Species-specific allelopathic effects of the diatom skeletonema costatum

We examined allelopathic effects of Skeletonema costatum on the growth of Heterosigma akashiwo, a major competitor of S. costatum. Growth of H. akashiwo was inhibited when it was exposed to S. costatum filtrates collected in the stationary and decline growth phases. During the decline phase, the filtrate had the strongest inhibitory effect. Deactivation of the allelopathic effect of S. costatum was not observed when the filtrate was preserved at room temperature for 30 days. Additionally, crude extracts greater than 3,500 Da, obtained by dialysis, did not inhibit the growth of H. akashiwo; that is, S. costatum produces low molecular weight allelochemicals. Furthermore, we examined the allelopathic effects of S. costatum on the growth of other species and determined that these effects were both species-specific and dependent upon the cell density of the target species. It is likely that the allelochemicals produced by S. costatum are chemically stable low molecular weight substances, and the allelopathy of S. costatum has the potential to affect the succession of phytoplankton species during dense blooms of S. costatum and/or after a decline in the bloom of S. costatum.