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The role of complexation and competition on the biouptake of Eu by a unicellular alga

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Short‐term (60 min) Eu biouptake fluxes by the freshwater green alga, Chlamydomonas reinhardtii, were investigated in the presence and absence of ligands (e.g. malic acid and citric acid) and a second rare earth metal (Sm). Data were interpreted in the context of the biotic ligand model (BLM), which uses experimentally determined stability constants in order to take into account the competition and complexation of the metal of interest. In the absence of ligands or competitors, Eu biouptake was well described by a Michaelis‐Menten equation with the constants: Jmax=1.7×10‐14 mol cm‐2 s‐1 and Km=10‐7.0 M (corresponding to an affinity constant of 107.0 M‐1). Biouptake of Eu (or Sm) decreased as the concentration of a competing rare earth element (REE, i.e. Sm (or Eu)) was increased, as predicted by the BLM. On the other hand, when hydrophilic complexes were formed with citric and malic acid, Eu biouptake was much greater than predicted on the basis of free ion concentrations alone. Overall, the results showed that for C. reinhardtii: the REE were likely to share a common biouptake pathway; biouptake of a one REE was reduced when another was present and REE complexes were bioavailable. Environ Toxicol Chem © 2014 SETAC

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