Growth competition between toxic dinoflagellate, Alexandrium, and diatom.

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Bialgal cultures between A.tamarense and diatom such as Thalassiosira, Cheatoceros, or Asterionella showed the growth inhibition to each other. Adegree of inhibition depended upon proportion of cell density of the species mixed. On the other hand, different growth pattern was observed in the bialgal culture between A.tamarense and Skeletonema costatum. In this case, both species did not show significant growth inhibition to each other even when the relative cell density is significantly different. These results well coincided with species succession pattern observed in Ofunato Bay, indicating that these growth competition is one of the important factors which control a composition of phytoplankton community.Environmental factors such as pH,light intensity and nutrient concentration were monitored during the process of bialgal culture. However, no clear relationship was observed between change of these factor and growth competition, indicating that growth inhibition is caused by other m … More echanism. On the other hand, the cultured medium of A.tamarense clearly inhibit the growth of Thalassiosira sp., showing that some substance (s) secreted from A.tamarense cells involves in extinction of Thalassiosira observed in bialgal culture described above. Growth of A.tamarense was also suppressed by the cultured medium of Thalassiosira and Cheatocers, whereas no effect was observed in the case of Skeletonema. These results suggest that allelochemic substance plays anpimportant role in growth competition between A.tamarense and diatoms. In natural environment, biomass of diatoms is overwhelming to compare that of A.tamarense. Therefore, growth inhibition by diatoms to A.tamarense seems to be more crucial for controlling a species composition. Allelochemic substance of A.tamarense to Thalassiosira was characterized as lipid soluble substance. Substance of Thalassiosira showing allelopathy effect on A.tamarnese was also lipid soluble one. Chromatographic behavior revealed that the substance is a kind of hydrocarbon. Less