Biphasic elimination and uptake kinetics of Zn and Cd in the earthworm Lumbricus rubellus exposed to contaminated floodplain soil

To study the interaction between chemical availability in a floodplain soil and physiological compartmentalization of internalised metals, bioaccumulation experiments were performed with the earthworm Lumbricus rubellus. Uptake and elimination kinetics of Zn and Cd were determined using radioisotopes, allowing for non-destructive measurements in time and allowing flux measurements of elements under homeostatic control. Two distinct compartments could be identified in the earthworm, with different affinities for Zn and Cd. The first compartment is thought to represent the pool of loosely-bound metals, whereas the second one represents a tightly bound storage fraction. A model based on this view provided an accurate description of the data set. Turnover rates were faster for Zn than for Cd. While the loosely-bound metal compartment determined Zn accumulation patterns, the magnitude of Cd accumulation was dominated by the behaviour of the storage compartment. Net accumulation over 2 weeks of Fe, Ni, Cu, Zn, Cd, Pb in the earthworms was at least two times higher than the absolute amount of metals present in the pore water of the soil, except for Ca. This supports the hypothesis that replenishment of the pore water concentration by desorption of metals from solid soil particles feeds metal uptake and that bioavailability cannot be seen as a static equilibrium. It is concluded that bioaccumulation estimates should be based on fluxes and account for the physiology of internalized metals. At least two internal and external compartments are needed to describe metal kinetics in an accurate way.