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

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 <similar to>4X) as a function of the chloride concentration (5 muM-->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 AgCl2- 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., greater than or equal to 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.