Interactions between the red tide dinoflagellates Heterocapsa circularisquama and Gymnodinium mikimotoi in laboratory culture

Abstract

Abstract Interactions between Heterocapsa circularisquama and Gymnodinium mikimotoi, causative red tide dinoflagellates, were investigated using bialgal cultures. G. mikimotoi was killed by H. circularisquama when the initial cell density of each species was set at 200 cells ml−1. However, cells of H. circularisquama transformed to temporary cysts when the initial cell density of G. mikimotoi was increased to 2000 cells ml−1. Thus the interaction between H. circularisquama and G. mikimotoi was found to be dependent upon the initial cell densities of the two species. Culture filtrate of H. circularisquama induced no inhibitory effect on the growth of G. mikimotoi. Similarly when separated by a membrane filter, G. mikimotoi grew well when cultured with H. circularisquama. G. mikimotoi appear to be killed by cell contact with H. circularisquama. In growth experiments using a culture filtrate of G. mikimotoi and cultures separated by a membrane filter, G. mikimotoi was shown to secrete a substance that inhibited the growth of H. circularisquama. However, the inhibitory effect of the medium was found at higher cell densities of G. mikimotoi than in the bialgal cultures at which the growth of H. circularisquama was suppressed and formed temporary cysts. It is likely that the inhibitory effect of G. mikimotoi on H. circularisquama in the bialgal cultures occurred mainly by direct cell contact. The growth of H. circularisquama and G. mikimotoi in the bialgal cultures was simulated using a mathematical model to quantify the interaction. The degree that G. mikimotoi was inhibited by H. circularisquama was found to be three times larger than the inhibitory effect of G. mikimotoi on H. circularisquama.