Changes in subcellular metal partitioning in the gills of freshwater bivalves (Pyganodon grandis) living along an environmental cadmium gradient

To test the response of the freshwater bivalve Pyganodon grandis to increas ed metal exposure in the field, specimens were collected from 10 lakes loca ted along a known metal gradient in a mining area in northwestern Quebec. T otal gill concentrations of metallothionein (MT) were determined by the Hg- 203 saturation method for molluscs from each lake, and the distribution of Cd among various cytosolic ligands, including MT, was determined by size-ex clusion chromatography. Gill MT concentrations responded to environmental e xposure to Cd but not to Cu or Zn exposure; these spatial variations along the Cd gradient were more important than the seasonal summer variations in gill MT concentrations. Bivalves exposed to concentrations of dissolved fre e Cd2+ higher than similar to 1 nM in the external medium exhibited a marke d increase of Cd in the low relative molecular mass ligand pool, Symptoms o f toxic effects at different levels of biological organization were associa ted with this biochemical anomaly.