Mechanism Of Chemical Defense Against Grazing In Karenia Mikimotoi

Zooplankton grazing was shown to have a great impact on phytoplankton community development and harmful algae blooms (HABs) formation. It is essential to explore the grazing deterrence of HABs organisms. In this paper, toxicity of Karenia mikimotoi, a major HABs-causing dinoflagellate in Chinese coasts, to Moinan mongolica was evaluated by a series of experiments, including mixed diet experiments, isolated culture experiments and impacts of different portions of K. mikimotoi on M. mongolica. Secondly, changes in hemolytic toxins, reactive oxygen species (ROS) and neutral lipid in K. mikimotoi in the present of M. mongolica were analysed, where four diet treatments were used:(1) mixed culture (K. mikimotoi+M. mongolica); (2) starved (=M. mongolica medium only); (3) isolated (K. mikimotoi and M. mongolica was cultured in the same beaker but separated by a filter; (4) control (=K. mikimotoi medium only). Finally, effects of hemolytic toxin and Eicosapntemacnioc Acid (EPA) on M. mongolica were observed.The results showed that the grazing rate of M. mongolica to Platymonas subcordiformis decreased significantly with the increment of cell density of K. mikimotoi in mixture culture, suggesting that K. mikimotoi might inhibit the grazing of M. mongolica to P. subcordiformis. survival ship of M. mongolica fed with K. mikimotoi was lower than those of the control(M. mongolica cultured in f/2 medium only), starved group (M. mongolica cultured with K. mikimotoi in the same beaker but separated by a filter) and isolated group (M. mongolica cultured with the mixed culture of K. mikimotoi and M. mongolica in the same beaker but separated by a filter). This indicated that toxicity of K. mikimotoi to M. mongolica might be from intracellular substances. However, various partion from K. mikimotoi cultures were non-toxic to M. mongolica and P. subcordiformis. So, it would probably be reasonable to ascribe the toxicity of K. mikimotoi on M. mongolica to some unstable intracellular toxins or direct contact with live alga cells.survival ships of M. mongolica in starved group and mixed culture group were shown to decrease with time, and lower than that of control in 72 h, suggesting that K. mikimotoi was able to defend M. mongolica. Besides, hemolytic activity per cell of K. mikimotoi in isolated group was significantly higher than the control, demonstrating that M. mongolica might accelerate the production of hemolytic toxins in K. mikimotoi. Conversely, neutral lipid of starved group was significantly lower than that of the control, implying that M. mongolica might induce K. mikimotoi to turn neutral liquid into fatty acid to strengthen the ability to prevent grazing. Hemolytic toxin and EPA all decreased the survival ship of M. mongolica, suggested the toxicity of hemolytic toxin and EPA to M. mongolica.In conclusion, the toxicity of K. mikimotoi on M. mongolica derived from intracellular substances. M. mongolica grazing could induce the production of hemolytic toxin and decomposition of neutral lipid, which might strengthen the ability of K. mikimotoi to prevent grazing. The hemolytic toxins and EPA might have an important role in chemical defenses in K. mikimotoito deter grazing.