Allelopathic effects of Alexandrium spp. on Prorocentrum donghaiense

Effects of toxic Alexandrium species on Prorocentrum donghaiense were studied in co-cultures and filtrates in order to shed some light on the competition between the co-occurring harmful algae. The three live cultures of Alexandrium tamarense and Alexandrium minutum negatively affected the growth of P. donghaiense, while only A. minutum was affected by P. donghaiense when they were in 1:3 ratio of cells. Each species was affected by the co-occurring algae as could be observed by microscopy. The allelopathic effects observed in co-culture did not correlate with the measured content of the intracellular PSP toxins but matched the observed hemolytic properties of the culture filtrates, suggesting that certain hemolytic substances other than PSP toxins are responsible for the allelopathic effect of Alexandrium.