BIOKINETICS AND METALLOTHIONEIN-LIKE PROTEINS RESPONSE IN OYSTERS FACING METAL CHALLENGES IN AN ESTUARY

The discovery of colored oysters in an estuary in Southern China raised questions of how these oysters could survive in such an unpredictable estuary. In the present study, the authors conducted a transplant experiment using oysters Crassostrea hongkongensis in the estuary to quantify the changes of metal biokinetics (Cd and Zn) and the responses of metallothionein-like proteins. Oysters in the transplantation experiment accumulated extremely high concentrations of Cd, Cu, Cr, Ni, and Zn. The present study provided the field evidence of the time-course coupling relationship between Cd and Zn bioaccumulation. Over the 2-mo transplantation, the uptake rate constants of Cd and Zn varied greatly during the early stage of exposure but subsequently became comparable among the different locations. The dietary Zn assimilation remained relatively constant, whereas the Cd assimilation increased gradually with increasing period of exposure. No notable difference was seen in dietary metal assimilation among the different locations. In contrast, the efflux rates quantified by the end of transplantation were 26% to 42% higher for Cd and 12% to 37% higher for Zn than that in the unexposed oysters. The authors further demonstrated that the incoming Cu and Zn were not stored in metallothionein-like pools and that metallothionein-like proteins synthesis and breakdown were little affected by metal exposure. Overall, the authors' transplantation experiments suggested that oysters displayed a rather weak ability to modify their biokinetics and metallothionein turnover under metal exposure.