ARE THE SAME COMPOUNDS IN MICROCYSTIS RESPONSIBLE FOR TOXICITY TO DAPHNIA AND INHIBITION OF ITS FILTERING RATE

Three strains of Microcystis when supplied to Daphnia in mixtures with Scenedesmus differed in their power to inhibit filtering rates. The axenic strain PCC 7806 had the strongest effect (93% inhibition with 50% Microcystis in the food). This strain was toxic to daphnids since Daphnia died faster under these conditions than without any food. Strain PCC 7806 also exhibited strong toxicity against mice (LD50 22 mg/kg). When the toxicity against Daphnia and mice was tested with cells of Microcystis PCC 7806 which has been subjected to various extraction procedures, the extraction of freeze-thawed cells with water slightly reduced the mouse toxicity, eliminated Daphnia toxicity and reduced the inhibition of the filtering rate. After treatment with a lipophilic solvent, the cells of PCC 7806 still showed low mouse toxicity, but no longer inhibition of the filtering rate and no toxicity to Daphnia. Thus, our data suggest that Daphnia toxicity and inhibition of the filtering rate are caused by different compounds, but we cannot rule out the possibility that the factors toxic to mice and inhibitory to the filtering rate are identical.