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.

One of the major challenges in environmental analytical chemistry is to develop methods for determining metal speciation in natural waters that contain low metal concentrations and dissolved organic matter (DOM). Because of its complex heterogeneous nature, metal binding to DOM cannot be predicted accurately using equilibrium models. Two independent speciation methods, the equilibrium ion-exchange technique (IET) and equilibrium dialysis (EqD), were used to determine silver binding by standard Suwannee River humic acid. Both approaches gave very similar results, although for a given silver loading, the concentration of free silver obtained by IET was somewhat higher than that determined by EqD. Our results suggest that any high-affinity binding sites present within the humic acid are likely saturated at [Ag(T)] > 10(-9) M. This comparison of free metal ion concentrations with two independent methods provides useful speciation information in the absence of reliable complexation constants for the reaction of silver with humic acid.

Chiral pesticides are used widely in the world, and at present, older racemic products are being replaced by enantiopure products because of accelerated development of asymmetry synthesis techniques. Pesticides as xenobiotic released into environment impose a great stress on nontarget organisms. Although it is a necessary procedure for pesticides to have a registration based on toxicological data from nontarget organism, until now ecological risk assessment about metalaxyl only depend on racemic products. Hence, we investigated the acute, chronic, and sublethal toxicity of R-metalaxy and rac-metalaxyl on aquatic organisms such as D. magna (Daphnia magna), algae (Scenedesmus quadricanda), and adult zebrafish (Danio rerio). The results showed a significant difference in toxicity between R-metalaxyl and rac-metalaxyl. R-Metalaxy was about 20-fold more toxic to algae than rac-metalaxyl with IC(50) of 222.89 +/- 1.18 mg/L and 19.95 +/- 1.12 mg/L, respectively. Similarly, R-metalaxyl was about fourfold toxic to D. magna than rac-metalaxyl according to the individual 24-h-LC(50) values, and sixfold toxic than rac-metalaxyl based on 24-h-EC(50) values. In the light of 48-h-LC(50) and EC(50), this difference in toxicity was more significant. As for adult zebrafish, there was no pronounced difference in acute toxicity, in addition, at sublethal level a different pattern in inducing Na(+),K(+)-ATPase activity between them was found. In general, R-metalaxyl seemed more toxic to aquatic organisms than rac-metalaxyl.