Allelopathic effects of fresh tissue and dry powder of two bloom-forming green macroalgae Ulva pertusa and Ulva prolifera on microalga Prorocentrum donghaiensewere evaluated at different concentrations. The filtrates of macroalga culture medium were investigated for their inhibitory effect on P. donghaiense using initial or semi-continuous filtrate addition. Preliminary results using the extracts prepared with four solvents confirmed that allelochemicals were present in the tissue of the two macroalgae. In addition, the effects of the two macroalgae on the microalgae were tested. The results show that the growth of P. donghaiense was strongly inhibited by fresh tissue and dry powder of U. pertusa and U. Prolifera, and that the culture medium filtrates of the two macroalgae exhibited an apparent growth inhibition effect under initial or semi-continuous addition, which suggested that the cells of P. donghaiense are sensitive to the allelochemicals. Methanol extracts of the macroalgae were found to strongly inhibit the growth of P. donghaiense, while extracts made with three other organic solvents (acetone, ether and chloroform) did not. This suggests that the allelochemicals might be relatively high polar compounds. The methanol extracts were further partitioned using petroleum ether, ethyl acetate, butanol and distilled water by liquid-liquid fractionation. Bioassays of each fraction were carried out on activity against P. donghaiense, and the results indicated that the extracts in petroleum ether and ethyl acetate phases had strong algicidal activity.
Key words: Allelopathy, Ulva pertusa, Ulva Prolifera, macroalgae, red tide microalgae,Prorocentrum donghaiense, microcosm experiment, extracts.
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