Kinetics of Cadmium Uptake and Subcellular Partitioning in the Earthworm Eisenia fetida Exposed to Cadmium-Contaminated Soil

To understand the bioaccumulation kinetics of cadmium (Cd) at the subcellular level, toxicokinetics and subcellular fractionation of Cd were determined for the terrestrial earthworm Eisenia fetida exposed to Cd-contaminated red soil. The bioaccumulation factor was 1.74 after exposure to soil containing 1 mg Cd/kg dry weight for 21 days. Four distinct compartments with different Cd-binding affinities were obtained by sequential centrifugations, including the cytosolic fraction (G), the organelles fraction (F), the granular fraction (D), and the tissue, cell membrane, and intact cell fraction (E). Most of the accumulated Cd in the organisms existed as fraction G. Cd bound to the tissue and cell membrane fraction (E) was < 8% of total Cd. Only approximately 3% of Cd existed in fraction F, and < 1% was found in fraction D. Accumulation and elimination kinetics of Cd in different fractions were accurately described by one-compartment models, and kinetic parameters (uptake rate constant k (1) and elimination rate constant k (2) ) were derived. Fractions G and F were responsible for the linear accumulation pattern during 3 weeks of exposure, whereas fractions D and E showed a nonlinear uptake curve, and steady state was achieved after 7 and 14 days of exposure, respectively.