title: The role of complexation and competition in the biouptake of europium by a unicellular alga.
abstract: Short-term (60 min) europium (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, samarium (Sm). Data were interpreted in the context of the biotic ligand model, which uses experimentally determined stability constants 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 maximal uptake flux (Jmax ) and Michaelis-Menten constant (Km ) of Jmax  = 1.7 × 10(-14)  mol cm(-2)  s(-1) and Km  = 10(-7.0)  M (corresponding to an affinity constant of 10(7.0)  M(-1) ). Biouptake of Eu (or Sm) decreased as the concentration of a competing rare earth element (i.e., Sm or Eu) increased, as predicted by the biotic ligand model. 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 rare earth elements were likely to share a common biouptake pathway; biouptake of one rare earth element was reduced when another was present, and rare earth element complexes were bioavailable.
authors.affiliation: Key Laboratory of Pollution Process and Environmental Criteria, Ministry of Education, College of Environmental Science and Engineering, Nankai University, Tianjin, China; Department of Chemistry, Biophysical Environmental Chemistry Group, University of Montreal, Montreal, Quebec, Canada.
authors.forename: Guang
authors.initials: G
authors.lastname: Yang
authors.forename: Qiao-Guo
authors.initials: QG
authors.lastname: Tan
authors.forename: Lin
authors.initials: L
authors.lastname: Zhu
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Wilkinson
mesh_terms: D016825:Chlamydomonas reinhardtii / Q000187:drug effects* / Q000378:metabolism; D004777:Environment; D004784:Environmental Monitoring; D005063:Europium / Q000032:analysis*; D007477:Ions; D007700:Kinetics; D008024:Ligands; D008670:Metals / Q000378:metabolism; D008962:Models, Theoretical
chemical_list: D007477:Ions; D008024:Ligands; D008670:Metals; D005063:Europium
keywords: Bioaccumulation; Biotic ligand model; Competition; Complexation; Rare earth metal