Inhibitory Effects And The Mechanism Of The Marine Algae Enteromorpha Intestinalis On The Growth Of The Red Tide Microalgae

In recent decades, harmful algal blooms （HABs） have occurred frequently in the world and havebecome one of the hot topics associated with marine ecological research. They caused negative effectsdirectly or indirectly on the marine environment, fisheries resources and aquaculture. Early studies havebeen focused on the control of HAB algae by use of clay, hydroxyl radical, and biological agents. Althoughthese control methods seem effective, they might have dangerous sconsequences to the environment. Inthis study, we reported the inhibitory effects and possible mechanism of the E. intestinalis fresh tissue andtheir water phase extracts on the growth of the red tide microalgae Prorocentrum micans and Heterosigmaakashiwo.1. The inhibition effect of fresh tissues and water extracts of U. intestinalis were tested for their algicidalactivities against Prorocenrum micans and Heterosigma akashiwo. The results indicated that fresh tissuesand water extracts can inhibit the growth of red tide microalgae, and the inhibition activities increasedalong with the concentration increased. The concentrations of t fresh tissues and water extracts respectivelyachieved above16mg/mL and30μg/mL could kill the Heterosigma akashiwo. And the rate of inhibition ofP. micans reached89.2%and87.5%.2. The inhibitory effects of the marine green macroalgae Enteromorpha intestinalis on thegrowth of the red tide microalgae Prorocentrum micans were studied and the possible algicidalmechanisms was investigated by analysis of the activities of antioxidant enzymes, alteration ofmembrane permeability and ultrstructure of P. micans cells. Results indicated that the growth of P.micans was significantly inhibited by high concentrations of water phase extracts of E. intestinalis.The IC₅₀of the water phase extracts of E. intestinalis on P. micans for six days was0.017mg·mL^-1.The activity of antioxidant enzymes such as superoxide dismutase, catalase and glutathioneperoxidase significantly elevated at4h, and decreased gradually until24h, and finally lower thanthat of the control group at36h. The level of malondialdehyde was significantly elevatedindicating lipid peroxidation and oxidation stress by the extracts of E. intestinalis treatment. Themembrane permeability was altered by the extracts of E. intestinalis treatment as the intracellularconcentration of Na⁺, K⁺, Mg²⁺and Ca²⁺decreased significantly after36h treatment. The cellularorganelles including chloroplasts and Golgi complex were seriously damaged. Clearly, thephysiological and morphological changes in the P. micans cells induced by the co-culture with theE. intestinalis fresh tissues or water phase extracts caused the death of the red tide microalgae P.micans.3. The inhibitory effects of α-linolenic acid on the red tide microalgae Heterosigma akashiwo were studied and the possible algicidal mechanism was investigated through the membrane permeability,antioxidant enzymes, and photosynthesis of the microalgae cells. Results showed that the growth of H.akashiwo was inhibited obviously by high concentrations of α-linolenic acid. The seven days’ IC₅₀was2.4μL·L^-1. The membrane permeability was destroyed by the α-linolenic acid treatment with the intracellularconcentration of Na⁺, K⁺, Mg²⁺and Ca²⁺decreased significantly after36h. The soluble protein content ofalgal cells decreased during the experimental process with the antioxidant enzymes superoxidedismutase （SOD）, peroxidase （POD） and catalase （CAT）content elevated significantly at8h, thendecreased gradually until24h, and finally lower than the control group at36h. The malondialdehyde（MDA） content indicated the lipid peroxidation stress and the membrane damage degree by theα-linolenic acid treatment. It significantly elevated at12h, and then decreased gradually until36h. Fv/Fm, φPS Ⅱ, qP and NPQ have a significant impact,.NPQ has increased significantly, and others weresignificantly decreased, the decline also increases as the time long. The results of this study revealed thatthe α-linolenic acid could destroy the structure of red tide algal cells by changing the cell membranepermeability and free radical reactions, and in this way the inhibitory effects on the algae were achieved.