TRACE-METALS IN THE MARINE BIVALVE MACOMA-BALTHICA IN THE WESTERSCHELDE ESTUARY, THE NETHERLANDS .3. VARIABILITY OF THE ROLE OF CYTOSOL IN METAL UPTAKE BY THE CLAMS

Tellinid clams Macoma balthica were sampled every 2 months for 2.5 yea rs at two locations from the Westerschelde estuary (The Netherlands) a nd submitted to in vitro short-term exposure to Cd, Cu and Zn. Total a nd heat-stable cytosolic metal concentrations have been measured befor e and after exposure to study the effects of the sampling season on me tal uptake by the bivalve. We observed much higher uptakes of Cd and, to a lesser extent, Cu in winter than in summer, while Zn appears to b e constantly regulated. These phenomena are amplified in the cytosol, especially for Cd, a non-essential element, for which the concentratio ns after exposure can reach 3.5-4 mu g . g(-1) (dry wt.) in winter, bu t only a maximum of 0.5 mu g . g(-1) in summer, all natural values bei ng between 0.01 and 0.1 mu g . g(-1) on average. The contents (mu g) o f the various elements, either at the total or at the cytosolic level, are not constant, hence showing that the seasonal body weight fluctua tion of the clams (dilution or concentration effect) is not the only p arameter responsible for the metal uptake variability. Furthermore, we have shown that the proportion of cytosolic metal is not constant, bu t increases with the absolute cytosolic concentration up to 35-40%. Th erefore, the cytosolic phase of the cells plays a growing role in Cd, Cu and Zn storage as long as their total concentration increases.