Cadmium-handling strategies in two chronically exposed indigenous freshwater organisms -: the yellow perch (Perca flavescens) and the floater mollusc (Pyganodon grandis)

Laboratory experiments on a variety of aquatic organisms suggest that metallothionein-like proteins (MT) play an important role in the regulation of essential metals, and in the sequestration and detoxification of non-essential metals (e.g., Cd). However, the importance of metallothionein production relative to alternative strategies of metal detoxification, and its effectiveness in metal detoxification, remain largely unexplored in field situations. In the present study we explored metal-handling strategies in an adult benthic bivalve (Pyganodon grandis) and in juvenile yellow perch (Perca flavescens), exposed to Cd in their natural habitat. The two biomonitor species were collected from lakes located along a Cd concentration gradient. Ambient dissolved Cd concentrations were determined by in situ dialysis as a measure of metal exposure. Sub-cellular Cd partitioning was determined in target tissues (bivalve gills and digestive gland; perch liver) by differential centrifugation, and metallothionein was measured independently by a mercury-saturation assay in the bivalve tissues. Malondialdehyde concentrations were measured as a potential indicator of oxidative stress. Ambient dissolved Cd concentrations ranged from 0.06 to 0.57 nM in the nine lakes from which bivalves were collected, and from < 0.3 to 6.7 nM in the eight lakes from which yellow perch were sampled. Bioaccumulated Cd also varied from lake to lake, more so for the bivalve than for the yellow perch; the [Cd](max)/[Cd](min) ratios for the various tissues decreased in the order: bivalve gill Cd (28) > bivalve digestive gland Cd (18) > perch hepatic Cd (14). In the two lakes that were common to both the bivalve and perch studies, i.e. lakes Opasatica and Vaudray, accumulated Cd concentrations were consistently higher in the bivalve than in the perch. Cadmium-handling strategies were similar in the bivalve digestive gland and perch liver, in that Cd was mainly associated with the heat-stable protein (HSP) fraction. Furthermore, in these organs the contributions from the "mitochondria" and "lysosomes + microsomes" fractions were consistently higher than in the gill tissue. In the bivalve gill, the HSP fraction could only account for a small proportion (10 +/- 3 %) of the total Cd burden, and the metal was instead largely sequestered in calcium concretions (58 +/- 13%).