Grazing rate of the mixotrophic dinoflagellate Akashiwo sanguinea is associated with hemolytic activity under varying irradiance and nutrient levels

Harmful algal blooms formed by the mixotrophic dinoflagellate Akashiwo sanguinea are toxic to multiple aquatic organisms, although the relationship between toxicity and grazing is unknown. In this study, the effects of irradiance and nutrient levels on the grazing rate of A. sanguinea on the cryptophyte Rhodomonas salina were investigated using an optimized stable isotope method. Under light-limited conditions (0–30 μmol photons m −2  s −1 ), A. sanguinea exhibited higher hemolytic activity and grazing rate. Under the same level of irradiance, the growth and respiration rates were higher in co-culture treatments, while the oxygen release rate was higher in monoculture treatments. The growth rate, hemolytic activity, and grazing rate of A. sanguinea increased significantly with higher nutrient levels or an unbalanced N:P ratio. The hemolytic activity and grazing rate were higher in urea treatment, while the growth rate was higher in ammonia treatment. This study suggests that toxicity is involved in the feeding process of the mixotrophic dinoflagellate A. sanguinea ; the unique feeding flexibility adapting to low irradiance, increased nutrients, unbalanced N:P ratio, and organic nitrogen urea gains an advantage in interspecific competition among phytoplankton.