Isolation and characterization of microcystin-producing cyanobacteria from Dau Tieng Reservoir, Vietnam

Dau Tieng Reservoir supplies drinking water for millions of people in southern Vietnam. In recent years, increased nutrient loading of the reservoir coupled with year-round warm weather has tended to enhance the growth of cyanobacteria, which are capable of producing hepatotoxin microcystins (MCs). In this study, cyanobacterial assemblages, isolated strains, and bloom samples from the Dau Tieng Reservoir were used for species identification, MC synthetase gene (mcy) analysis, and MC determination. Microcystin concentrations in both isolated strains and bloom samples were quantified by high performance liquid chromatography, whereas toxic and nontoxic strains were distinguished by three molecular markers: mcyA, mcyB, and mcyD. We isolated and characterized a total of 68 cyanobacterial strains belonging to 5 genera: Microcystis (59 strains), Dolichospermum (4 strains), Arthrospira (2 strains), Pseudanabaena (2 strains) and Cylindrospermopsis (1 strain). Cyanobacterial populations from the reservoir consisted of toxic and nontoxic genotypes. Dolichospermum nygaardii was morphologically identified and described for the first time in Vietnamese waters. Our results showed that mcyD is the best molecular marker for determination of toxicogenic strains. Microcystis included toxic and nontoxic genotypes. The MC concentrations from bloom samples and cultures were as high as 669 and 2129 mu g g(-1) dry weight, respectively. The results argue strongly for the implementation of a monitoring program for cyanobacteria and their toxins in the water to minimize potential health risks to animal and human populations.