Thiosulfate Enhances Silver Uptake by a Green Alga: Role of Anion Transporters in Metal Uptake

Abstract
Short-term (Chlamydomonas reinhardtii was measured in the laboratory in defined inorganic media in the presence or absence of ligands (chloride and thiosulfate). In contradiction to the free-ion model of metal uptake, silver accumulation by the alga proved to be sensitive to the choice of ligand used to buffer the free silver concentration. For a low fixed free Ag+ concentration of 10 nM, silver uptake in the presence of thiosulfate (0.11 μM) was 2× greater than in the presence of chloride (4 mM). When sulfate was removed from the exposure medium (i.e., 81 μM → 0 μM), silver uptake in the presence of thiosulfate was even more markedly enhanced (more than 4× greater than in the presence of chloride). Varying the sulfate concentration in the exposure medium only affected silver uptake if thiosulfate was present. We conclude that silver-thiosulfate complexes are transported across the plasma membrane via sulfate/thiosulfate transport systems and that sulfate acts as a competitive inhibitor of this uptake mechanism.