Toxicité, transfert et gestion subcellulaire de l’yttrium (Y) chez trois organismes d’eau douce

Metals included in the Rare Earth Elements (REE) group, a key resource in green energy technologies, may soon be exploited in Canada. This, combined with their increasing industrial use, could lead to contamination of freshwater ecosystems by these metals. REE risk assessment is therefore of emerging concern. Yttrium (Y) is the only heavy REE amongst the four most abundant REE. It is also the one that has been the subject of the least toxicity studies even though previous studies have shown a higher toxicity for heavy REE than for light REE. Hence, this thesis aims to evaluate in the laboratory the toxicity and trophic transfer of Y, in relation to its subcellular fractionation, in three freshwater organisms: Daphnia magna, Chironomus riparius and Oncorhynchus mykiss. Analysis of metal subcellular distribution provides promising insights in ecotoxicology. Nonetheless, depending on the organism and the homogenisation step chosen, the fractionation protocol may lead to an erroneous separation of the different subcellular components. Thus, the first part of this thesis optimizes the fractionation protocol for our three organisms. Regardless of the protocol used, we observed significant gaps between predicted fractions in literature and fractions validated in our trials. This section concludes with the establishment of fractionation protocols adapted to each of our organisms and the recommendation to use validated fractions instead of predicted ones in future subcellular fractionation studies. The chronic sublethal toxicity of Y was assessed in the second part of this thesis through bioassays on our three organisms. C. riparius, the benthic species, was the only one presenting adverse effects at exposure levels close to those already reported in natural ecosystems. In addition, we observed different subcellular Y management strategies for each organism. For instance, D. magna, by accumulating Y mainly in its detoxified fractions, was able to bioaccumulate up to 100 times more Y than the other two organisms. The last part of this thesis investigates the trophic transfer potential of Y. Experiments consisting in the exposure of trouts either to Y-spiked daphnids or Y-spiked water were performed. As predicted from Y subcellular partitioning in D. magna, Y trophic transfer was low in our trials. Inside O. mykiss, Y was distributed following this order: iv guts > gills > liver > muscle. However, unlike other organs, the liver was the only one for which no Y-depuration was measured. We concluded on the importance of the liver in REE toxicity analysis. This thesis contributes to optimize metal subcellular fractionation method and data produced by this approach. It shows the importance of understanding the mechanisms behind metal toxicity and its transfer in the food chain. Finally, by measuring Y toxicity thresholds and Y trophic transfer potential, this thesis contributes to the risk assessment of Y on freshwater ecosystems.

Keyphrases: Rare earth, Yttrium, Ecotoxicology, Bioassays, Trophic transfer, Subcellular fractionation, Daphnia magna, Chironomus riparius, Oncorhynchus mykiss