Biouptake of a rare earth metal (Nd) by Chlamydomonas reinhardtii - Bioavailability of small organic complexes and role of hardness ions.

A green alga, Chlamydomonas reinhardtii, was used to verify whether a simple Biotic Ligand Model (BLM) could be used to predict carefully controlled short-term biouptake for the lanthanide, Nd. In the absence of ligands or competitors, Nd biouptake was well described by a Michaelis-Menten equation with an affinity constant, KNd, of 106.8 M-1 and a maximum internalization flux of Jmax = 1.70 × 10-14 mol cm-2 s-1. For bi-metal mixtures containing Nd and Ca, Mg, Sm or Eu, Nd uptake could also be well modelled by assigning experimentally determined affinity constants of KCa = 102.6 M-1, KMg = 103.4 M-1, KSm = 106.5 M-1 and KEu = 106.5 M-1. The similar values of Km and Jmax for the three rare earth elements (REEs): Sm, Eu and Nd is consistent with them sharing a common metal uptake site. On the other hand, in the presence of the small organic ligands (citric or malic acid), neither, free or total Nd concentrations could be used to quantitatively predict Nd internalization fluxes. In other words, in order to predict biouptake by simple BLM determinations, it was necessary to consider that the Nd complexes were bioavailable. The data strongly suggest that risk evaluations of the REE will require a new paradigm and new tools for evaluating bioavailability.