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

Abstract

Short-term (< 1 h) silver uptake by the green alga 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 microM) was 2x greater than in the presence of chloride (4 mM). When sulfate was removed from the exposure medium (i.e., 81 microM-->0 microM), silver uptake in the presence of thiosulfate was even more markedly enhanced (more than 4x 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.

When are metal complexes bioavailable?

“…As seen above, our knowledge with respect to metal bioavailability has greatly evolved over the past 25 years, progressing largely (but not entirely) from equilibrium concepts that mainly considered the contribution of the free ion [1] to dynamic systems where several factors, including the intrinsic properties of the metal and ligand, [34,80] their respective concentrations, organism characteristics such as size, [64,81] media properties and the kinetic behaviour of the metal-ligand system [78,82] are considered. In many cases, equilibrium models are a reasonable simplification of the real-world system, such that simple relationships should be obtained that relate metal speciation to biological effects.…”

“…For example, silver-thiosulphate complexes are taken up by algal cells (Chlamydomonas reinhardtii) at higher fluxes than free Ag. [34] In this case, evidence that uptake was occurring over the sulphate transport system was provided by the observation uptake was significantly inhibited by the addition of sulphate. Nonetheless, the toxicity induced by the silver-thiosulphate complexes was not as important as that due to silver ions, [35] again demonstrating the importance of the intracellular fate of metal complexes once assimilated.…”

Predicting the Bioavailability of Metals and Metal Complexes: Critical Review of the Biotic Ligand Model

“…7). For example in the presence of cadmium and zinc complexes of citrate, glycine, and histidine (Cyprinus carpio [112] ), magnesium, calcium, cobalt, and manganese phosphate complexes (Escherichia coli [113] ), silver thiosulphate complexes, [114] and cadmium citrate complexes (S. capricornutum [115] ), metal uptake fluxes were observed to be larger than expected based upon the concentration of free metal ion.…”

Long-Term Transformation and Fate of Manufactured Ag Nanoparticles in a Simulated Large Scale Freshwater Emergent Wetland

“…32 Silver thiosulfate complexes may also form, but a model compound for this complex was not included in the EXAFS study. Fortin and Campbell (2001) 55 found that silver uptake in aquatic systems was greater in the presence of thiosulfate than based on the free Ag + water concentrations, and importantly, their studies suggest the presence of an inorganic anion transport system where a Ag−thiosulfate complex could give rise to increased uptake in higher organisms, including fish. This suggests that partial sulfidation, which should dramatically decrease Ag + ion release, does not completely prevent uptake of Ag by organisms.…”