Abstract: To investigate the mechanisms underlying blooms of the toxic harmful algal bloom-forming dinoflagellate Karenia selliformis, which has recently been reported in Chinese coastal waters, we examined its physiological responses to nitrogen (N) and phosphorus (P) limitation. Cultures grown in L1 medium served as the control, and four treatments were established by reducing nitrate or phosphate concentrations: moderate N limitation (LN), severe N limitation (LLN), moderate P limitation (LP), and severe P limitation (LLP). Cell density, specific growth rate, chlorophyll a content, chlorophyll fluorescence parameters, and changes in N and P nutrient concentrations in the culture medium were measured. Both N and P limitation significantly suppressed K. selliformis proliferation, and the inhibitory effects increased with the degree of nutrient limitation, as indicated by reduced maximum cell densities, shortened exponential growth phases, and lower specific growth rates. N and P limitation had distinct temporal effects on photosynthetic performance. Under severe N limitation, chlorophyll a content, maximum photochemical efficiency (Fv/Fm), and maximum electron transport rate (ETRmax) decreased significantly during the early phase of stress. In contrast, cells under P limitation maintained a certain level of photosynthetic activity during the early phase, whereas prolonged severe P limitation resulted in gradual decreases in Fv/Fm and ETRmax. K. selliformis also exhibited a pronounced coupling response to N and P limitation: when one nutrient was limiting, the overall apparent consumption of the other non-limiting nutrient in the culture system also decreased markedly. This study preliminarily reveals the distinct physiological responses of K. selliformis to N and P limitation, providing a physiological basis for understanding the effects of changes in nutrient availability and stoichiometry in coastal waters on the initiation, maintenance, and decline of K. selliformis blooms, as well as a theoretical reference for the monitoring and early warning of such harmful algal blooms.

Key words: Karenia selliformis, nitrogen limitation, phosphorus limitation, nutrients, harmful algal blooms