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

Specimens of the Asiatic clam Corbicula fluminea were transplanted from a clean lacustrine site to four stations along a polymetallic pollution gradient in the river Lot, France, downstream from an old Zn ore treatment facility (see Part I). From April to September 1996, we studied Cd and Zn bioaccumulation and the metallothionein‐like metal‐binding protein (MT) concentrations 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 station Riou‐Mort, MT concentrations did not increase despite very rapid metal accumulation; all mollusks died between days 49 and 85, suggesting that the 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 case the MT concentrations also increased progressively with positive correlations 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 (as a stress response) and reduced growth. Subcellular metal partitioning, as determined by size‐exclusion chromatography, revealed that most of the Cd was sequestered by MT (70% of cytosolic Cd). In contrast, most of the Zn was bound to low molecular weight proteins (70%, ≤ 6.5 kD), the MT fraction representing only 12% of cytosolic Zn. These data show the marked role of MT toward Cd bioaccumulation and toxic effects on this freshwater bivalve species.