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

There is increasing evidence to indicate that metal pre-exposure may mediate the subsequent metal uptake from the dietary or dissolved phase by marine invertebrates. In this study, we examined the detoxification and the effects of Ag, Cd, or Zn pre-exposure on the metal uptake of the clam Ruditapes philippinarum. The relationships between metal body burden and metal uptake of Ag, Cd, Hg, and Zn were also concurrently examined. Radiotracers ^110mAg, ¹⁰⁹Cd, ²⁰³Hg, and ⁶⁵Zn were used to chase the metal uptake in the clams. The clams accumulated Ag and Cd over time, but there was not a significant Zn accumulation. Metallothionein played a role in the detoxification of Ag and Cd, but the insoluble fraction was also the detoxification pool for Cd and Zn. Generally, metal pre-exposure did not greatly affect metal uptake. Ag pre-exposure had no effect on dietary Ag uptake. Cd and Zn pre-exposure increased Zn dietary uptake by 10%. After 2 wk Ag exposure and 3 wk of Cd or Zn exposure, Cd and Hg dissolved uptake was reduced by 2 to 6 times. However, these effects mainly appeared during the earlier part of the exposure period (i.e. up to Weeks 2 or 3). The effects of metal exposure on uptake of dissolved Cd and Hg were related to the coupling relationships of dissolved metal uptake between the 2 metals and other metals (Ag and Cd or Hg; Cd and Hg; Zn and Hg), The effects on dietary and dissolved uptake of all metals were independent of Ag, Cd or Zn body burden; thus, metal body burdens may not necessarily indicate the potential metal uptake of marine clams in the environment. Our data have important implications for kinetic modeling predicting metal uptake by marine clams.