First report on the identification of microcystin in a water bloom collected in Belgium.

A toxic cyanobacterial bloom dominated by Microcystis aeruginosa occurred in 1995 in three adjacent ponds near Liège (Belgium) where at the same time conspicuous bird deaths were observed. The toxicity assay using primary rat hepatocytes indicated a high hepatoxicity. A 4 h incubation yielded a LD50 of 0.23 mg bloom material (dry weight)/ml cell culture medium. Toxicity was due to hepatotoxins of the microcystin class, microcystin-LR and-RR being the major microcystins present as determined by RP-HPLC absorption spectra, 1H NMR, and ESMS spectra. Additionally, the bloom sample contained small amounts of microcystin-YR. The microcystin content of the dry bloom biomass was 870 micrograms/g (on the basis of the hepatotoxicity assay) and 556 micrograms/g (on the basis of the RP-HPLC peak area). A higher yield of microcystins was obtained by acetic acid extraction instead of methanol extraction, whereas different extraction temperatures (20 degrees C, 40 degrees C) had no effect on the yield.