UPTAKE OF LIPOPHILIC ORGANIC CU, CD, AND PB COMPLEXES IN THE COASTAL DIATOM THALASSIOSIRA-WEISSFLOGII

Short-term uptake experiments using the coastal diatom Thalassiosira weissflogii demonstrated that low molecular weight, lipophilic, organic Cu, Cd, and Pb complexes enter the cell by diffusion across the plasma membrane. This passive uptake mechanism acts in parallel to facilitated transport, which involves an equilibrium between free metal ions in solution and surface transport sites. Neither the observed permeability coefficents nor the calculated theoretical permeability coefficents for these lipophilic organic complexes account for the increased cellular Cu concentration observed over 5 h. Furthermore, permeability measurements cannot explain the discrepancy in cellular concentrations between Cu, Cd, and Pb in the lipophilic organic metal treatments. Rather, we hypothesize that the rate-limiting step for the uptake of lipophilic organic metal complexes by unicellular algae is the ability of the metal to dissociate from the transport ligand and chelate intracellular ligands and not their flux across the membrane.