The Biotic Ligand Model Can Successfully Predict the Uptake of a Trivalent Ion by a Unicellular Alga Below pH 6.50 but not Above: Possible Role of Hydroxo-Species

In many reported cases, the biotic ligand model (BLM) has been shown to predict the bioavailability of divalent metals toward aquatic biota successfully. However, studies on the bioavailability of nonessential trivalent metals, including aluminum (Al), are relatively scarce. In the present study, short-term scandium (Sc) internalization fluxes (J(int)) were measured in Chlamydomonas reinhardtii in order to explore the applicability of the BLM to trivalent metals. Scandium was selected for its chemical similarities with Al and for its convenient radio-isotope (Sc-46). Apparent affinity constants of Sc3+ with membrane transport sites (KSc-Rcell app) were surprisingly high, ranging from 10(8.08) M-1 to 10(13.95) M-1 over the pH range from 4.50 to 7.90. The competition of H+ for binding with Sc3+ transport sites explained this trend within the pH range of 4.50 to 6.00, but not from pH 6.50-7.90. In this latter pH range, predicted fluxes were smaller than observed fluxes and this divergence increased with pH, from a factor of 4 to approximately 1000. Above pH 6.50, the calculated supply of Sc3+ to the biointerface by physical diffusion of the free Sc3+ ion and by the dissociation of its hydroxo-complexes (ScOH2+, Sc(OH)(2)(+) and Sc(OH)(3)) was insufficient to support the high observed internalization fluxes. We speculate that this failure of the BLM could be due to the transmembrane transport of undissociated Sc hydroxo-complexes. Scandium uptake could be modeled reasonably well using a simple semiempirical equation considering equal contributions from Sc3+, ScOH2+, Sc(OH)(2)(+), and Sc(OH)(3) and no H+ competition. Our work highlights the importance of studying the possible role of hydroxo-species in trace metal uptake.