Sub-cellular partitioning of metals (Cd, Cu, Zn) in the gills of a freshwater bivalve, Pyganodon grandis: role of calcium concretions in metal sequestration

Cytosolic Cd concentrations were significantly related to the dissolved Cd concentrations at each site, but cytosolic Cu and Zn (essential metals) were not related to their respective ambient dissolved metal concentrations. Metallothionein concentrations increased along the metal contamination gradient and were related to cytosolic Cd (and Zn) in a concentration-dependent manner. However mass balance calculations showed that binding to metallothionein could only account for a small proportion of total gill metal (∼10% Cd; ∼3% Cu; ∼1% Zn). Under these chronic exposure conditions, the three metals (Cd, Cu and Zn) were mainly located in calcium concretions present in the gills (respectively 58 ± 13% of the total gill Cd, 64 ± 6% of the total gill Cu and 73 ± 6% of the total gill Zn). The overall contribution of granules to the total gill dry weight remained relatively constant among the different lakes, suggesting that lake-to-lake variations in granule synthesis were independent of the metal contamination gradient, i.e., these constituent elements of unionid gills act as non-inducible metal sinks at the cellular level. Metal concentrations increased proportionally in both the granules and the MT pool along the polymetallic gradient, suggesting a constant partitioning between these two compartments. Overall, despite an increase in Cd in the “mitochondria” fraction, metal sequestration mechanisms seem to be reasonably effective in detoxifying cadmium: in the cytosol, Cd concentrations in the potentially metal-sensitive HDP fraction remained relatively low and constant, even in specimens collected from the most contaminated lakes.