Several cases have been described in the literature where different ligands enhanced lead (Pb) bioavailability over what would have been expected on the basis of equilibrium models such as the biotic ligand model (BLM). These exceptions compromise the development of BLMs for this metal, and mechanistic knowledge of the involved processes is still insufficient. The present study shows that the hydrophilic organic ligand citrate enhances Pb internalization by Chlamydomonas reinhardtii for both the wild and the wall-less strains. Despite the high Pb internalization fluxes shown by this alga, which may be near the limits set by diffusion through the boundary layer, and its capacity to assimilate citrate (and potentially Pb-citrate complexes), neither of these mechanisms could quantitatively account for the observed increase in Pb internalization in the presence of this ligand. However, algal-driven pH increases in the boundary layer and concomitant changes in Pb speciation successfully explained the observed results. This study suggests that information on bulk solution chemistry is not enough to predict metal bioavailability for organisms that can substantially modify the chemical composition of their boundary layer, as observed for C. reinhardtii.
