In order to elucidate the production of an autoinhibitor by Skektonema costatum and its producing phase, four phases in the growth process were selected. The autoinhibitor was extracted from S. costatum and its cultured medium at each phase with ethyl acetate and subjected to the growth inhibition test for S. costatum. The autoinhibitor was produced gradually from the exponential growth phase of S. costatum and mostly accumulated in the cells up to the stationary phase. However, the inhibitor in the dying or dead cells was rapidly released into the medium, resulting in the increase in the concentration of the inhibitor in the medium. After almost all the cells died, the large portion of the inhibitor produced by S. costatum was found in the medium. This autoinhibitor also prevented the growth of other diatoms and Chattonella, but it did not show any inhibitory effect on the growth of dinoflagellates.
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