The role of cadmium complexation on the cadmium uptake in biota

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.nrpages: 165status: publishe

A resin buffered nutrient solution for controlling metal speciation in the algal bottle assay

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Werken in detentie. Een eindeloze werf of werken aan rechtvaardige en veilige toekomst

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Labile synthetic cadmium complexes are not bioavailable to Pseudokirchneriella subcapitata in resin buffered solutions

The Free Ion Activity Model (FIAM) predicts that cadmium (Cd) uptake by organisms is identical for solutions with the same free Cd2+ concentration and inorganic composition. Clear exceptions to the FIAM have been shown for Cd uptake by plant roots, periphyton and human cells where labile Cd complexes increase bioavailability and which has been attributed to their role in enhancing Cd diffusion towards the uptake cells. Here, we assessed the role of labile Cd complexes on Cd uptake by algae, for which diffusion limitations should be less pronounced due to their smaller size. Long-term (3 days) Cd uptake by the green algae Pseudokirchneriella subcapitata was measured in resin buffered solutions with or without synthetic ligands and at three Cd2+ ion activities (pCd 8.2–5.7). The free Cd2+ activity was maintained during the test using a metal-selective resin located in the algal bottles. 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 labile Cd complexes do not greatly enhance Cd bioavailability to the unicellular algae and calculations suggest that Cd transport from solution to these small cells is not rate limitingstatus: publishe

Larger Metal Affinity of Freshwater Dissolved Organic Matter in Wastewater Affected Rivers

In freshwater, complexation of trace metals by dissolved organic matter (DOM) alleviates metal toxicity. The toxic effect of metals is modelled by the biotic ligand model (BLM). Usually it is assumed that different aquatic DOM samples have equal metal affinity. However, our results showed a 4-fold (Cu) to 10-fold (Cd, Ni, Zn) variability in metal affinity. Metal affinity of DOM from water bodies that receive important wastewater inputs is higher than that of natural DOM. WHAM6 underestimates metal complexation in these waters by an average factor of 3. The BLM assumption of equal DOM quality may be violated in such water bodies, and the BLM is likely to set over-protective water quality criteria.status: publishe

A resin-buffered nutrient solution for controlling metal speciation in the algal bottle assay

Metal speciation in solution is uncontrolled during algal growth in the traditional algal bottle assay. A resin-buffered nutrient solution was developed to overcome this problem and this was applied to test the effect of chloride (Cl⁻) on cadmium (Cd) uptake. Standard nutrient solution was enriched with 40 mM of either NaNO₃ or NaCl, and was prepared to contain equal Cd²⁺ but varying dissolved Cd due to the presence of CdCl(n)(2-n) complexes. Both solutions were subsequently used in an algal assay in 100 mL beakers that contained only the solution (designated "-R") or contained the solution together with a cation exchange sulfonate resin (2 g L⁻¹, designated "+R") as a deposit on the bottom of the beaker. Pseudokirchneriella subcapitata was grown for 72 h (1.4 × 10⁵-1.4 × 10⁶ cells mL⁻¹) in stagnant solution and shaken three times a day. Growth was unaffected by the presence of the resin (p>0.05). The Cd concentrations in solution of the -R devices decreased with 50-58% of initial values due to Cd uptake. No such changes were found in the +R devices or in abiotic controls. Cd uptake was unaffected by either NaNO₃ or NaCl treatment in the +R device, confirming that Cd²⁺ is the preferred Cd species in line with the general concept of metal bioavailability. In contrast, Cd uptake in the -R devices was two-fold larger in the NaCl treatment than in the NaNO₃ treatment (p<0.001), suggesting that CdCl(n)(2-n) complexes are bioavailable in this traditional set-up. However this bioavailability is partially, but not completely, an apparent one, because of the considerable depletion of solution ¹⁰⁹Cd in this set-up. Resin-buffered solutions are advocated in the algal bottle assay to control trace metal supply and to better identify the role of metal complexes on bioavailability.status: publishe

Variability in Cd uptake by Pseudokirchneriella subcapitata among freshwaters containing dissolved organic matter with contrasting characteristics

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