Role of pH on indium bioaccumulation by Chlamydomonas reinhardtii

For divalent metals, the Biotic Ligand Model (BLM) has been proven to be an effective tool to predict biological effects by taking into account speciation calculations and competitive interactions. Nonetheless, the BLM has only rarely been validated for trivalent metals (e.g. rare earth elements), and the potential competitive effects of protons has been understudied. In this paper, the short-term biouptake of indium (In), a trivalent metal that is a byproduct of zinc extraction and used in numerous applications including the semiconductor industry, was evaluated under controlled conditions. Short-term (i.e. 60 min) indium biouptake by Chlamydomonas reinhardtii was measured as a function of pH in order to verify the validity of the BLM. At a given pH, In biouptake could be well described by the Michaelis-Menten equation with conditional stability constants of K_In,pH=4.0 = 10^6.7 M^-1, K_In,pH=5.0 = 10^8.6 M^-1, K_In,pH=6.0 = 10^9.3 M^-1 and maximum internalization fluxes of J_{max, pH=4.0} = 0.74 × 10^-14 mol cm^-2 s^-1, J_{max, pH=5.0} = 1.60 × 10^-14 mol cm^-2 s^-1, J_{max, pH=6.0} = 2.22 × 10^-14 mol cm^-2 s^-1. Although several potential mechanisms for the role of pH were examined, the results were best explained by a competitive interaction of H⁺ with the In uptake sites using overall stability constants of logK_In = 9.76 M^-1 and logK_H = 15.66 M^-1. Based on these results, pH will play a critical role in bioavailability measurements of the trivalent cations in natural waters.