Effects of co-existing microalgae and grazers on the production of hemolytic toxins in Karenia mikimotoi

Karenia mikimotoi （Miyake ＆ Kominami ex Oda） Hansen ＆ Moestrup is associated with harmful algal blooms in temperate and subtropical zones of the world. The hemolytic substances produced by K. mikimotoi are thought to cause mortality in fishes and invertebrates. We evaluated the composition of the hemolytic toxin produced by K. mikimotoi cultured in the laboratory using thin-layer chromatography. In addition, we evaluated the effect of co-occuring algae （Prorocentrum donghaiense and Alexandrium tamarense） and the cladoceran grazer Moina mongoliea on hemolytic toxin production in K. mikimotoi. The hemolytic toxins from K. mikimotoi were a mixture of 2 liposaccharides and 1 lipid. Waterborne clues from P. donghaiense and A. tamarense inhibited the growth of K. mikimotoi but increased the production of hemolytic toxins. Conversely, K. mikimotoi strongly inhibited the growth of caged P. donghaiense and A. tamarense. In addition, the ingestion of K. mikimotoi by M. mongolica induced the production of hemolytic toxins in K. mikimotoi. Taken together, our results suggest that the presence of other microalgae and grazers may be as important as environmental factors for controlling the production of hemolytic substances. K. mikimotoi secreted allelochemicals other than unstable fatty acids with hemolytic activity. The production of hemolytic toxins in dinofiagellates was not only dependent on resource availability, but also on the risk of predation. Hemolytic toxins likely play an important role as chemical deterrents secreted by K. mikimotoi.