Composition and Calcium Binding Properties of the Water Soluble Polysaccharides in the Calcareous Alga Halimeda opuntia (L) (Chlorophyta, Udoteaceae)

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Abstract
The mucilages of this alga contain acid polysaccharides, some of which are Ca binding. After deproteinization and gel filtration the residue of the aqueous algal extract has the composition: 2% complexed protein, 3–4% Ca, 0.4–0.5% SiO2, 0.03–0.04% Al, ∼ 0.005% Fe, rest: polysaccharide. The molecular weights of the major and minor polysaccharide fractions are approximately 5000 and 200000 respectively (Sephadex). Uronic acid and O‐ester sulfate give a total of 0.25 μequiv. H+/mg polysaccharide whereas the Ca binding capacity is 2.1 μequiv./mg polysaccharide. Ca complexing of the dibasic polyacid is restricted to the low molecular species and involves the 2 acidic and unidentified nonacidic groups. The complexing strength of the polysaccharide for Ca is similar to succinate. These Ca binding polysaccharides may play a role in algal calcification.