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.