Evaluation de la metallothioneine comme biomarqueur d'effets toxiques chez le bivalve d'eau douce Pyganodon grandis dans un environnement aquatique contamine par les metaux traces

Metallothionein (MT) is a low molecular-weight metal-binding protein synthesized by a wide variety of organisms as a defense mechanism against excess metals in the surrounding media. In the present thesis, we evaluate the use of MT as a biomarker for toxic effects in the freshwater bivalve Pyganodon grandis living in lakes impacted by anthropogenic metal inputs.; In a preliminary study, we proposed a selection procedure to reduce the background variability in screening for cadmium contamination in lakes: nine lakes were selected from an original set of 20, and were chosen so as to have water bodies with similar limnological characteristics but contrasting Cd levels. Our intention was to minimize the differences in habitat quality for the bivalves, and in this way facilitate the unequivocal identification of the biological responses of P. grandis to Cd contamination. In this study, we notably showed that the relative influence of limnological confounding factors (primarily pH and dissolved Ca) on Cd bioaccumulation and MT synthesis in natural populations of P. grandis was successfully reduced by the lake selection procedure. In the selected lakes, density, total biomass, production, turnover ratio (P/B) and reproductive success of P. grandis populations decreased with increasing concentrations of the free-cadmium ion concentration in the environment. Overall, the concentrations of Cd in the gill cytosolic high molecular weigh pool (presumably representative of metallo-enzymes) of the individual bivalves were the biomarker that was the most strongly correlated with population variables. Consistent with this, we observed a marked increase in mortality rates of P. grandis specimens translocated from an uncontaminated site to cadmium-contaminated sites, concomitant with an increase of the Cd associated with high molecular weight proteins (>25 kDa) in gill cytosols of bivalves. However, it is not possible from the present thesis, to positively assign to sub-cellular metal partitioning measurement any predictive role for population health, notably because of the influence of environmental confounding factors (i.e., the number of degree-days available in the littoral zone of the lakes). Based on the results of a long-term study of the variations in MT concentrations in indigenous bivalves of the studied lakes, we determined that metal contamination of our lakes had markedly decreased in the past 13 years, and consequently we hypothesized that the toxic effects of Cd might have been replaced by some natural factors as the main agent for structuring the clam populations in these lakes. This thesis casts legitimate doubt on environmental studies that do not consider habitat characteristics in their sampling protocols along with components related to anthropogenic activities. (Abstract shortened by UMI.)...