Species specificity and potential roles of Karlodinium micrum toxin

Karlodinium micrum is a toxic mixotrophic dinoflagellate that has been responsible for fish kills in coastal environments worldwide. The role that karlotoxins play in the life history of K. micrum is unknown, but may contribute to its bloom-forming ability. We tested the hypothesis that karlotoxins could inhibit the growth of other protists depending on the sterol composition of target cell membranes. We also examined the effect of toxin addition on feeding rates of K. micrum on a flagellated prey, Storeatula major. Dose-dependent effects of isolated karlotoxin (KmTX2) were tested in growth bioassays (24-48h) of K. micrum, three raphidophytes (Heterosigma akashiwo, Fibrocapsa japonica and Chattonella subsalsa), two cryptophytes (S. major and Pyrenomonas salina), and the dinoflagellates Amphidinium carterae, Pfiesteria piscicida and P. shumwayae. Growth of K. micrum, P salina, A. carterae and P piscicida were not affected by karlotoxin additions up to 1 000ng ml(-1). Other organisms showed growth inhibition at concentrations between 500ng ml(-1) and 1 000ng ml(-1). Predation by K. micrum on S. major was significantly higher in the presence of 25ng ml(-1) KmTX2. The results are consistent with a role for karlotoxin in allelopathic inhibition of competitors and/or prey immobilisation depending on sterol composition.