Role of microcystins in poisoning and food ingestion inhibition of Daphniagaleata caused by the cyanobacterium Microcystis aeruginosa

The effects of microcystins on Daphnia galeata, a typical filter-feeding gr azer in eutrophic lakes, were investigated. To do this, the microcystin-pro ducing wild-type strain Microcystis aeruginosa PCC7806 was compared with a mcy(-) PCC7806 mutant, which could not synthesize any variant of microcysti n due to mutation of a microcystin synthetase gene. The wild-type strain wa s found to be poisonous to D. galeata, whereas the mcy(-) mutant did not ha ve any lethal effect on the animals. Both variants of PCC7806 were able to reduce the Daphnia ingestion rate. Our results suggest that microcystins ar e the most likely cause of the daphnid poisoning observed when wild-type st rain PCC7806 is fed to the animals, but these toxins are not responsible fo r inhibition of the ingestion process.