Selenium is an essential micronutrient for most of living organisms. However, toxic effects in several ecosystems have been reported in the literature. Toxicity comprehension is difficult due to the complexity of Se oxidation states in the environment.
The aim of this thesis work was to acquire knowledge on the physiological and environmental factors involved in bioaccumulation and toxicity processes in the freshwater filter-feeding bivalve C. fluminea.
The aims were :
i) to define what the factors involved in Se bioaccumulation processes in the bivalve are,
ii) to characterize Se bioaccumulation at different biological organisation levels,
iii) to investigate Se toxic effects.
First experiments, carried out for short term exposure duration (3 days), have permitted to underline the importance of Se chemical speciation in bioaccumulation processes in C. fluminea. It has been shown that the organic form, selenomethionine, was much more bioavailable than the inorganic forms, selenite and selenate.
Moreover, the route of transfer was determinant in thoses processes. Inorganic forms have been better extracted by trophic route, whereas selenomethionine has been better extracted by the direct route. In our experimental conditions, ventilation of the bivalve has not been a limitant factor for Se bioaccumulation by the direct route, whereas it has been for accumulation by the trophic route. Ventilation has been largely modified by the presence of dissolved selenite and selenomethionine.
We have shown that the kinetics of selenomethionine bioaccumulation are much more fast than those of selenite. Moreover, when introduced as SeMet, internalised Se appeared to be relatively remanent in soft tissues of C. fluminea in comparison with Se internalised when introduced as selenite. Subcellular and molecular distributions of these forms were very different.
Finally, it has been shown that selenomethionine and selenite could generate weak alterations of the anti-oxidant status and the genetic expression in C. fluminea. On the other hand, important ultrastructural modifications of gill tissues have been observed after selenite and selenomethionine exposure.