Cell contact-dependent lethal effect of the dinoflagellate Heterocapsa circularisquama on phytoplankton–phytoplankton interactions

We used bi-algal culture experiments to investigate and verify the roles of growth interaction between Heterocapsa circularisquama and Prorocentrum dentatum in monospecific bloom formation. Growth of H. circularisquama was slightly inhibited when inoculated at 102 cells mL–1 along with P. dentatum at 104 cells mL–1. In other combinations of inoculation densities, P. dentatum density rapidly decreased to extremely low levels in the presence of H. circularisquama. We used a mathematical model to simulate growth and interactions of H. circularisquama and P. dentatum in bi-algal cultures. The model indicates that one species will always inhibit the growth of the other and that the relative initial cell densities of the species are critical in determining the outcome. When cultured together under conditions without cell contact, growth of H. circularisquama and P. dentatum was not inhibited. As with P. dentatum, the growth of Heterosigma akashiwo and Skeletonema costatum was inhibited in intact cell suspensions with H. circularisquama, but a nontoxic species, Heterocapsa triquetra, did not affect the growth of P. dentatum or the other species. Similarly, cell suspensions of H. circularisquama showed hemolytic activity toward rabbit erythrocytes, but those of H. triquetra did not. In addition, the cell-free supernatant of H. circularisquama cultures showed no significant hemolytic activity. These results suggest that H. circularisquama causes lethality in P. dentatum by direct cell contact in which live-cell-mediated hemolytic activity might be a contributing factor.