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Metal speciation dynamics and bioavailability: Zn(II) and Cd(II) uptake by mussel (Mytilus edulis) and carp (Cyprinus carpio).

In the analysis of metal biouptake from complexing environments, both chemical speciation and biological uptake characteristics have to be taken into account. The commonly used free ion activity model is based on equilibrium speciation and implies that diffusion of the bioactive free metal toward the organism is not rate-limiting. In the presence of complexes, however, sufficiently labile species might contribute to the biouptake via preceding dissociation. Coupling of the ensuing diffusional mass transfer flux of metal with the biouptake flux of free metal, the supposedly bioactive species, shows under which conditions labile metal complexes can contribute to the uptake. The goal of the present paper is to apply this type of analysis to experimental data on metal uptake by mussel (Mytilus edulis) and carp (Cyprinus carpio) in complexing environments. These biosystems have fairly well-characterized uptake parameters, but the uptake fluxes cannot be fully explained by considering equilibrium speciation only. For Zn(II) uptake by mussel, evidence was found for diffusional limitation at low concentrations, whereas for Cd(III) uptake by carp, diffusion is not limiting at all. The analysis provides an example of how a more comprehensive treatment of complex systems can be applied to real experimental data.