Sub-cellular partitioning of Cd, Cu and Zn in tissues of indigenous unionid bivalves living along a metal exposure gradient and links to metal-induced effects

We studied organ and sub-cellular distributions of several trace metals in a freshwater bivalve that has been proposed for use as a metal biomonitor. Specimens of Pyganodon grandis were collected from nine lakes located along a Cd, Cu and Zn concentration gradient (Rouyn-Noranda area, Quebec). Gills and digestive gland were isolated, homogenized and six sub-cellular fractions were separated by differential centrifugation and analysed for their Cd, Cu and Zn content. Metallothionein was quantified independently. Gill tissues contained abundant calcium concretions that accounted for over 60% of the total gill burden of each metal. Cadmium and Zn concentrations in this granule fraction reflected ambient metal concentrations. Metal concentrations in the digestive gland also responded to the metal contamination gradient, but to a lesser extent than the gills, reflecting the lower abundance of granules in the digestive gland. Metals (Cd, Cu) in this organ were present largely in the "heat-stable proteins" fraction, and metal concentrations in this fraction were strongly correlated with those of both metallothionein and, to a lesser extent, the "lysosomes+microsomes" and "mitochondria" fractions. In both organs, Cd concentrations in the "heat-denaturable protein" fraction remained low and constant, suggesting reasonably effective metal detoxification. Some evidence for oxidative stress was noted in the gills but not in the digestive gland. Overall, we conclude that in nature metals in P. grandis are bound differently in the gills and in the digestive gland and that metal detoxification in the former organ may be less effective than in the latter.