Toxicity to Daphnia of a compound extracted from laboratory and natural Microcystis spp., and the role of microcystins

SUMMARY 1. The microcystin content of a variety of Microcystis spp., from both laboratory strains and natural blooms, was analysed by HPLC. The microcystin content of laboratory strains ranged from 1.6 to 4.3μgmg−1 dry weight. Yearly and seasonal variation was detected in an analysis of bloom material collected from Bautzen Reservoir over a 3‐year period. The microcystin concentration in bloom material ranged from undetectable to 1.16 μg ml−1 dry weight. 2. Toxicity of laboratory and natural Microcystis to Daphnia pulicaria was determined using an established LC50 technique. Partially purified water extracts from different Microcystis samples exhibited a wide range of toxicity. The highest activity was found in natural Microcystis samples, with an LC50 of 36 μgm−1 dry weight of Microcystis, whereas one strain did not appear toxic at 1600 μg ml−1. 3. No correlation was found between the concentrations of microcystins of different laboratory and natural Microcystis strains and the toxicity of extracts to Daphnia pulicaria from the same strains. Therefore, we discriminated between hepatotoxic microcystins and the compound(s) that is toxic to Daphnia, here termed DTC (Daphnia‐toxic compound), which is independent of microcystins.