Allelopathic effect shown by seawater from sargassum horneri bed on the growth of skeletonema costatum

The necessary condition for bioassay of allelopathic effect using microalgae was investigated. It was found that nutrient condition of the sample seawater did not affect the growth of microalgae for a certain time when the bioassay was conducted by using microalgae collected during stationary growth phase in preculture. This finding is useful for bioassay of allelopathic effect which needs to distinguish the growth inhibition by allelopathic effect from that by nutrient condition associated with the sample. Although the time for bioassay enabling to neglect the nutrient condition of the sample was different from microalgae species, such growth characteristics were confirmed for three kinds of microalgae, Skeletonema costatum, Heterosigma akashiwo and Chattonella antiqua. A bioassay of allelopathic effect was tested for seawater obtained from inside and outside of seaweed bed of Sargassum horneri at Matsushima bay in Japan using S. costatum. Although the growth of S. costatum was not inhibited by seawater from outside of the seaweed bed, a significant inhibition effect was observed for seawater from inside of the seaweed bed. Moreover, it was found that the growth inhibitory effect of seawater from inside of the seaweed bed was disappeared by heat treatment. Similar results were also obtained for culture supernatant of S. horneri. These results indicate the excretion of allelochemicals by S. horneri, which inhibits microalgal growth in aquatic environment.