The present study investigated the potential allelopathic effects of the living green marine macroalga Ulva fasciata growth of several marine microalgae. Skeletonema costatum (Bacillariophyta, centric diatom), Nitzschia longissima (pennate diatom), Scrippsiella trochoidea & Alexandrium minutum (Dinophyta), and Heterosigma akashiwo (Raphidophyta) all being previously identified as bloom forming species in Alexandria coastal water, Egypt. The results showed reduction of microalgal growth as a result of treatment with fresh thalli compared to the respective control treatments. In addition, macroalgal growth increased with the increasing of treatment time in a dependent way. The degree of dominance and concentrations of fatty acids oscillated in all treatments. The fatty acids composition showed 11 pattern of Fatty acids (9 saturated& 2 unsaturated). The major saturated acids arranged by decreasing order are C15.0, C14.0, C16.0 and C13.0.
Key words: Allelopathic, Ulva fasciata, blooms, Skeletonema costatum, Nitzschia longissima, Scrippsiella trochoidea, Alexandrium minutum fatty acids
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