2023 年 21 巻 2 号 p. 119-128
This study investigated the relationship between nitrogen (N) and phosphorus (P) mass ratio in the medium and cellular N (Nc) and P (Pc) mass ratio in Microcystis cell. Monoculture experiments were conducted with the phosphorus concentration fixed at 0.110 mg L–1 and the initial N:P mass ratios of 1, 10, 18, 50, 70, and 100. The results showed that the cell density of Microcystis sp. was highest at the initial N:P mass ratio of 50. Although it has been disputed that a low total N:P (TN:TP) ratio (< 29) in water was a cause of the development of cyanobacterial blooms or a result of its occurrence, this study could support the latter trend. This assumption could be supported by the results of elemental analysis (i.e., cellular Nc:Pc mass ratio of Microcystis sp.). At the end of the culture experiment, the Nc:Pc ratios ranged from 13.2 to 20.3, which were similar to the initial value (12.7). In general, the dissolved N and P concentrations in lake water are completely consumed when cyanobacterial blooms occur. Therefore, the present study suggests that the low TN:TP ratio may reflect the cellular Nc:Pc ratio of Microcystis sp. following assimilation of N and P in lake water.
