Sou Nagasoe

Extracellular polysaccharide-protein complexes of a harmful alga mediate the allelopathic control it exerts within the phytoplankton community.

The goal of this study was to examine the significance of allelopathy by the raphidophyte Heterosigma akashiwo in a multispecies phytoplankton community in the field. Towards this aim, we sought allelochemicals of H. akashiwo, which had allelopathic effect both in laboratory experiments and in the field. As an initial step, we showed that the allelopathic effects of H. akashiwo filtrate were both species-specific and dependent upon the cell density of the target species. Secondly, we found for the first time that extracellular, high-molecular weight allelochemicals [that is, polysaccharide-protein complexes (APPCs)] were produced by a marine phytoplankton species, H. akashiwo. Thirdly, we indicated that the purified APPCs selectively inhibited the growth of the diatom Skeletonema costatum that is a major competitor of H. akashiwo, and thereby tended to promote the formation of monospecific H. akashiwo blooms. Furthermore, we demonstrated that the inhibitory effect of APPCs on the growth of the diatoms was determined by binding to the cell surface of the target species. Finally, we succeeded in the detection of APPCs in the field samples at concentrations exceeding their experimentally determined action threshold during the H. akashiwo bloom. Strategies for ecosystem control, including mitigation of harmful algal blooms (HABs), should take into account that red-tide organisms like H. akashiwo are already part of complex webs involving inter-specific allelopathic inhibition and ecosystem control during their dense blooms.

Growth inhibition of Gyrodinium instriatum (Dinophyceae) by Skeletonema costatum (Bacillariophyceae)

Interactions between the red tide-forming dinoflagellate Gyrodinium instriatum Freudenthl et Lee and the diatom Skeletonema costatum (Greville) Cleve were investigated in bialgal cultures. The growth rates and maximum cell densities of G. instriatum decreased dramatically in cultures containing S. costatum. In addition, morphologically abnormal cells of G. instriatum were observed in these cultures, and many cells lost their motility. Conversely, the growth of S. costatum was generally not influenced by the presence of G. instriatum, although small etiolated cells of S. costatum were occasionally observed when the initial cell density of G. instriatum was high. The growth of G. instriatum and S. costatum in the bialgal cultures was simulated using a mathematical model. This interactive model indicated that G. instriatum and S. costatum steadily approach a stable equilibrium point of about 5 × 103 cells ml−1 and 12 × 105 cells ml−1 respectively, when the two species coexist.

The role of interactions between Prorocentrum minimum and Heterosigma akashiwo in bloom formation

We examined the growth and interactions between the bloom-forming flagellates Prorocentrum minimum and Heterosigma akashiwo using bi-algal culture experiments. When both species were inoculated at high cell densities, growth of H. akashiwo was inhibited by P. minimum. In other combinations of inoculation densities, the species first reaching the stationary phase substantially suppressed maximum cell densities of the other species, but the growth inhibition effect of P. minimum was stronger than that of H. akashiwo. We used a mathematical model to simulate growth and interactions of P. minimum and H. akashiwo in bi-algal cultures. The model indicated that P. minimum always out-competed H. akashiwo over time. Additional experiments showed that crude extracts from P. minimum and H. akashiwo cultures did not affect the growth of either species, but both strongly inhibited the growth of the bloom-forming diatom Skeletonema costatum. Further experiments showed that it was unlikely that reactive oxygen species produced by H. akashiwo were responsible for the inhibition of P. minimum growth.