The Free Ion Activity Model is one of the most employed steady-state models for relating metal speciation in solution with metal bioavailability and toxicological effects on the aquatic biota. This study highlighted the advantages of this model to predict metal bioavailability by using unicellular green alga.
A rigorous study of Cd uptake (i.e., adsorption, internalization) by a walled and a wall-less strain of Chlamydomonas reinhardtii grown in TAP medium was performed. Free Cd concentrations were examined in the absence and presence of ligands forming both labile and inert hydrophilic complexes, from 10-11 M to 10-3 M. Cadmium adsorption by the walled and wall-less strains of Chlamydomonas reinhardtii was not predicted by a single site (Langmuirian) model. Indeed, no saturation of the cell wall was observed, even for Cd concentrations in excess of 5 x 10-3 M. A continual production of Cd binding sites appeared to be responsible for the observed increase of Cd adsorption with time. SDS PAGE and measurements of the protein content of algal supernatants demonstrated that organic matter was released by the algae, both in the presence and absence of Cd demonstrating that exudate release was not in response to Cd stress but rather a constitutive process. Both the nature (e.g.
polysaccharides, proteins) and the quantity of exudate production were influenced by the physicochemistry of the external medium (i.e., pH buffer). Moreover, measurements using the permeation liquid membrane (PLM) and anodic stripping voltammetry (ASV) demonstrated that dissolved cadmium was rapidly complexed by the organic exudates produced by the algae. Due to the continual production of the binding sites on the cell wall, the equilibrium between Cd adsorbed to the cell wall sites and Cd in solution (steady-state) was never achieved under the studied conditions. Nevertheless, this result did not have any implication in the applicability of the FIAM for Cd internalization, since transporters (specific sites) are responsible for the internalization and not the cell wall binding sites (non-specific sites).
A first order biological internalization, as predicted by the free ion activity model (FIAM), was observed for the entire range of Cd concentrations studied for both strains. A linear relationship with [Cd2+] in solution was seen also for the transporter bound Cd. The maximum Cd internalization flux, Jmax, for the walled strain was 5-fold higher (1.3x10-11 mol cm-2 min-1) than for the CW-2 strain (2.3x10-12 mol cm-2 min-1) and was not influenced by the presence of competitors such as Ca in the experimental solution. The conditional stability constant for the
adsorption of Cd to transport sites of the CW-2 strain was 5-fold higher (106.7 M-1) than for the wild type strain (106 M-1) demonstrating the higher affinity of the CW-2 strain for Cd and suggesting that the number of transport sites was greater for the WT strain. Due to the demonstrated limiting nature of Cd internalization, differences smaller than 10-fold in Cd diffusive fluxes due to the presence of a cell wall should have had no effect on Cd uptake.
Competition experiments demonstrated that Mo, Mn, Cu, Co, Zn, Ni, Ca and Pb competed, at least partially, for the Cd binding sites while no inhibition was observed for similar concentrations of Mg and Fe. Stability constants for the competitive binding of Ca, Zn and Cu to the Cd transport site were determined to be 104.5 M-1, 105.2 M-1 and 105.6 M-1, respectively.
Protons also appeared to compete with Cd uptake sites as uptake could generally be predicted quantitatively in their presence and KH2-Rcell was determined to be 1011.3 M-1. Finally, in the presence of low concentrations (<20 mg L-1) of Suwannee River fulvic acid, Cd internalization fluxes could be predicted from [Cd2+], in accordance with the FIAM.
The vast majority of the results presented above indicated that Cd uptake by C. reinhardtii can be best predicted by models based upon the thermodynamic equilibria of trace metals at the surface of the microorganism. This would suggest that the FIAM or BLM models would be reasonable tools for evaluating Cd bioavailability to C. reinhardtii in freshwaters. Although slight deviations from the steady-state models were observed for high concentrations of organic matter, Cd or protons, these concentrations are generally not seen in the majority of freshwaters. Nonetheless, due to the strong interaction of trace metals, hardness metals and protons at the biological surface, the knowledge of a large number of stability constants and physicochemical parameters will be necessary to quantitatively predict uptake (and thus biological effects) in natural aquatic systems.
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