The allelopathic effects of Enteromorpha linza on Heterosigma akashiwo

Harmful algal blooms(HABs) of the coastal raphidophyte Heterosigma akashiwo havebeen the focus of many studies,primarily due to the devastating effects this alga has had on the aquaculture industry.Heterosigma akashiwo is notorious for ichthyotoxicity and widely distributed in the world.Heterosigma akashiwo has caused great economic loss in many countries.Allelopathy is a prevalent natural phenomenon in aquatic ecosystems and also a means of biological control of water weeds.This paper studies the allelopathic effects of the macroalga Enteromorpha linza on the microalga Heterosigma akashiwo,in order to provide some theoretical proof for biological control of harmful algae blooms.Allelopathic effects of fresh tissue and dry powder of Enteromorpha linza on Heterosigma akashiwo were studied using coexistence culture systems.Different initial inoculation concentrati-ons(0,0.625,1.25,2.5,5 and 10 g wet/L) of macroalga fresh tissue and(0,0.15,0.3,0.6,1.2 and 2.4 g dry/L) of macroalga dry powder were used in both coexistence culture systems.The results of the coexistence assays showed that the growth of Heterosigma akashiwo was strongly inhibited by fresh tissue and by dry powder of Enteromorpha linza.The cells of Heterosigma akashiwo died completely within five days at the two highest concentrations(5 and 10 g wet/L) of macroalga fresh tissue.At a threshold level of 1.2 g dry/L of dry powder of Enteromorpha linza,H.akashiwo cells were completely killed when their initial density was lower than 1×10~5 cells/ml.The effects of the macroalga culture medium filtrate on the Heterosigma akashiwo were also investigated to confirm the existence of allelochemicals.The growth of H.akashiwo was not significantly(p(0.05)) lowered by macroalga culture medium filtrate under initial filtrate addition.On the contrary,the growth of H.akashiwo was strongly inhibited by macroalga culture medium filtrate under semicontinuous filtrate addition.It was speculated that continuous addition of allelochemicals was important to effectively control the growth of H.akashiwo.Heterosigma akashiwo was not inhibited when inoculated in the boiled macroalga culture filtrate,which suggested that allelochemicals from the fresh tissue of Enteromorpha linza were unstable and degradable at higher temperature.These assays prove that Enteromorpha linza has allelopathic effects on(H.akashiwo.) These results not only provide insight into the interactions between macroalgae and microalgae in coastal areas but also lead us to isolate and characterize these allelopathic substances in future.