Photosensitizing hemolytic toxin in Heterocapsa circularisquama, a newly identified harmful red tide dinoflagellate

Red tides of Heterocapsa circularisquama (H. circularisquama), recently identified as a novel species of dinoflagellate, have frequently caused mass mortality of several species of bivalves in Japan, while no harmful effects of this flagellate on fish have been reported so far. We found that the cell-free ethanol extract prepared from H. circularisquama caused hemolysis of rabbit erythrocytes and demonstrated cytotoxic effects in HeLa cells and on the microzooplankton rotifer (B. plicatilis) in a dose- and time-dependent manner. Interestingly, the hemolytic activity and cytotoxic effects of the extract were completely dependent on the presence of light. When the experiments were conducted in the dark, no hemolysis was observed even at very high concentration of the extract. These results suggest that H. circularisquama has photosensitizing hemolytic toxin which can be easily extracted into ethanol. This may be the first report documenting the occurrence of photosensitizing hemolytic toxin in marine phytoplankton species.