Absorption of divalent trace metals as analogues of calcium by Australian freshwater bivalves: an explanation of how water hardness reduces metal toxicity

Abstract

A competitive inhibition experimental design, incorporating radiotracer labelling of metals and the geochemical simulation of their speciation at two varying Ca water concentrations, was employed to conclusively demonstrate that the divalent trace metals Pb, Mn, Cd and Co, were absorbed from the aquatic medium as metabolic analogues of Ca by two species of Australian freshwater bivalves (Hyridella depressa and Velesunio ambiguus). Several important implications stem from this mechanistic interpretation of metal uptake by aquatic organisms. Because of the general positive empirical relationship established between metal uptake/accumulation and acute/sub-chronic toxicity, the ameliorative effect of an increased water hardness on metal toxicity most likely results from the competitive binding of Ca (>Mg) at the Ca channels of the cell membrane. This conclusion is consistent with empirical studies and also with the basic chemical properties of Ca and Mg, that are relevant to their behaviour at the Ca channel. It follows that Ca water concentration, rather than total water hardness, should be utilised in water quality guidelines as the variable that governs the maximum permissible concentration of certain trace metals that can be sustained by freshwater life.