Depuration of metals by the green-colored oyster Crassostrea sikamea

Abstract

The accumulation of metals (especially copper) in oysters has led to green-color now being found in Chinese estuaries. In the present study, the authors quantified the depuration of 8 metals (Ag, Cd, Co, Cr, Cu, Ni, Pb, and Zn) in green-colored oysters (Crassostrea sikamea) collected from an estuary that is heavily contaminated by metals as a result of industrial effluent releases. The oysters were depurated under laboratory conditions for 4 mo; the accumulated concentrations and the subcellular distribution of metals were measured at different time intervals. Results showed that the green color of oysters faded to light yellow (nearly normal) after 4 mo of depuration. Depuration of metals could be described by a first-order kinetic process. The calculated overall depuration rate constants of metals were in the range of 0.008 d-1 to 0.024 d-1, with a biological retention half-life of 30 d to 70 d. The depuration rates of green-colored contaminated oysters were significantly higher for Cd, Cu, Cr, and Ni than the rates of oysters from a less contaminated site, whereas the depuration rates of Ag, Co, Pb, and Zn were comparable between the 2 populations. When corrected for the change of oyster tissue weight, the actual efflux rate constants of the metals (0.0708-0.1014 d-1) were much higher than the overall depuration rate constants. Cellular debris and metallothionein-like proteins were the important fractions binding with the metals in the oysters. Significant changes in metal subcellular distribution were observed during the 4-mo depuration for Ag, Cd, Cu, and Zn. Metallothionein-like protein became more important in sequestering the metals during the depuration period, with a concomitant decrease in metals associated with the cellular debris fraction.