Effect of salinity on interspecific competition between the dinoflagellate Alexandrium catenella and the raphidophyte Heterosigma akashiwo

We investigated the effect of salinity on interspecific competition between the dinoflagellate Alexandrium catenella and the raphidophyte Heterosigma akashiwo using bi-algal cultures under several culture conditions. When initial cell densities of A. catenella and H. akashiwo were both 1 × 10² cells ml.1 at salinity 30, the growth of H. akashiwo was strongly suppressed by A. catenella, and no intact H. akashiwo cells were observed. In contrast, the growth of A. catenella was suppressed when initial cell densities of A. catenella and H. akashiwo were 1 × 10² and 1 × 10⁴ cells ml^-1, respectively. Interestingly, the growth of A. catenella was notably suppressed in bi-algal cultures at salinity 20 when initial cell densities of A. catenella and H. akashiwo were both 1 × 10² cells ml^-1. In addition, growth of H. akashiwo in bi-algal cultures with A. catenella under cell-contact conditions was significantly inhibited, whereas growth of H. akashiwo was not inhibited by A. catenella in non-contact culture conditions. Furthermore, live-cell-mediated hemolytic activity of A. catenella was detected, but no hemolytic activity was detected in A. catenella culture filtrate. In contrast, growth of A. catenella was significantly inhibited by 50 to 100 μg ml^-1 of a crude extract, including allelochemicals, from H. akashiwo culture filtrate. Therefore, salinity and interspecific interactions via allelopathy and/or cell contact might be important factors deter - mining the growth dynamics of A. catenella and H. akashiwo in the field.