Competitive interactions among the rare earth metals (lanthanum, cerium, and yttrium) lead to quantitative decreases in biouptake by Chlamydomonas reinhardtii

With increasing mining and production of the rare earth elements (REE), it is important to quantify their potential environmental impacts. The objective of this study was to quantitatively relate REE biouptake to exposure under carefully controlled laboratory conditions. Internalization fluxes of three REE cations (La³⁺, Ce³⁺, and Y³⁺) were measured at environmentally relevant concentrations (10^-9 - 5 × 10^-6 M) in order to determine the stability constants of their interaction with biological uptake sites in Chlamydomonas reinhardtii (logK_La = 7.9 M^-1; logK_Ce = 7.6 M^-1; logK_Y = 7.8 M^-1). Biouptake for cation pairs and ternary metal mixtures confirmed a competitive (antagonistic) interaction among the three REE that was well predicted using constants from the single metal experiments, indicating that the REE shared a common uptake site. Competition for Ca²⁺ and Mg²⁺ was also quantified, and while less important than for the REE, the constants (logK_Ca = 4.7 M^-1; logK_Mg = 3.8 M^-1) indicated that water hardness will nearly completely reduce biouptake and effects of the REE for environmentally relevant concentrations of the hardness ions (e.g. biouptake reduced by ∼78 % for a [Ca]/[REE] of 10⁴ and ∼97 % for [Ca]/[REE] of 10⁵).