Calcium is an important element in water bodies that plays a pivotal role in numerous biological processes; however, its effects on bloom-forming cyanobacteria have been studied only rarely. In the present study, the effects of calcium on Microcystis aeruginosa, a major bloom-forming cyanobacterium, were investigated. Significant decreases in growth, chlorophyll a and maximum electron transport rate were shown when calcium was absent, while similar reductions in these parameters were also found at high concentrations of calcium (240 mg/l). The expression of psbA, grpE and recA was up-regulated significantly for 0 and 240 mg/l calcium treatments. The activity of catalase (CAT) also increased remarkably under these two treatments, suggesting that Microcystis suffered from stress under both calcium absence and under high calcium concentrations. Additionally, the polysaccharide synthesis gene, epsL, was up-regulated at high concentrations of calcium. These data suggest that calcium plays an important role in the growth of Microcystis, and that enhancement in polysaccharide synthesis in response to increases in calcium concentration might be responsible for bloom formation.

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