DETERMINATION OF SOME PHYSICOCHEMICAL PARAMETERS OF MICROCYSTINS (CYANOBACTERIAL TOXINS) AND TRACE-LEVEL ANALYSIS IN ENVIRONMENTAL-SAMPLES USING LIQUID-CHROMATOGRAPHY

Some physicochemical parameters of three microcystin standards, known to be potent hepatotoxins produced by cyanobacteria, were determined u sing well defined chromatographic measurements. The logarithm of their retention factor on octadecylsilica (ODS) with water as the eluent, w hich is an estimation of the hydrophobicity of a molecule, was assesse d at pH 7 at 3.9, 4.2 and 4.4 for microcystins-YR, -LR and -RR, respec tively. Though being rather hydrophobic. microcystins also possess pol ar functions, namely carboxylic acids, amino and amido groups. The ion ization of carboxylic groups occurs at pH values of 3.3-3.4. In enviro nmental waters, microcystins are neutral or anionic. They are readily soluble in water, the solubility of microcystin-1R being higher than 1 g/l. Owing to their hydrophobicity and their po!,ar functions, microc ystins remain in the aqueous phase rather than being adsorbed on sedim ents or on suspended particulate matter. In a river water spiked with microcystins at 5 mu g/l, only 10% was adsorbed on particles and 7% on the sandy sediment after three days. A method using solid-phase extra ction on ODS followed by high-performance - or micro - liquid chromato graphy was optimized to detect microcystins at trace level in water. A clean-up was introduced to eliminate part of the interfering compound s coextracted during the sample percolation. Good recoveries (75-80%) were obtained. The method was linear, reproducible (with relative stan dard deviations ranging from 5 to 8%) and enabled the determination of microcystins at levels as low as 30 ng/l in drinking water and 100-20 0 ng/l in surface waters. The production of toxins by two strains of c yanobacteria was evaluated. Variations in the microcystin-1R content i n the cells and in the medium of Microcystis aeryginosa PCC7806 were r ecorded over a five-week period. Toxin production was not correlated t o the biomass but depended on the growth stage and was maximal at the end of the exponential growth phase. The release of toxin in water occ urred essentially in old cultures where microcystin-1R was determined at concentrations of 170 and 280 mu g/l in the media of M. aeruginosa PCC7806 and M. aeruginosa PCC7813, respectively. Other microcystins ar e likely to be synthesized by these strains. But owing to the lack of standards, mass spectrometric detection is required for further identi fication. This study points out the need of having other standards for water quality monitoring. (C) 1998 Elsevier Science B.V.