The basis for increased metallothionein in a natural population of Crassostrea virginica

Metallothionein (MT) is elevated in a natural population of the oyster Cras sostrea virginica that is exposed to cadmium. However, induction of MT is n ot a likely cause for this elevation. This was concluded after re-examinati on of the results of a study in which MT mRNA and MTs were analysed in oyst ers collected from a cadmium-contaminated environment. MT mRNA concentratio ns did not differ in oysters collected from a cadmium exposure gradient and could not account for increases in MT, although laboratory experiments had clearly shown that induction of MT in response to cadmium is seen at both transcriptional and translational levers. It appears that, at the low conce ntrations of cadmium encountered in natural environments even when contamin ated, the underlying basis for the elevation of MT is its stabilization by cadmium and its accretion while being synthesized at basal rates, rather th an induction of MT per se. This observation has implications for the effica cy of using either MT mRNA or MT levels in molluscs in assessing metal expo sure or its biological effects in contaminated natural environments.