Understanding the Differences in Cd and Zn Bioaccumulation and Subcellular Storage among Different Populations of Marine Clams

Abstract

The marine clams Mactra veneriformis were collected from three different locations in a contaminated bay in Northern China. Another species of clams Ruditapes philippinarum was collected from the same contaminated bay as well as from a relatively clean site in Hong Kong. The indices of Cd and Zn bioaccumulation (assimilation efficiency, dissolved uptake rate, and efflux rate), tissue concentration, subcellular distribution, metallothionein (MT) content, and clearance rate of the clams were subsequently quantified in these populations in the laboratory. In the two species of clams, the population with a higher Cd tissue concentration assimilated Cd and Zn more efficiently, in correlation with an increase in the Cd associated with the metallothionein-like protein (MTLP) fraction. The subcellular partitioning of Zn was similar among the different populations. The dissolved uptake rates of Cd and Zn were not influenced by the different tissue concentrations of metals in the clams. However, the clam R. philippinarum from the contaminated site reduced their Zn uptake rate constants in response to increasing Zn concentration in the water. Differences in Cd and Zn tissue concentrations had little influence on the metal efflux rate constant and the clams' clearance rate. Our results indicate that the higher Cd and Zn tissue concentrations observed in these two species may be partially caused by the high levels of metal assimilation. Populations living in contaminated environments may be able to modify their physiological and biochemical responses to metal stress, which can subsequently alter trace metal bioaccumulation to aquatic animals. The relative significance of dietary uptake and the potential trophic transfer of metals in the contaminated areas may be substantially different from those in the clean environments.

Detoxification and effects of Ag, Cd, and Zn pre-exposure on metal uptake kinetics in the clam Ruditapes philippinarum

“…It is likely that the induced MT level was still not high enough to affect dietary uptake. The highest MT level in this study was 20.3 ± 9.7 µg g -1 compared to 46.2 ± 5.7 µg g -1 observed for the same species concomitant with an increase in Cd AE (Shi & Wang 2004). However, it has also been suggested that metal AE is not related to MT levels, since repartitioning of metals into MTLP requires a long period of time e.g.…”

“…The increase in Cd AEs in green mussels following Cd exposure was mainly attributed to the induction of MT synthesis that sequestered Cd and increased the cytosolic Cd concentration (Blackmore & Wang 2002). Similarly, the higher Cd AE observed in polluted populations of the clams Ruditapes philippinarum and Mactra veneriformis was related to the higher fraction of Cd associated with the MTLP (Shi & Wang 2004). However, in our present study, the Cd AE of clams after Cd preexposure remained unchanged, irrespective of the level of MT synthesis.…”

“…In contrast to Ag and Cd, the Zn concentration in R. philippinarum was stable, and mainly bound to the insoluble fraction (> 80%). A high percentage of Zn has also been found in the metal-rich granules and the insoluble fraction in the mussel Perna viridis (Blackmore & Wang 2002), clams R. decussatus (Hamza-Chaffai et al 2000, Baudrimont et al 2003, Bebianno & Serafim 2003 and R. philippinarum (Shi & Wang 2004). Although the toxicological importance of the insoluble fraction in R. philippinarum is unknown, the inert granules or mineral concretions in the insoluble fraction may immobilize Zn for detoxification similar to the process in other mollusks (Mason & Jenkins 1995).…”

Metallothionein induction and bioaccumulation kinetics of Cd and Ag in the marine fish Terapon jarbua challenged with dietary or waterborne Ag and Cu

“…Another subsample of pre-exposed fish was dissected into 3 parts (gills, viscera and muscle), and the metallothionein concentrations were measured using a modified silver saturation method (Scheuhammer & Cherian 1991, Shi & Wang 2004. Briefly, the tissues were homogenized in cold 0.25 M sucrose and ultrasonicated.…”

Utilising caging techniques to investigate metal assimilation in Nucella lapillus, Mytilus edulis and Crassostrea gigas at three Irish coastal locations

“…Prior metal exposure may induce specific metal detoxification processes or physiological and biochemical changes that can subsequently affect the uptake of metals (Wang and Rainbow, 2005). Shi and Wang (2004) compared the cadmium biokinetics in clams Ruditapes philippinarum from a previously contaminated site and a "clean" site and found that the contaminated clam population had a higher metallothionein (MT) concentration compared to the uncontaminated population. No significant difference in the dissolved uptake rate constants, efflux rate constants and the clearance rates of the two were observed between the populations and the contaminated clam population had significantly higher cadmium and zinc AEs compared to the uncontaminated population.…”