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We studied the effects of fresh tissue and culture medium filtrate of two species of macroalgae, Ulva pertusa (Chlorophyta) and Gracilaria lemaneiformis (Rhodophyta) on growth of Heterosigma akashiwo (Raphidophyceae) in co-culture. Both U. pertusa and G. lemaneiformis, and especially their fresh tissues, significantly impede the growth of H. akashiwo. Carbonate limitations and the presence of environment bacteria are not necessary for the negative effects of macroalgal on H. akashiwo. The simultaneous nutrient assays show that nitrate and phosphate are almost exhausted in the G. lemaneiformis co-culture system, but remain at acceptable levels in the U. pertusa system, when all cells of H. akashiwo are completely dead. When f/2 medium is supplied daily to G. lemaneiformis culture, the growth of H. akashiwo is greatly inhibited but not completely terminated. Furthermore, different amounts of fresh seaweed tissue, and culture medium filtrate prepared from different macroalgal concentrations are analyzed to determine their effects on the growth of H. akashiwo. The results show a positive correlation between the initial macroalgal concentration and the negative effects they exert on the co-cultured microalgae. Results suggest that the allelopathic effects of U. pertusa may be essential for negative effects on H. akashiwo; however, the combined roles of allelopathy and nutrient competition may be responsible for the negative effect of G. lemaneiformis the release of allelochemicals by U. pertusa. |
