Toxicity to embryo and adult zebrafish of copper complexes with two malonic acids as models for dissolved organic matter

The toxicity to embryo and adult zebrafish (Danio rerio) of Cu complexes with two substituted malonic acids, benzyl- and n-hexadecyl-, chosen as models for low-molecular-weight natural dissolved organic matter, were investigated. Toxicity test solutions at pH 6.5 +/- 0.1 with the required Cu speciation were designed using the chemical speciation model MINTEQA2, and the speciation was confirmed by Cu ion-specific electrode. In the absence of malonic acids, concentrations of Cu2+ up to 1.13 mu mol/L increased the embryo hatching time from approx. 2 d in control solutions (no Cu or malonic acid) and solutions containing malonic acids without Cu to approx. 8 d. The Cu-benzylmalonic acid complex (n-octanol/water distribution coefficient [p(ow)] = 0.00) in the presence of inorganic Cu species did not delay hatching beyond that attributable to Cu2+. I, contrast, 0.60 mu mol/L Cu-n-hexadecylmalonic complexes (p(ow) = 6.89) delayed hatching by 5.5 d in excess of that attributable to 1.13 mu mol/L Cu2+, assuming that the hatching delays caused by the different Cu species were additive, possibly because of Cu entry into the embryo as the lipophilic Cu-n-hexadecylmalonic complex. None of the Cu-malonic acid complexes was acutely toxic to adult zebrafish at concentrations up to 1.4 mu mol/L, possibly because Cu was removed from the Cu-malonic acid complexes by stronger chelating groups at the gill surface. Substituted malonic acids with similar proton and Cu association constants can be readily prepared with a variety of simple substituents, radiolabeled if required. Our results show that these acids could be useful ligands for investigating intracellular transport and metabolism of metal-organic complexes.