Response of metallothionein concentrations in a freshwater bivalve (Anodonta grandis) along an environmental cadmium gradient

Surficial (oxic) sediments, overlying water, and specimens of the freshwater bivalve Anodonta grandis were collected by divers at 11 littoral lacustrine sites located along a geochemical gradient of pH, [Cd], Dl, and [Z n in an area influenced by mining and smelting activities. The molluscs were dissected (gills; 1 hepatopancreas; remaining tissues) and analyzed for Cd, Cu, Zn, and metallothioneins (MT). Tissue concentrations of MT in A. grandis varied between 100 and 440 nmol metal binding sites (g dry wtl. Metallothionein levels in the gills, the remaining tissues, and the whole organism were significantly correlated (P < 0.01) with tissue Cd concentrations. In contrast, correlations between [MT] and tissue levels of Cu or Zn were weak or nonexistent. The intersite variations in MT concentrations were best related, not to total dissolved metal concentrations at the time of sampling, nor to extractable metal concentrations in the sediments, but to the free Cd2+ concentration at the sediment-water interface, as estimated from sediment-water sorptive equilibria. These observations suggest that Cd2’ activity is the key environmental factor to which metallothionein levels in A. grandis are responding in the studied lakes. The mollusc condition indices (C.I. = 1,000 x total dry wt of flesh of the animals/total intervalval volume) deteriorated as Ld concentrations in the tissues increased. Freshwater and marine molluscs are recognized for their capacity to concentrate metals from their environment (Phillips 1980). Their tolerance of the resulting metal burdens has been attributed to the existence of an effective detoxification mechanism involving the trapping of the incoming metals by specific ligands present in the cytosol (Viarengo 1989). Possible candidates for this role include metallothioneins (MT), low-molecular-weight, cysteine-rich metal-binding proteins with high affinity for group IB and IIB metal ions. Stability constants measured in vitro for metal ions such as Cd2+, Cu2+, Hg2+, and Zn2+ are 1 To whom correspondence should be addressed. Acknowledgments This research was supported by the World Wildlife Fund (Environment Canada/Natural Sciences and Engineering Research Council of Canada, NSERC), by a NSERC Strategic Grant and by the Fonds pour la Formation de Chercheurs et 1’Aide a la Recherche du Quebec. Y.C. received a NSERC postgraduate scholarship. We thank R. Rodrigue and F. Fournier for their efforts in the field, M. Bordeleau for assistance in the laboratory, and J. C. Auclair for advice on the statistical analyses. Special recognition is extended to J. F. Klaverkamp and M. Dutton for their help in learning the details ofthe 203Hgsaturation assay for metallothionein. The sediment Hg analyses were performed by the Laboratoire d’Ecologie fondamentale et Ecotoxicologie of the Universite de Bordeaux, France. Comments on draft versions of this paper were provided by L. Hare and three anonymous reviewers. usually several orders of magnitude higher than those of the comparable metal complexes with other cellular ligands. Given its ubiquity in the animal kingdom and its strong affinity for “soft” metal cations, MT is well positioned to play a metal detoxification role (Stegeman et al. 1992). Consistent with such a role is the observation of enhanced metal tolerance associated with the induction of metallothionein. Thus, prior exposure of the marine mussel M’tilus edulis in the laboratory to Cu, Cd, or Zn at concentrations sufficient to induce metallothioneinlike proteins conferred increased tolerance to inorganic Hg (Roesijadi and Fellingham 1987). Similar results have been reported for freshwater fish (Klaverkamp and Duncan 1987; NRCC 1985). However, most of the available evidence supporting the idea that aquatic organisms synthesize metallothionein as a defense against toxic metals derives from laboratory trials where exposure conditions can differ greatly from those encountered in natural environments. In particular, metal concentrations shown to induce MT synthesis in laboratory experiments are often orders of magnitude higher than those found in even the most contaminated aquatic systems where the studied organisms are found. Other putative functions have been proposed for metallothionein: as a