TRACE-METALS IN THE MARINE BIVALVE MACOMA-BALTHICA IN THE WESTERSCHELDE ESTUARY, THE NETHERLANDS .2. INTRACELLULAR PARTITIONING OF COPPER, CADMIUM, ZINC AND IRON - VARIATIONS OF THE CYTOPLASMIC METAL CONCENTRATIONS IN NATURAL AND IN IN-VITRO CONTAMINATED CLAMS

The intracellular distribution of copper, cadmium, zinc and iron in th e marine bivalve Macoma balthica has been studied for 18 months at two locations from the Westerschelde estuary. Metal concentrations in the soluble fraction (cytosol) and in the particulate phase of the soft t issues were measured by atomic absorption spectrometry. Considering al l data, heat-stable cytosolic metal accounts for 10-33% of total Cu, a lmost-equal-to 0-28% of total Cd, 7-17% of total Zn and 2-8% of total Fe. The case of non-essential cadmium is extremely interesting since t he average difference in total concentration between the two stations (factor of 4) induces a much more marked average difference in cytosol ic metal (factor of 15). Short term metal exposures carried out in the laboratory show a high sensitivity of this clam to Cd, a moderate sen sitivity to Cu and no uptake of Zn. For the essential elements, the in tracellular partitioning is not significantly affected. On the contrar y, the extra load of cadmium provokes a large increase of the proporti on of cytosolic metal, especially obvious when the natural cadmium con centration is low.