Growth interactions between the raphidophyte Chattonella antiqua and the dinoflagellate Akashiwo sanguinea

We investigated the growth interactions between the raphidophyte Chattonella antiqua and the dinoflagellate Akashiwo sanguinea by using bi-algal cultures under axenic conditions. When initial cell densities of C. antiqua and A. sanguinea both were 1×10² cells ml^-1, C. antiqua grew faster and its growth inhibitory effect on A. sanguinea was stronger than that of A. sanguinea on C. antiqua. In other combinations of inoculation densities, the species that was inoculated at lower cell density (1×10² cells ml^-1) stopped growing soon after the start of the experiments, and cell density of this species gradually decreased after the other species reached early stationary phase. The mathematical model used to simulate the growth and interactions between the two species predicts that C. antiqua and A. sanguinea will approach an unstable equilibrium point of about 2.16×10⁴ cells ml^-1 (C. antiqua) and 1.46×10⁴ cells ml^-1 (A. sanguinea). Thus, one of the species always inhibits the growth of the other and their initial cell densities are critical in determining the successful species in growth competition. When one of these two species was inoculated at the cell density of 2×10³ cells ml^-1 in bi-algal culture experiments under noncontact and contact conditions, its growth inhibitory effect toward the other species was greater under contact conditions than under noncontact conditions. Furthermore, enriched filtrate from dense culture of C. antiqua resulted in both reduced maximum yield and maximum growth rate of A. sanguinea, whereas that of A. sanguinea resulted only in reduced maximum yield of C. antiqua but did not affect its growth rate.