We report on (i) in vitro growth conditions for different growth stages of Heterosigma akashiwo and (ii) isolation, purification and structural investigation of the exopolysaccharide fraction (EXO). Cell polysaccharide fractions were analyzed for chemical composition, and we performed a comparison of the lipid components between cultures subjected to either a light/dark cycle of 12 h/12 h or 24 h of light. Man, Rha and Gal were the main monosaccharides in the EXO fraction. Following the purification step, the EXO fraction yielded EXO1.0 and EXO1.5 fractions. A linkage analysis of the EXO1.0 fraction indicated the presence of a highly branched polysaccharide with a (1→4)-linked mannan backbone, and analysis of EXO1.5 indicated the presence of a complex rhamnan consisting mainly of 2- and 4-linked Rha units with the non-reducing end of Gal and Rha. In cultures subjected to a 12 h/12 h light/dark cycle, palmitic and myristic acids were the most abundant lipids, and no polyunsaturated acid was detected. However, in cultures subjected to 24 h of light, no myristic acid was detected, and stearic and palmitic acids were the most abundant fatty acids. A polyunsaturated fatty acid and fatty acids with 20 or more carbon atoms also occurred in this fraction.
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