Allelopathic interactions between the bacillariophyte Skeletonema costatum and the raphidophyte Heterosigma akashiwo

We investigated growth interactions between Skeletonema costatum (Greville) Cleve and Heterosigma akashiwo (Hada) Hada ex Hara et Chihara using bi-algal cultures under axenic conditions. When inoculated at high cell densities, growth of both species was coincidentally suppressed. In other combinations of inoculation density, the species first reaching stationary phase substantially reduced maximum cell densities of the other species. When cultured together under conditions without cell contact, growth of S. costatum and H. akashiwo were both suppressed. Furthermore, despite re-enrichment with nutrients, filtrates from dense cultures of S. costatum and H. akashiwo reciprocally reduced their maximum cell densities. In additional experiments, growth of Chaetoceros muelleri was also suppressed with filtrates from the above cultures, but growth of Prorocentrum minimum was not. Therefore, growth interactions between these species strongly suggest the involvement of allelopathic substances secreted by both species. Finally, growth and interaction of S. costatum and H. akashiwo in bi-algal cultures were simulated using a mathematical model. This model indicated that S. costatum and H. akashiwo steadily approach a stable equilibrium point of about 3.4 × 10⁵ cells ml^-1 and 4.8 × 10⁵ cells ml^-1, respectively, when the 2 species coexist.