Effects of calcium on the uptake and elimination of cadmium and zinc in asiatic clams

We examined the effects of Ca, one of the major ions contributing to water hardness, on the uptake and elimination of Cd and Zn in the Asiatic clam Corbicula fluminea, a common bivalve species in the rivers and streams of Southern China. Over the wide range of dissolved Cd (4.5 to 446 nM) and Zn (15.3 to 1529 nM) concentrations, uptake of both metals increased with increasing dissolved metal concentration, showing Michaelis-Menten-type saturation kinetics. At each tested metal concentration, increased Ca concentration led to a significantly lower metal uptake. When the Ca, Cd, Zn concentrations were expressed as free-ion activities, Cd and Zn uptake data fitted the Michaelis-Menten inhibition model well. We also quantified the effects of the two Ca blockers on metal uptake by the clams. Verapamil significantly inhibited the uptake Cd and Zn, but the inhibitive effect of lanthanum on metal uptake was more evident for Cd than for Zn. Ca did not significantly affect the assimilation efficiency of either metal from ingested phytoplankton, nor did it affect the elimination of the two metals during a 1-month depuration period. Our study has shown that Ca inhibited the uptake of metals from water; such an effect could be predicted based on the free-metal ion activities. Ca did not seem to have a direct effect on metal assimilation from food or metal elimination from the bivalve.