Studies on the Water Bloom of Toxic Alga, Microcystis Viridis

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We studied the water bloom of toxic alga Microcystis viridis in the field and laboratory experiment. A field study was carried out from summer to fall between 1988-1990 to clarify the change in the dominant species of Microcystis in Lake Kasumigaura. M. aeruginosa f. aeruginosa and M. flos-aquae were dominant in te summer, but M. viridis became dominant in the fall. A high correlation was found between the standing crop of M. aeruginosa and the water temperature, but the standing croup of M. viridis and M. wesenbergii showed no correlation in the water temperature. Effects of temperature and illumination on the specific growth rates and the algal yield of M. viridis N-1 and M. viridis NIES 102 were studied with a batch culture. Maximum specific growth rates of M. viridis N-1 and M. viridis NIES 102 were observed at 25 and 30^0C, respectively. M. viridis could grow at lower temperatures than the other species of Microcystis. M. viridis N-1 could not grow at the continuous illumination o … More f 2,000 lx but could grow at the periodic illumination. The specific growth rate of M. viridis was lower than those of other species of Microcystis. These results suggested that M. viridis was more difficult to dominate than other Microcystis in summer.Moreover, we studied the effect of heavy metals on the growth of Microcystis spesies. Toxicity of copper, cadmium and cobalt on the growth of blue-green algae M. aeruginosa K3A, M. viridis, Anabaena affinis and green algae Chlorella ellipsoidea was determined in the liquid medium. The EC_<50> (effective concentration, 50% inhibition of the specific growth rate of control) of copper for M. aeruginosa K3A, M. viridis, A. arrinis, C. ellipsoidea were 0.019muM, 0.023muM, 0.018muM and 0.58muM, respectively, Copper showed the severest toxicity to the growth of these glgae compared with cadmium and cobalt. The sensitivity to copper of three species of blue-green algae was 30 times higher than that of the green algae C. llipsoidea. Chelating agents such as EDTA and EGTA showed the stroug reducing effect on the copper toxicity. Copper concentratin in the surface water of Lake Kasumigaura was 0.017muM, which was the almost same to the value of EC_<50> for blue-green algae. However, the EC_<50> of copper for M. aeruginosa K3A in the lake water was found to be 3 times larger than in the liquid medium. From these facts, it is assumed that natural ligands in the lakewater stimulated the growth of blue-green algae to make algal bloom. Less