Characteristics of microcystin production in the cell cycle of Microcystis viridis

The correlation between the content of three microcystins (types LR, RR and YR) and the cell cycle of an axenic strain of Microcystis viridis, NIES-102, was investigated under conditions of high (16 mg L(-1)) and low (1.0 mg L(-1)) nitrate (NO(3)-N) concentrations. Each phase of the cell cycle was identified using a flow cytometer equipped with a 488-nm argon laser using SYTOX Green dye, which binds specifically to nucleic acids and can be exited by the wavelength (Ex/Em: 504/523 nm on DNA). Microcystin concentration showed a positive linear correlation with DNA concentration. The microcystin content of the cells changed remarkably as the cell cycle process proceeded, with maximum content in the G(2)/M phase and minimum content in the G(0)/G(1) phase. Under a condition of high NO(3)-N concentration, the ratio of the total content in the G(0)/G(1) phase to that in the G(2)/M phase was about 6:1. In contrast, for the two batch cultures the total content was 1.3-fold greater in the G(2)/M phase. The compositions of the three microcystins also changed along with the cell cycle process, although there was little difference in composition that was related to NO(3)-N concentration. Therefore, there were distinctive compositions specific to each phase of the cycle, and the cell cycle of the M. viridis strain was more strongly responsible for both the quantity and the types of microcystin production than was the effect of NO(3)-N concentration.