Allelopathic activity of exometabolites excreted by different algal species produced the competitive interactions may have led to adaptations enabling bloom-forming phytoplankton to dominate pelagic communities. Therefore, in vitro study was performed on the extracellular products (protein, carbohydrate, polysaccharides, phenolic compounds, alkaloids and fatty acids) during exponential and decline growth phase of the marine diatom Skeletonema costatum on the growth of Nannochloropsis oculata and Tetraselmis chuii. The results showed that the culture filtrate of decline growth phase contains higher protein, carbohydrate, polysaccharides, phenolic compounds and alkaloids than exponential growth phase. The results of fatty acids analysis showed the presence of palmitic acid (48.73 and 23.39 mg.ml-1) and linoleic acid (35.29 and 46.48 mg.ml-1), respectively during exponential and decline phase. These results suggest that the metabolites released from Sk. costatum in both growth phases would be the target of allelopathic inhibition of the growth of the two tested algae with different degrees. Therefore, future investigations on the characterization of the chemical compounds involved in the allelopathic process is needed to advance the study of the mode of action of phytoplankton allelochemicals in mediating phytoplankton interactions.
Key words: Skeletonema costatum, Allelopathy, culture filtrate, Phenolic compound, Fatty acids
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