Ni uptake by a green alga. 2. Validation of equilibrium models for competition effects

It is generally admitted that the presence of major cations and H+ can attenuate trace metal uptake. Recent models such as the biotic ligand model (BLM) aim to quantify and predict this effect by determining stability constants for each of the major competitors for any given interaction of a trace metal with a biological organism. In this study, short-term Ni internalization fluxes (J(int)) were used to quantitatively assess the binding of H+, Mg2+, Ca2+ (K(H-Rs), K(Mg-Rs), K(Ca-Rs)), and trace metals to transport sites (R(s)) leading to Ni biouptake by Chlamydomonas reinhardtii. H+ and Mg2+ are shown to compete directly for the entry of Ni with affinity constants that are of the same order of magnitude (K(Mg-Rs) = 10(5.1) M(-1); K(H-Rs) = 10(5.3) M(-1)) as that measured for Ni (K(Ni-Rs) = 10(5.1) M(-1)). The Ni internalization fluxes were also strongly linked to the Mg cell status. In contrast, the role of Ca2+ could not be explained by a simple competitive equilibrium with the Ni transport sites. Aluminum (K(Al-Rs) = 10(8) M(-1)), Zn (K(Zn-Rs) = 10(6.5) M(-1)), and Cu (K(Cu-Rs) = 10(6.6) M(-1)) were all shown to compete strongly with Ni for uptake. In addition to the determination of uptake constants, these studies provide insight into the transport mechanisms of Ni by the green alga, C. reinhardtii.