STRUCTURE AND ION-EXCHANGE PROPERTIES OF THE CELL-WALLS OF BROWN-ALGAE - ECOPHYSIOLOGICAL SIGNIFICANCE

The cell walls of marine algae differ from land plant cell walls in their relative abundance of negatively charged matrix polysaccharides. Moreover, cell walls of most marine plants - including higher plants - contain sulfated polysaccharides whereas cell walls of land plants - including freshwater algae - do not. This suggests that sulfated polysaccharides play specific physiological functions in the intertidal or marine environment. It is commonly assumed that they are involved in hydric or ionic cell regulation. Our approach to assess the role of sulfated polysaccharides in ion regulation was to study the chemical composition and the cation-exchange properties of isolated cell walls from eight intertidal brown algae: Pelvetia canaliculata, Fucus spiralis, F. ceranoides, F. vesiculosus, Ascophyllum nodosum, F. serratus, Bifurcaria bifurcata (Fucales) and Laminaria digitata (Laminariales). These algae are regularly distributed throughout the intertidal zone. Their walls are mainly carboxylated or sulfated polysaccharides, i.e. alginates and fucans, respectively. A clear correlation exists between their zonation and their wall chemical composition: the higher the species on the shore, the higher is its content of sulfated fucans. We review here the cation-binding properties of cell walls isolated from the above species and equilibrated against seawater at various salinities or in the presence of various mixtures of calcium and sodium chloride. No significant differences were observed in the ion-exchange behaviour of the isolated cell walls of the various species under investigation. The examination of the discrimination of calcium between alginate and fucans, however, suggests the existence of a gradient of calcium distribution throughout the cell wall, with higher proportions of calcium bound to the inner wall layers. The physiological significance of such a gradient in relation to the marine or intertidal environment and the adaptative role of cell walls of brown algae in ionic regulation are discussed.