Silver uptake by the green alga Chlamydomonas reinhardtii in relation to chemical speciation: Influence of chloride

Abstract

Short‐term (<1 h) silver uptake by the green alga Chlamydomonas reinhardtii was measured in the laboratory in defined inorganic media over a range of silver and chloride concentrations. For a low, fixed, free Ag+ concentration (e.g., 8 nM), silver uptake increases markedly (up to ∼4×) as a function of the chloride concentration (5 μM→4 mM Cl−); 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 or for the facilitated uptake of the anionic AgCl−2 complex. 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, 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, and silver uptake is sensitive to the free‐ion concentration. However, such a high concentration of silver is not likely to be encountered in the environment, even in wastewater effluents.

When are metal complexes bioavailable?

“…Cd E Zn E Mn, with an Ag uptake flux that is nearly one order of magnitude higher than that for Pb. [71] Indeed, as mentioned earlier, AgCl complexes were shown to contribute to biouptake for C. reinhardtii [14] and the authors concluded that the increased bioaccumulation was due to the lability of AgCl n (nÀ1)À complexes, rather than to passive diffusion of the uncharged metal species through the biological membrane. This interpretation was supported by subsequent reanalysis of the data by Pinheiro et al [72] Under conditions of a diffusion limitation, both the ligand properties and the size of the organisms will influence metal bioavailability through their influence on mass transport.…”

“…A diffusive limitation is usually identified by comparing the metal internalisation flux with the calculated maximum diffusive flux. [14,61,63] Diffusive limitation can also be inferred from fluxes determined by the diffusive gradients in thin films (DGT) technique, [64] from free-metal-ion gradients close to biological surfaces, as measured by microelectrode arrays, [65,66] or from changes in biouptake under different flow conditions. [64,67,68] In the environment, a mass transport limitation (and thus potential contribution of labile complexes) is often observed: (i) for very low concentrations of metals (picomolar to nanomolar levels) [14,61,64] ; (ii) in constrained media such as sediments, soils or biofilms, where metals have much lower diffusion coefficients [69] ; or (iii) for nutrients and metals with high biouptake fluxes (e.g.…”

“…PO 4 3À , Ag). [14,70] For example, for Chlamydomonas reinhardtii, metal uptake occurs in the order: Ag . Pb E Cu .…”

“…Indeed, although several authors have attributed increased bioaccumulation to the passive diffusion of AgCl 0 in marine diatoms and salmonids, for the green alga Chlamydomonas reinhardtii, Fortin and Campbell were able to show that enhanced Ag biouptake resulted from increased mass transport and dissociation of the complex under conditions of diffusion limitation (see Dissociation of labile metal complexes section below). [13][14][15] In these cases, metal complexes are thought to be taken up in a similar manner to hydrophobic organic contaminants, which are simply partitioned between the water and the phospholipid bilayer of the cell membrane. In such a case, metal complexes could diffuse into the cytosol where the metal could dissociate from the complex and bind with intracellular ligands.…”

Development of biochemical methods to estimate the subcellular impact of uranium exposure on Chlamydomonas reinhardtii

“…Cells were grown axenically in a medium with reduced nitrate and phosphate concentrations (HSM-U) in order to avoid uranium complexation. The composition of HSM-U [7] compared to original HSM media [8] is presented in table 1. Asynchronous and axenic cultures of exponentially-growing cells were maintained at 24 ± 2°C, continuous illumination (100 ± 10 µmol.m -2 .s -1 ), agitation (130 rpm), and pH (5 and 7 for contamination and stock media, respectively).…”