Cadmium replaces calcium in the cell wall of Ulva lactuca

Electron microscopy, in conjunction with X-ray microanalysis, was used to investigate the effects of exposure to cadmium on the elemental composition of the macroalga Ulva lactuca. The cell wall was the only region of the cell to show any marked change in chemical composition as a result of exposure to cadmium, with less calcium evident in cadmium-treated thallus compared with untreated thalli. The cell wall of U. lactuca is a complex structure made up of polysaccharides consisting of many-branched chains composed mostly of rhamnose and galactose subunits. Some of the hydroxyl groups on the subunits are substituted by sulphate groups. Berate is associated with the rhamnose subunits, which contain no sulphate groups, and calcium binds to berate, cross-linking the rhamnose groups. The berate-calcium complex adds rigidity to the cell wall; the replacement of calcium by cadmium will, therefore, influence the rigidity of the thallus. The ecological significance of this work is discussed with respect to the ability of the alga to withstand grazing or emersion.