Many of the apparent exceptions to the Free-Ion Model (FIM) of metal toxicity involve either ligands that are assimilable on their own (so-called "accidental" metal transport), or ligands that form neutral lipophilic metal complexes. The objectives of this research project were (i) to determine whether enhanced silver uptake may occur in the presence of inorganic ligands such as chloride or thiosulfate, and (ii) to elucidate the mechanism(s) responsible for this behavior. The unicellular green alga, Chlamydomonas reinhardtii, was chosen as our test organism. Short-term (< 1 h) silver uptake by the alga was measured in the laboratory in defined inorganic media over a range of silver concentrations in the presence and absence of ligands (chloride and thiosulfate). For a low fixed free Ag+ concentration (e.g. 8-10 nM), silver uptake increases markedly (up to ~4X) as a function of chloride (from 0.005 mM to 4 mM Cl-) and thiosulfate (up to ~25X; from 0 nM to 114 nM) concentrations; the free-ion model would have predicted a constant silver uptake rate. No evidence could be found for the passive diffusion of the neutral AgCl0 complex but evidence for the facilitated uptake of the anionic AgS2O3- complex was discovered. The enhanced uptake observed in the presence of chloride is related to the very high silver uptake rates demonstrated by the test alga, which lead to diffusion limitation in the boundary layer surrounding the algal cell. In such a situation, silver speciation becomes irrelevant since equilibrium is never reached at the surface of the plasma membrane and metal accumulation is proportional to the total metal concentration (i.e., to the concentration gradient between the bulk solution and the algal surface). At higher silver concentrations (e.g., 10^-7 M), diffusion in the phycosphere is no longer rate-limiting, the chloride stimulation disappears. Thiosulfate enhanced silver uptake was shown to be sulfate sensitive, indicating a competitive effect between sulfate and silver thiosulfate complexes for transport across a common anion transport system.