A mechanistic model of copper accumulation in Hyalella azteca.

A mechanistic model is proposed for describing the accumulation of copper in Hyalella azteca. The metal is assumed to diffuse passively into and out of the animal. Once inside, it binds primarily to a ligand (X), the function of which is probably to supply copper to copper-requiring essential macromolecules. Once X is saturated, no further copper is accumulated. The rate of approach to the steady-state is much faster during uptake than during depuration because the number of binding sites (Xt) is limited. Diffusion across the animal's body surface does not seem to be rate-limiting. The binding strength of copper to X (KCu-X) is stronger than the binding of copper to fish gills, but this is not necessarily a valid comparison because KCu-X is the product of several constants, including the equilibrium for diffusion across the animal's surface and the strength of the internal binding site. Prolonged exposure to elevated copper in the water gradually reduces the concentration of Xt, primarily through growth dilution. Regulation of body copper appears to be through control of the concentration of Xt, rather than through control of copper influx or efflux rates, and chronic mortality is not affected by changes in Xt.