Uptake And Depuration Of Heavy Metals By Contaminated Oysters

Heavy metals have long been recognized as one of the most important pollutants in the Chinese coastal waters because of their potentials for accumulation in marine organisms.Thus,they can pose a great threat to marine environments and mariculture of China.Bivalves as the dominant filter-feeders play an important role in keeping the ecological balance of esturine ecosystem and in the biomonitoring of trace metal pollution.Given the increasing pollution of heavy metals in the estuary of China and the phenomenon of deterioration of shellfish resources,we studied two common oyster speices,Crassostrea hongkongensis and Crassostrea sikamea in estuarine environments of China in this study.Firstly,we studied the biokinetics of Cd and Zn as well as the turnover of metallothioneins(MTs)in the oysters collected from different locations with different contamination histories.We then quantified the depuration of 8 metals in the green colored oysters.Finally,we determined the tissue-specific depuration patterns of 6 metals in the blue-colored oysters.Our results have great implications for the biological monitoring,ecological risk assessment of trace metal pollution in esturine environment,as well as the protection of shellfish resources.1.We collected three groups of oysters from estuaries with different contaminated histories and contrasted their differences in the biokinetis of Cd and Zn,coupled with metallothionein turnover measurements.The oysters from the contaminated estuaries were enriched with a suite of toxic metals,specifically Cd,Cu,Zn.The uptake rate constants quantified for Cd and Zn were higher than those of most invertebrates.The assimilation efficiencies(AEs)of metals,determined for four species of phytoplankton and natural sediment,were in the range of 14.5-62.6%for Cd and 36.5-60.6%for Zn,respectively.The measured efflux rate constants in the oysters were in the range of 0.013-0.020 d-1 for Cd and 0.025-0.031 d-1 for Zn.Our measurements suggested that oysters collected from the contaminated sites had a high metal concentration in the estuarine oysters may be achieved by a high ingestion rate,a high concentration of metals in the natural seston and a high uptake rate from the dissolved phase.Cellular debris and MTLPs fractions were the main subcellular fraction binding the metals,but metal-rich granules were important for Cr,Ni,Ag and Pb.In the contaminated oysters,the faster MTLP synthesis could recycle more metals from other intracellular pools,effectively reducing the efflux rate.Our study explored the strategies of the estuarine oysters in accumulating high metal concentration in their tissues.2.Oysters are the hyper-accumulators of several toxic metals,and green colored oysters are now found in Chinese contaminated estuary due to their accumulation of toxic metals(especially copper).In this study,we specifically quantified the depuration of eight toxic metals(Ag,Cd,Co,Cr,Cu,Ni,Pb,and Zn)in Crassostrea sikamea collected from an estuary heavily contaminated by metals due to industrial effluent releases.The oysters were depurated under the laboratory conditions for 16 weeks,and the accumulated concentrations as well as the subcellular distribution of metals were measured at different time intervals.Results showed that the green color of oysters fade to light yellow(nearly normal)after four months of depuration.Depuration of metals could be described by a first-order kinetic process.The calculated efflux rate constants of metals were in the range of 0.008-0.024 d-1,with a biological retention half-life of 30-70 days.The efflux rates of green colored contaminated oysters were significantly higher for Cd,Cu,Cr,and Ni than those from a less contaminated site,whereas the efflux rates of Ag,Co,Pb and Zn were comparable between the two populations.The cellular debris and metallothionein-like proteins were the important fractions binding with the metals in the oysters.Significant changes of metal subcellular distribution were observed during the 4-month depuration for Ag,Cd,Cu,and Zn.Metallothionein-like protein became more important in sequestering the metals during the depuration period,with concomitant decrease of metals associated with the cellular debris fraction.3.Blue oysters are now found in Chinese contaminated estuary due to their hyper-accumulation of toxic metals(especially copper and zinc).In this study,we specifically quantified the depuration of six toxic metals(Ag,Cd,Cr,Cu,Pb and Zn)in Crassostrea hongkongensis collected from an estuary heavily contaminated by metals(Baijiao)and relatively uncontaminated site(Jiuzhen).The oysters were depurated under the laboratory conditions for 16 weeks,and the accumulated concentrations as well as the subcellular distribution of metals were measured at different time intervals.Results showed that the color of oysters fade to light blue but still be unhealthy at the end of experiment.The efflux rates of Cr were the fastest(0.49-0.61 d-1),and Cu and Zn were the slowest,the rates being less than 0.3 d-1(Cu 0.16-0.29 d-1,Zn 0.13-0.24 d-1).The efflux rates varied among metals,but were comparable among different tissues for a metal.At the end of 16-week depuration,the concentrations of Ag in the oysters from Baijiao decreased nearly to the initial levels of Jiuzhen;Cd and Pb in viscera were also close to the initial levels of referenced oysters.However,the concentrations of Cr,Cu and Zn were still higher than those of controls.Cellular debris,MRG and MTLP were the important fractions binding with the metals in the contaminated oysters.Different from the subcellular distributions in oysters from Jiuzhen,there were not consistent trends in the oysters from Baijiao.The percentages of metals in TAM increased during the 16-week depuration.The temporal trends also fluctuated.These results suggested that long-term accumulation and elimination experiments should be carried out to better understand the "regulation" in C.hongkongensis.The challenge is to develop methods facilitating the elimination of metals and transfer in subcellular fractions in bivalves,thus to meet the permissible levels for shellfish consumption.