Antialgal Substances from Isochrysis galbana and Its Effects on the Growth of Isochrysis galbana and Six Species of Feed Microalgae

Antialgal substances produced by Isochrysis galbana affects the growth of other micro-algal cells. In this study, the productions of antialgal substances from Isochrysis galbana and their possible chemical structures were elucidated. The effects of antialgal substances on the growth of Isochrysis galbana and other microalgae (Chaetoceros muelleri, Chaetoceros gracilis, Phaeodactylum tricornutum, Nitzschia closterium, Platymonas elliptica and Dunaliella sauna) were studied. The medium for culturing Isochrysis galbana was determined initially and then crude antialgal substances (CEAE) were extracted using ethyl acetate. Subsequently, fractionated using Sephadex G-15 gel filtration chromatography and analyzed by gas chromatography. The contents of the fatty acids were determined by using the normalization method. The results indicated that as Isochrysis galbana culture ages, the concentration of the antialgal substances in the culture medium increases, suggesting that Isochrysis galbana gradually produces antialgal substances from the exponential growth phase to the death phase. The growth inhibitory rates of Isochrysis galbana by CEAE during the three growth phases were 7.17%, 34.2% and 64.1%, respectively. The CEAE showed significant growth inhibition to Isochrysis galbana and the six species of feed microalgae. At the concentration of 21.6 mg/L, the growth inhibitory rate of Isochrysis galbana, Chaetoceros muelleri, Chaetoceros gracilis, Phaeodactylum tricornutum, Nitzschia closterium, Platymonas elliptica and Dunaliella salina by CEAE was 67.2%, 63.3%, 59.6%, 61.1%, 66.2%, 68.3% and 48.5%, respectively. Further investigation found that chlorophyll content significantly decreased in the cells of all tested microalgae, which indicated the chlorophyll synthesis was inhibited by CEAE. The CEAE at 21.6 mg/L, significantly inhibited the protein synthesis in the cells of Isochrysis galbana, Chaetoceros muelleri, Chaetoceros gracilis, Phaeodactylum tricornutum and Nitzschia closterium approximately by 49%. Of the three fractions, F- I and F- II showed inhibitory activity against the growth of the six species of microalgae, but the inhibition activity of was lower. The fatty acids in CEAE, F- I and F- II were stearic acid (C18:0) and oleic acid (C18:1), respectively. Results suggested that stearic acid (C18:0) or oleic acid (C18:1) or the mixture of both stearic acid (C18:0) and oleic acid (C18:1) probably was the substance that could inhibit the growth of the six species of microalgae.