COMPLEXATION OF IRON(III) BY NATURAL ORGANIC-LIGANDS IN THE CENTRAL NORTH PACIFIC AS DETERMINED BY A NEW COMPETITIVE LIGAND EQUILIBRATION ADSORPTIVE CATHODIC STRIPPING VOLTAMMETRIC METHOD

A highly sensitive voltammetric technique was developed to examine Fe speciation in seawater. The technique involves adding an Fe(III)-compl exing ligand, salicylaldoxime, which competitively equilibrates with i norganic and organic Fe(III) species in ambient seawater. The Fe(III)- salicylaldoxime complex then is measured by adsorptive cathodic stripp ing voltammetry (ACSV). This new method revealed that 99.97% of the di ssolved Fe(III) in central North Pacific surface waters is chelated by natural organic ligands. The total concentration of Fe-binding ligand s is approximately 2 nM, a value greatly in excess of ambient dissolve d iron concentrations. The titration data can be modeled as consisting of two classes of Fe-binding ligands, a strong ligand class (L(1)) wi th an average surface-water concentration equal to 0.44 nM with a cond itonal stability constant K-L1/Fe'(cond) = 1.2 X 10(13) M(-1), and a w eaker ligand class (L(2)) with an average concentration equal to 1.5 n M with K-L2/Fe'(cond) = 3.0 X 10(11) M(-1). The low concentration of d issolved Fe present in surface waters (similar to 0.2 nM), coupled wit h the excess of strong Fe-chelators, results in extremely low equilibr ium concentrations of dissolved inorganic iron, [Fe'] approximate to 0 .07 pM. In the deeper waters there is a 2 nM excess of Fe-binding liga nds with a stability constant similar to that of the L(2) class of lig ands observed in surface waters, resulting in dissolved Fe(III) existi ng primarily in the chelated form in deep waters as well. The stabilit y constants of the natural ligands are comparable to the model ligands desferal, a siderophore, and the prosthetic heme group, protoporphyri n-IX. The high degree of organic complexation of iron makes it critica lly important to reevaluate our perceptions of the marine biogeochemis try of iron and the mechanisms by which biota can access this chelated Fe.