The toxic effects of the typically noxious bloom-forming dinoflagellate Karenia mikimotoi were studied using the allelopathic experimental system under controlled laboratory conditions. The potency of intact cell suspensions with whole cells, cell-free culture filtrate in different growth phases, and lysed cells with ultrasonication were compared, and the growth and reproduction of the marine rotifer Brachionus plicatilis were used as endpoints to evaluate toxic differences. The intact cell suspension resulted the most significant growth inhibition, including lethality, on the growth of B. plicatilis (p < 0.05). Lysed culture medium treated with ultrasonication and the cell-free culture filtrates at either the exponential or stationary phase exhibited limited negative impacts compared to the control according to changes in the population growth rate (r) and survival rate (p > 0.05). Reproduction presented a similar tendency to change, and the number of eggs produced per individual, as well as spawning period decreased in the whole cell and lysed cell suspensions. The key parameters in the lift table include the net reproductive rate (R₀) and the intrinsic rate of increase (rm), which were more sensitive to treatment and were significantly suppressed compared to that of the control. The addition of the ROS inhibitor N-acetylcysteine (NAC) could not change the growth or reproduction patterns. Moreover, substantial hemolytic toxicity was found in the treatment of the intact cell suspension (p < 0.05), while limited toxicity was found in other treatments compared to that of the control. K. mikimotoi was speculated to secrete allelopathic substances onto the cell surface, and direct cell contact was necessary for allelopathic toxicity in B. plicatilis. Reactive oxygen species (ROS)-independent hemolytic toxicity was assumed to be the explanation for what was observed.
Keywords: Brachionus plicatilis; Karenia mikimotoi; ROS-dependent pathway; allelopathic effect; harmful algal blooms (HABs); hemolytic activity.