Validation of the Biotic Ligand Model in Metal Mixtures: Bioaccumulation of Lead and Copper

Abstract

The biotic ligand model (BLM) has the potential to predict biological effects and bioaccumulation in metal mixtures. Pb and Cu uptake by the green alga Chlamydomonas reinhardtii have been quantified in single-metal exposures and in metal mixtures in order to test some of the key assumptions of the BLM. Stability constants for the interaction of the metals with biological uptake sites were determined from measured short-term internalization fluxes. In the absence of competition, a value of 10(5.8) M(-1) was obtained for Cu, while 10(5.9) M(-1) was obtained for Pb. Competition experiments did not show a straightforward antagonistic competition as would be predicted by the BLM. Only at high Cu(2+) concentrations (>1 microM) did Cu behave as a competitive inhibitor of Pb transport. Surprisingly, low concentrations of Cu(2+) had a synergistic effect on Pb uptake. Furthermore, Cu uptake was independent of Pb when Cu concentrations were below 10(-7) M. In order to explain the observed discrepancies with the BLM, membrane permeability and Cu transporter expression levels were probed. The expression of ctr2, a gene coding for a Cu transporter, increased significantly in the presence of Pb, indicating that bioaccumulation is much more dynamic than assumed in the equilibrium models.

Bioaccumulation and biosorption of copper and lead by a unicellular algae Chlamydomonas reinhardtii in single and binary metal systems: A comparative study

“…The values of J int were about 1.5 times smaller for Pb than for Cu. The stability constants determined for the interaction of Cu (10 5.8 ) and Pb (10 5.9 ) with biological transport sites, showed that the metal affinity was similar for both ions (Chen et al, 2010). The stability constant of 10 5.9 M À1 for Pb is similar to the value of 10 5.5 M À1 determined for Chlorella vulgaris under similar conditions (Slaveykova and Wilkinson, 2002).…”

“…This implies that the ability of algae to uptake copper is greater in the presence of lead. Increased uptake in the presence of a potentially competing ion was often observed in the literature but it is not consistent with BLM predictions (Worms and Wilkinson, 2007;Chen et al, 2010). Based upon equilibrium principles, the amount of metal that binds to the cell surface can be determined by a competition between toxic metal ions and other metal cations in solution for the biotic ligand (sensitive site at the membrane surface).…”

“…This can explain the higher concentrations of internalized copper by C. reinhardtii with respect to the amount of internalized Pb. Linear uptake curves of Pb and Cu by C. reinhardtii have been shown also by Chen et al (2010). The authors have studied the variation of the internalization fluxes (J int ) as a function of free ion concentration in the exposure solution.…”

“…For pH adjustment, concentrated solutions of nitric acid (Suprapur, Sigma) were used. Under these conditions, in the absence of added ligand, free metals (Pb 2þ , Cu 2þ ) represented >96% of the total metal in solution (Chen et al, 2010). All culture media and experimental solutions were sterilized prior to use.…”

“…The bioavailability of trace metals, including their toxicity, is thought to be related to their ability to cross biological barriers (e.g. plasma membrane) and it is most often predicted by the concentration or flux of internalized metal (Chen et al, 2010;Hill et al, 1996;Kola et al, 2004;Wang and Dei, 2006;Worms and Wilkinson, 2007). However, the comparison between bioaccumulation and biosorption of heavy metals by algal cells are very limited in the literature.…”