Comparative radiotracer study of cadmium uptake, storage, detoxification and depuration in the oyster Crassostrea gigas: potential adaptive mechanisms

International audienceThe bioaccumulation of cadmium in the oyster Crassostrea gigas originating from a clean (Bourgneuf Bay) and a chronically Cd-contaminated area (Gironde estuary) experimentally exposed to 109Cd-labeled bulk seawater (dissolved and particulate pathways combined) was examined over 21 d. A single-component first-order kinetic model describing the behavior of the bioaccumulation factor (BAF) throughout the experiment showed that the estimated Cd BAF at 21 d was 47% higher for oysters originating from the contaminated estuary than for oysters from the clean area, suggesting an influence of the previous chronic exposure to Cd contamination in the estuarine environment. From the experimental results, the potential adaptive mechanism suggested cannot be attributed to a reduction in Cd permeability but rather to a higher Cd turnover due to the synergy between lysosomes and metallothioneins which, through chelation, are responsible for the reduction in bioavailability and toxicity of cd in oysters. The lower BAF observed for soft parts of oysters previously exposed to chronic Cd contamination corresponded to a faster response to the experimental Cd contamination due to the presence of pre-existing metallothioneins induced by the Cd present in the estuarine environment. Furthermore, based on a 2-component exponential loss kinetic model, Cd complexation to metallothioneins and lysosomes was probably responsible for the slow turnover in the long-term compartment of loss (biological half-life, Tb1/2 = 495 and 198 d for the Bourgneuf and the Gironde oysters, respectively). Of the total Cd accumulated, 40 to 60% was in the soluble form and 30 to 40% of this fraction had been detoxified by the Gironde oysters through chelation to metallothioneins or to lysosomes, which means that approximately 12 to 24% of the total Cd accumulated was potentially bioavailable to humans through oyster consumption. However, through depuration, it was also more efficiently eliminated from oyster soft parts (the edible portion) previously exposed to Cd than from control oysters. Therefore, in the light of these results, it is suggested that the way in which regulatory thresholds of Cd in oysters are presently calculated should be reconsidered and should take into account the level of Cd already detoxified by the oysters through complexation processes