Strain-specific growth and microcystin responses of Microcystis to phosphorus availability under fixed nitrogen supply

Microcystis blooms are strongly shaped by nutrient availability, yet different strains may respond differently to the same phosphorus regime. In this study, we compared the growth, nutrient dynamics, and microcystin production of four Microcystis strains isolated from the Yeongsan and Nakdong Rivers, South Korea, under four phosphorus concentrations with a fixed nitrogen supply over 15 days. The strains were confirmed as members of the genus Microcystis by light microscopy and 16S rRNA gene phylogeny. Responses to phosphorus differed markedly among strains. AG61403 maintained moderate growth across the phosphorus gradient, whereas AG61402 showed strong biomass accumulation at intermediate phosphorus supply. Dissolved phosphorus was depleted more rapidly than dissolved nitrogen in actively growing cultures, generating strain-specific changes in dissolved N:P ratios. Microcystin concentrations did not increase proportionally with biomass accumulation. Cell-normalized total microcystin generally declined during rapid growth, especially under phosphorus-sufficient conditions and most clearly in AG61403, the only strain with quantifiable microcystin throughout the experiment. These findings suggest that bloom magnitude and cell-normalized total microcystin can respond differently to phosphorus enrichment, depending on Microcystis strain composition. Incorporating strain-level variability may help refine nutrient-based assessments of cyanobacterial bloom risk.