Effect of salinity on the bioaccumulation and depuration of cadmium in the pacific cupped oyster, Crassostrea gigas

As a euryhaline species, the oyster Crassostrea gigas can adapt rapid and dramatic salinity fluctuations, and show physiological mechanisms of adaption to tolerant salinity changes. They are continuously exposed to Cd because they are filter feeders and their mobility is poor. In order to understand the influence of salinity on the molecular responses of C. gigas exposed to Cd, oysters were acclimatized to different salinities (13, 20, 27, and 34) for 14 days and then exposed to 10 μg/L Cd for 28 d, followed by a depuration period of 35 d. Control groups were kept at the same salinities without Cd. Oysters were sampled for chemical analysis by inductively coupled plasma mass spectrometry (ICP-MS) and for mRNA quantification by qPCR. The rank order of the mean concentrations of Cd in oyster tissues was digestive gland > mantle > adductor muscle. Use of a two-compartment model showed that in the three tissues, Cd uptake rates (k1) in digestive gland (13.525-35.430 d−1) also increased as salinity decreased. However, no difference was observed in Cd uptake rates of C.gigas in the three higher salinities in mantle (11.703-17.250 d−1). Cd depuration rates (k2) (0.0139 - 0.0127 d-1 in mantle and 0.0111-0.0134 d−1 in digestive gland) followed a reverse trend. There was not a relationship between k2 and salinity in adductor muscle. In response to Cd contamination, MT was significantly up-regulated by Cd at all salinities, and P-gp was significantly up-regulated in mantle, while down-regulated in digestive gland, which means a disruption of the protein synthesis at high concentration. At depuration phase, MT level was higher in digestive gland and mantle, and its expression was higher at S13 than that at S34. No relationship was found between the P-gp gene expression level and concentrations of cadmium in tissues in either accumulation phase or depuration phase. In accumulation phase, the MT gene expression level was positively correlated with the concentration of cadmium in both the digestive gland and the mantle, while the relationship was weakened in depuration phase, suggesting an effort to create a detoxification mechanism.