Influence of Initial Cell Densities of Cyclotella sp. on the Dominance of Microcystis aeruginosa under High N:P Ratio

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This study examined the characteristics of M. aeruginosa (UTEX 2061) dominance under the existence of a competitor, Cyclotella sp. (CCAP 1070/4), at high N:P ratio through a series of culture experiments, and revealed the effects of initial cell density of Cyclotella sp. on the dominance of M. aeruginosa as well. At N:P ratio of 140 (P = 0.1 mg/L), M. aeruginosa could defeat Cyclotella sp., even though the initial cell density for M. aeruginosa was less than that for Cyclotella sp. Although it is generally accepted that the low N:P ratio favors the dominance of cyanobacteria, the results obtained in this study clearly indicate that the growth of M. aeruginosa would not depend on the low N:P ratio, and that M. aeruginosa could potentially dominate over other algal species in high N:P ratio. At N:P ratio of 1400 (P = 0.01 mg/L), M. aeruginosa could become a superior competitor when the same initial cell density of 5,000 cells/mL for M. aeruginosa and Cyclotella sp. was employed. The condition of higher initial cell density for Cyclotella sp. than that for M. aeruginosa was also examined. This experimental situation led to the dominance of Cyclotella sp. when a share of phosphorus concentration, Ps, was lower than the half saturation constant for M. aeruginosa.