BIOACCUMULATION AND METALLOTHIONEIN RESPONSE IN THE ASIATIC CLAM (CORBICULA-FLUMINEA) AFTER EXPERIMENTAL EXPOSURE TO CADMIUM AND INORGANIC MERCURY

The involvement of metallothioneins (MTs) in cadmium (Cd) and inorgani c mercury (Hg[II]) bioaccumulation by the freshwater bivalve Corbicula fluminea was experimentally investigated after 0, 15, 30, and 45 d of exposure from the water column source. Three levels of contamination were studied for each metal: 0, 5, and 35 mu g Cd/L and 0, 1.45, and 5 mu g Hg/L, with two replicates per condition. Forty eight experimenta l units (EUs) were conducted simultaneously. The mollusks were fed twi ce a week by additions of phytoplanktonic algae. Quantification of MTs was done by Hg-saturation assay, using cold Hg(II). A partial purific ation of these proteins was conducted by gel-filtration chromatography , followed by Cd determinations in the different eluted fractions. Res ults at the whole organism (soft tissues) and organ or tissue group (g ills, mantle, foot, visceral mass) levels show high metal concentratio ns, with a fourfold greater accumulation of inorganic Hg than Cd after 30 d exposure at the same concentration of 5 mu g/L. Gills and viscer al mass were the principal storage compartments. A significant increas e in MT concentrations was revealed in these two organs after exposure to Cd: ratios between the MT concentrations in contaminated and contr ol mollusks were 2.4 and 2.8, respectively, for 5 and 35 mu g Cd/L. Cd burdens in the cytosol and in less than or equal to 18-kDa protein fr actions, similar to purified mammal MTs, correspond to 30 and 14% of t he total Cd accumulated in the whole organisms. No significant increas e in MT biosynthesis was observed after exposure to inorganic Hg, desp ite the high metal concentrations in the organs.