Toxic compounds isolated from Microcystis PCC786 that are more active against Daphnia than two microcystins

Microcystis strain PCC7806 was toxic to Daphnia pulicaria. The toxicity of a crude water extract from lyophilized cells of Microcystis was determined in order to characterize the toxic compound. The LC50 of the crude extract was 47.4 µl ml−1. The crude extract was fractionated by solid‐phase extraction and tested for toxicity. The microcystins in the different fractions were analyzed by HPLC. The crude extract contained 22 µg ml−1 microcystin‐LR and 12.3 µg ml−1 3‐desmethylmicrocystin‐LR, the C‐18 eluate contained 16.3 and 9.1 µg ml−1 of these compounds, respectively. Neither compound was detected in the C‐18 cartridge‐passed crude extract. Nevertheless the C‐18 cartridge‐passed fraction was toxic to Daphnia. Hence I conclude that the two microcystins in the crude water extract are not the compounds in Microcystis PCC7806 which are toxic to Daphnia.