Detection and variation of microcystin contents of Microcystis blooms in eutrophic Lalla Takerkoust Lake, Morocco

Abstract

Hepatotoxic microcystins produced by cyanobacteria represent a significant health hazard. In Morocco, although blooms of cyanobacteria are known to occur frequently in some water-bodies, studies on toxicology and toxinology of these potentially harmful algae have been scarcely developed. This paper presents results of the detection of Microcystin-LR (Mcyst-LR) and other microcystin variants and their content variations from Microcystis natural blooms and from the isolated strain culture. During the cyanobacteria dynamic study, 11 bloom samples were collected between 1994 and 2000 and were assessed for toxicity and quantification of microcystins, respectively, by mice bioassay and the enzyme-linked immunosorbent assay. The bloom samples exhibit a high toxicity with mice intraperitoneal injection (i.p.) LD50 < 100 mg/kg and the total microcystin contents were 496–8800 μg/g dry weight with a coefficient of variation of less than 11%. However, the Microcystis isolated strain cultured on Z8 medium under controlled laboratory conditions produced an amount of 707 μg/g dry weight, its toxicity level corresponds to a LD50 i.p mice of 31 mg/kg. The detection of microcystin variants and the identification of Mcyst-LR from some natural bloom samples collected in 1994, 1996 and 1999 have been carried out by high performance liquid chromatography with a photodiode array detector, which confirmed the identification from each bloom sample of at least four microcystin variants, among which the Mcyst-LR one represents a relative proportion of 30% to 62% of total microcystin contents. Only one variant of microcystin that was not a Mcyst-LR has been detected from a Microcystis isolated strain. The toxicity level of the material depended either on the amount of microcystins or the number of these variants and their proportion. According to these results, the survey of cyanobacteria and cyanotoxins monitoring in water used for human consumption is needed to avoid any sanitary risk.