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

Abstract

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 (Jint) 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 Sc(3+) 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 Sc(3+) 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 Sc(3+) to the biointerface by physical diffusion of the free Sc(3+) ion and by the dissociation of its hydroxo-complexes (ScOH(2+), 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 Sc(3+), ScOH(2+), 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.

Development of a Multi-Species Biotic Ligand Model Predicting the Toxicity of Trivalent Chromium to Barley Root Elongation in Solution Culture

“…In the present study, there was a linear relationship between H + activity and Cr 3+ toxicity over the whole pH range, and the values of EC50{CrOH 2+ } ranged from 398 to 1663 nM with the increasing pH from 4.50 to 6.25, indicating that the effect of pH on Cr metal toxicity was a significant competition effect as well as speciation effect between protons and metal ions. This finding was consistent with that reported by Cremazy et al [34], who studied the uptake of a trivalent ion scandium (Sc) by Chlamydomonas reinhardtii , and found H + competitive for binding with Sc 3+ transport sites within the pH range of 4.50 to 6.00, and also suggested that reasonable fit for BLM could also be obtained as a function of the first hydroxo-species ([ScOH 2+ ]) along with proton competition. The results from Table 2 showed that Cr 3+ had a higher affinity to the biotic ligand than CrOH 2+ , which can be correlated to the charge of the ion.…”

Use of bioreporters and deletion mutants reveals ionic silver and ROS to be equally important in silver nanotoxicity

“…Besides ionic silver, complexed silver species such as AgCl 2 − (aq) can also act as a labile source of silver ions to bacteria via thermodynamic equilibration between the bulk solution and the cell surface. This observation is consistent with the Biotic Ligand Model [39]. In turn, any uptake by cells will drive a constant and steady flux from nanoparticles, potentially leading to locally elevated concentrations in the vicinity of the nanoparticles [40].…”

When are metal complexes bioavailable?

“…The formation of ternary complexes has been postulated to occur for both divalent metals, including Pb [43,49,50] and Zn, [44,51] and trivalent metals such as Al, [42,52] Sc, [53] Eu [47] and Tm. [45] Indeed, Aristilde et al [44] suggested that the formation of ternary complexes may be universal, irrespective of the specificity of the metal or ligand.…”