Field transplantation of the freshwater bivalve Corbicula fluminea along apolymetallic contamination gradient (river Lot, France): II. Metallothionein response to metal exposure

Specimens of the Asiatic clam Corbicula fluminea were transplanted from a c lean lacustrine site to lour stations along a polymetallic pollution gradie nt in the river Lot, France, downstream from an old Zn ore treatment facili ty (sce Part I). From April to September 1996. we studied Cd and Zn bioaccu mulation and the metallothionein-like metal-binding protein (MT) concentrat ions by subsampling the cages at t = 0, 21, 49, 85, 120, and 150 d. Marked differences were observed among the four stations. At the most polluted sta tion Riou-Mort, MT concentrations did not increase despite very rapid metal accumulation; all mollusks died between days 49 and 85, suggesting that th e metal detoxification mechanisms were overwhelmed at this station. At the next station downstream, the final levels of bioaccumulated metal after 150 d were as high as those at the Riou-Mort station (t = 21 d). but in this c ase the MT concentrations also increased progressively with positive correl ations between MT and metal concentrations; no mortality was observed, but a significant growth inhibition was revealed in comparison to the reference site, with a lack of correlation between MT las a stress response) and red uced growth. Subcellular metal partitioning, as determined by size-exclusio n chromatography, revealed that most of the Cd was sequestered by MT (70% o f cytosolic Cd). In contrast, most of the Zn was bound to low molecular wei ght proteins (70% less than or equal to 6.5 kD), the MT fraction representi ng only 12% of cytosolic Zn. These data show the marked role of MT toward C d bioaccumulation and toxic effects on this freshwater bivalve species.