Title: The role of cadmium complexation on the cadmium uptake in biota
Other Titles: De invloed van cadmiumcomplexatie op de cadmiumopname door biota
Authors: Verheyen, Liesbeth; S0108112
Issue Date: 28-Jun-2013
Abstract: Cadmium (Cd) is a toxic trace metal with widespread occurrence. The risk of Cd is largely affected by its bioavailability in the environment, i.e. the total Cd concentrations in the environment (soil, water, food,…) are poorly explaining uptake and effects in different biota across different compartments. The complexation of ionic Cd2+ by ligands in solution changes its bioavailability. Two common equilibrium models of bioavailability, the free ion activity model (FIAM) and biotic ligand model (BLM), assume mostly that mainly ionic Cd2+ and not its complexes determine the uptake. These models are widely applied to aquatic and terrestrial organisms. However, uptake of Cd in higher plants was recently shown not to obey this mechanism because labile complexes of Cd enhanced Cd uptake. It was demonstrated that this is related to kinetic limitations, more in particular the process by which labile complexes increase the flux of metals over unstirred layers adjacent to cells, only some tens of micrometer thick.The objective of this study was to assess the validity of equilibrium (FIAM) and/or kinetic models to describe the Cd bioavailability, i.e. Cd uptake in cells under contrasting scenarios. More specifically, this work addressed abiotic and biotic boundary conditions at which the FIAM starts to fail and kinetic considerations have to be invoked. In addition, this work assessed the validity of the equilibrium models in the presence of natural dissolved organic matter (DOM) sampled from different sources and with associated differences in Cd2+complexation properties. Experiments were designed with Caco-2 cells, a model systems for human gut cells and with the freshwater algae Pseudokirchneriella subcapitata (Korschikov). The Cd uptake by Caco-2 cells was higher from a solution in the presence of complexes than from a solution with the same free Cd2+ activity but without complexes, illustrating that the FIAM does not apply. The contribution of the complexes decreased with increasing Cd2+ concentration. At low Cd2+ concentration (1 nM), chloride complexation with Cd2+ forming CdCln2-n contributed to the uptake almost to the same extent as the free ion. At large Cd2+ concentration (10 µM), the contribution of the complexes was much smaller. Modelling suggested that these treatment effects were the result of alleviating the diffusion limitation of the free metal ion to the cell surface over an unstirred layer of about 2 mm.The technique of the algal bottle assay to asses metal uptake by algae was refined to better control Cd speciation during algal growth. A resin-buffered nutrient solution was developed and this was applied to test the effect of chloride (Cl-) on cadmium (Cd) uptake. Standard nutrient solution enriched with 40 mM of either NaNO3 or NaCl contained equal Cd2+ but varying dissolved Cd due to the presence of CdCln2-n complexes. The refined algal bottle test was compared to the traditional algal bottle test by growing green algae in the standard nutrient solutions in the absence (designated ‘-R’) or in thepresence (designated ‘+R’) of a cation exchange resin. The Cd concentrations in solution of the –R treatments decreased with 50-58 % of initial values due to Cd uptake. No such changes were found in the +R treatments. Cadmium uptake was unaffected by either NaNO3 or NaCl treatment in the +R treatment, confirming that Cd2+ is the preferred Cd species in line with the FIAM. In contrast, Cd uptake in the –R treatments was two-fold larger in the nutrient solution with NaCl than in the nutrient solution with NaNO3 in contrast with what FIAM would predict. The effect of synthetic ligands and DOM (20 mg C L-1) on the Cd uptake by algae was assessed with the refined algal bottle assay. Long-term (3 days) Cd uptake was measured in resin buffered solutions with or without synthetic ligands and at three different Cd2+ ion activities (pCd 8.2-5.7). Total dissolved Cd increased up to 35-fold by adding the synthetic ligands at constant Cd2+ activity. In contrast, Cd uptake by algae increased maximally 2.8 fold with increasing concentration of the synthetic ligands and the availability of the complexes were maximally 5.2% relative to Cd2+ for NTA and CDTA complexes. It is concluded that synthetic labile Cd complexes do not greatly enhance Cd bioavailability to the green algae and calculations suggest that Cd transport from solution to these small cells is not rate limiting. Hence, Cd uptake by algae in the presence of synthetic ligands generally obeys the FIAM in a test in which Cd2+ is buffered by a resinNatural dissolved organic matter (DOM) can have contrasting effects on metal uptake in algae because of complexation reactions and because of DOM adsorption to algal surfaces, thereby affecting the metal ion uptake process. Six different DOM samples were collected and isolated from natural freshwater systems and isolated by reverse osmosis and one 13C enriched DOM sample was isolated from soil to identify DOM adsorption to algae. In the presence of the resin, Cd uptake was unaffected by the presence of DOM or increased maximally 1.63-fold. In the absence of the resin, Cd uptake increased by DOM up to factors 2.4 but that was mostly due to the lack of buffering of solution Cd. The 13C analysis revealed that 6% of algal C was derived from DOM. Hence, Cd2+ and not DOM-complexed Cd is the main bioavailable form of Cd as predicted by FIAM, however the lack of instantaneous buffering of Cd2+ , as in resin free systems, may result in DOM enhancing Cd bioavailability in natural systems by acting as a mobile metal carrier locally buffering Cd2+.In summary, equilibrium models such as FIAM and BLM are not valid whenever the free ion activity is not constant in time and space. In such conditions, uptake is better related to the total initial concentrations of labile metal than to the initial free metal ion concentration. The FIAM and BLM are valid models at high, toxic metal concentrations where no such gradients in time and space are present. For small cells such a microalgae, the FIAM applies when the Cd2+ ion isbuffered in the medium because concentration gradients to small cells with high specific surface area are generally small.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Division Soil and Water Management

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