Effect of Alexandrium tamarense on three bloom-forming algae

We investigated the allelopathic properties of Alexandrium tamarense（Laboar） Balech on the growth of Prorocentrum donghaiense Lu,Chattonella marina（Subrahmanyan） Hara et Chihara and Heterosigma akashiwo（Hada） Hada in a laboratory experiment.We examined the growth of A.tamarense,C.marina,P.donghaiense and H.Akashiwo in co-cultures and the effect of filtrates from A.tamarense cultures in various growth phases,on the three harmful algal bloom（HAB）-forming algae.In co-cultures with A.tamarense,both C.marina and H.akashiwo were dramatically suppressed at high cell densities; in contrast,the growth of P.donghaiense varied in different inoculative ratios of A.tamarense and P.donghaiense.When the ratio was 1：1（P.donghaiense：A.tamarense）,growth of P.donghaiense was inhibited considerably,while the growth of P.donghaiense was almost the same as that of the control when the ratio was 9：1.The growth difference of P.donghaiense,C.marina and H.akashiwo when co-cultured with A.tamarense indicated that the allelopathic effect may be one of the important factors in algal competition and phytoplankton succession involving A.tamarense.In addition,the filtrate from A.tamarense culture had negative impacts on these three HAB algae,and such inhibition varied with different growth phases of A.tamarense in parallel with reported values of PSP toxin content in Alexandrium cells.This implied that PSP toxin was possibly involved in allelopathy of A.tamarense.However,the rapid decomposition and inactivation of PSP toxin above pH7 weakened this possibility.Further studies on the allelochemicals responsible for the allelopathy of A.tamarense need to be carried out in future.