Biotic Ligand Model Does Not Predict the Bioavailability of Rare Earth Elements in the Presence of Organic Ligands

Due to their distinct physicochemical properties, rare earth elements (REEs) are critical to high-tech and clean-energy industries; however, their bioavailability is still largely unexplored. In this paper, the bioavailability of several REEs has been carefully examined for the freshwater alga, Chlamydomonas reinhardtii. In the presence of organic ligands (L), the biouptake of REEs was much higher than that predicted by the biotic ligand model (BLM). Enhancement of the biouptake flux was observed for six ligands (metal = thulium) and six REEs (ligand = citric acid), indicating that this could be a common feature for these metals. In order to explore the mechanism for the enhanced uptake, Tm internalization was carefully evaluated. The Tm internalization flux (J(int)) followed first-order (Michaelis-Menten) kinetics with a calculated maximum internalization flux (J(max)) of (1.1 +/- 0.08) x 10(-14) mol(.)cm(-2)s(-1) and an affinity constant for the reaction of the metal with the transport sites (KTmR) of 10(7.1) M-1. In the presence of citric acid, malic acid, or NTA, the J(int) for Tm was more than 1 order of magnitude higher than that predicted by the BLM when algae were exposed to a constant 10(-9) M Tm3+. The bioavailability of the metal complexes could not be explained by a piggyback internalization (through an anion channel) or the contribution of labile complexes. The enhanced biouptake was attributed to the formation of a ternary Tm complex {L-Tm-R} at the metal transport site. In the natural environment where organic ligands are ubiquitous, classic models are unlikely to predict the bioavailability of REEs to aquatic organisms.