Metal Detoxification in Freshwater Animals. Roles of Metallothioneins

Abstract

In laboratory experiments with a variety of aquatic animals, the toxicity of non-essential metals normally exhibits a threshold response. At low exposure concentrations (low internal doses), the organism can detoxify the incoming metal and thus tolerate the exposure, whereas at higher concentrations, i.e., above some threshold, the detoxification mechanism is no longer able to protect the organism completely, the incoming metal binds to metal-sensitive sites within the cell, and deleterious effects begin to occur. Several metal-detoxification strategies have been identified in laboratory experiments, including metal sequestration in insoluble granules and metal complexation by metallothionein or metallothionein-like peptides (MTLP), but the ability of these mechanisms to prevent metals from binding to metal-sensitive sites in the intracellular environment has not been rigorously tested in field situations. In this chapter we briefly summarize the laboratory evidence supporting the threshold model for metal toxicity and then present the results of our field studies on chronically exposed freshwater animals (Pyganodon grandis, a unionid bivalve mollusc; Perca flavescens, a percid fish), studies that were designed to test the threshold model under field conditions. The two biomonitor species were collected from lakes located along a metal concentration gradient, as defined by ambient dissolved metal concentrations. Subcellular metal partitioning was determined in target organs (bivalve gills and digestive gland; perch liver) by differential centrifugation, and MTLP were measured independently by a mercury saturation assay. From these studies we extract evidence for the inadequacy of the threshold model in the case of chronic/life-long metal exposures (i.e., evidence for incomplete detoxification even at low metal exposures in the field). We speculate that under chronic exposure conditions the animals establish a trade-off between the “cost” of detoxifying non-essential metals and the “cost” of allowing some of these metals to spill over onto metal-sensitive sites. Finally, we discuss the implications of the apparent absence of an exposure threshold below which metal detoxification is complete.