High grazer toxicity of [D-Asp(3) (E)-Dhb(7)]microcystin-RR of Planktothrix rubescens as compared to different microcystins

Planktothrix rubescens, the dominant cyanobacterium in Lake Zurich, is gene rally considered to be toxic to zooplankton. The major toxin was determined by NMR spectroscopy and chemical analysis to be [D-Asp(3), (E)-Dhb(7)]micr ocystin-RR. The compound was isolated in high purity, and its 24-h acute gr azer toxicity was compared with microcystin-LR, microcystin-RR, microcystin -YR, and nodularin using a Thamnocephalus platyurus bioassay. Based on LC50 values [D-Asp(3),(E)-Dhb(7)]microcystin-RR was the most toxic microcystin tested. Nodularin was slightly more toxic under the conditions of the assay . The large number of individuals available for the grazer bioassay allowed the determination of dose-response curves of the different microcystins. T hese curves showed marked differences in their steepness. Microcystin-RR, w hich had nearly the same LC50 as microcystin-LR and microcystin-YR, exhibit ed a very flat dose-response curve. This flat curve indicates that, for som e individuals, lower concentrations of this microcystin are much more toxic than are the other two microcystins. Mortality of 100% requires much highe r concentrations of microcystin-RR, indicating the resistance of some anima ls to the toxin. The purified [D-Asp(3), (E)-Dhb(7)]microcystin-RR exhibite d a higher molar absorption coefficient determined by quantitative amino ac id analysis than the coefficients generally used for other microcystins. Th is observation has consequences for the risk assessment for microcystins an d makes a structural determination of microcystins an absolute requirement. The presence of the dehydrobutyrine residue may be the reason for the high er specific toxicity of [D-Asp(3), (E)-Dhb(7)]microcystin-RR when compared to the N-methyldehydroalanine-containing microcystins. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.