PERSISTENCE OF CYCLIC PEPTIDE TOXINS IN DRIED MICROCYSTIS-AERUGINOSA CRUSTS FROM LAKE MOKOAN, AUSTRALIA

Dried Microcystis aeruginosa Kuetzing emend. Elenkin crusts estimated to be 5-6 months old from the shore of Lake Mokoan were toxic by mouse bioassay (LD100 100-140 mg dry wt/kg mouse). Fresh bloom material fro m the lake was also highly toxic (LD100 25-35 mg dry wt/kg mouse). Mic rocystin high performance liquid chromatography (HPLC) profiles of the crust and fresh material were very similar, with 24 compounds having UV spectra consistent with microcystin LR. Five of the major microcyst ins were purified and analysed by electrospray/mass spectrometry. The molecular weights of these microcystins [910, 924, 982, 982 (two compo unds), and 986] do not correspond with known microcystins. All five co mpounds were hepatotoxic to mice with LD100 values ranging from 85 to 140 mug microcystin/kg mouse). Total microcystin contents (expressed a s microcystin LR equivalents) determined by HPLC correlated with the m ouse bioassay analyses (crust 2.1 mug microcystin/mg dry wt; fresh 4.1 mug microcystin/mg dry wt). These results suggest that microcystin is protected from degradation while encapsulated within the dried Microc ystis crusts. Leaching experiments demonstrated that re-wetting of the crust material leads to rapid release of microcystins into the surrou nding water. These observations have important management implications for lakes and reservoirs where crusts of cyanobacterial material form on the shoreline. (C) 1995 by John Wiley & Sons, Inc.