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. It is generally accepted that cyanobacteria can cause major disruptions of the aquatic ecosystem (Christoffersen 1996). In particular, the effects on the grazer Daphnia, which is a key species in freshwater food chains (Lampert 1987), have been documented extensively (e.g., Lampert 1987; DeBenardi and Giussani 1990; DeMott 1999; Müller-Navarra et al. 2000). Cyanobacteria have strong negative effects on Daphnia and are undoubtedly an inadequate food source (Lampert 1987). There is, however, much less consensus on the causal factors; hence, several causes for the negative effects of cyanobacteria on Daphnia have been proposed. First, many cyanobacteria possess toxins that might cause death in Daphnia (DeMott et al. 1991; DeMott and Dwahale 1995). Second, a ‘‘bad taste factor’’ or feeding deterrents might reduce the food intake of Daphnia (Nizan et al. 1986; Haney et al. 1995). Third, cyanobacteria might lack essential fatty acids or lipids, and the poor nutritional value could have severe effects on Daphnia growth and reproduction (MüllerNavarra 1995; DeMott and Müller-Navarra 1997; Von Elert and Wolffrom 2001). Finally, morphological features, such as size or mucus, might hamper ingestion or digestion of cyanobacteria by Daphnia (Fulton and Paerl 1987; DeBenardi and Giussani 1990). All these factors could contribute to reduced growth, lower reproduction, and, eventually, a Daphnia population decline thereby reducing the efficiency of the energy transfer from primary producers up the trophic levels. As a result, an accumulation of cyanobacteria occurs and notorious blooms are formed that are indicative of water quality deterioration. Microcystis aeruginosa is one of the most common cya1 Corresponding author ([email protected]).