins called microcystins. The effects of four M. aeruginosa strains, including one free of microcystins, on the growth of Daphnia magna were examined. Population growth was measured in Daphnia fed each M. aeruginosa strain alone and fed mixtures of each strain with the green alga Scenedesmus obliquus to avoid nutritional deficiency. The presence of Microcystis in the food resulted in decreased Daphnia population growth rates. A significant correlation (P < 0.001) was found between the amount of microcystins (MC) in the food and the population growth rate, but growth inhibition in Daphnia fed with the MC-free strain could not be explained by the toxicity of the microcystins. Inasmuch as there was greater reduction in growth of the Daphnia fed mixtures of the MC-free strain with Scenedesmus than of the Daphnia fed only the same amount of Scenedesmus as in the mixtures, other factors associated with this Microcystis strain must have inhibited growth. However, morphology and nutritional insufficiency could be excluded as causal factors. The strong relationship between Daphnia feeding activity and population growth suggests that feeding inhibition may have caused the observed detrimental effects on the animals. Copyright 2003 Wiley Periodicals, Inc.