Complexation of iron(III) by natural organic ligands in the central north Pacific and the equatorial Pacific: A re-evaluation of oceanic iron chemistry

This dissertation presents the development of a new analytical technique for determining the concentration and chemical speciation of iron in seawater. This first-row transition metal plays a key role in the biochemistry and physiology of oceanic phytoplankton and is the most important bioactive trace metal in the oceans. My new method involves the addition of an Fe(III)-complexing ligand, salicylaldoxime, to competitively equilibrate with natural inorganic and organic Fe(III) species in seawater. The Fe(III)-salicylaldoxime complex then is measured by adsorptive cathodic stripping voltammetry (ACSV) in the differential pulse mode on a hanging mercury drop electrode. This highly sensitive technique was first applied to chemical speciation measurements of central North Pacific surface waters and indicated that under equilibrium conditions, 99.97% of dissolved iron was chelated by nanomolar concentrations of strong Fe(III)-binding natural organic ligands. The total concentration of Fe(III)-binding organic ligands was {dollar}sim{dollar}2 nM, a value in excess of ambient dissolved iron concentrations. The titration data was modeled as consisting of two classes of organic ligands, a strong class (L{dollar}sb1){dollar} with an average surface-water concentration of 0.44 nM with a conditional stability constant {dollar}Ksbsp{lcub}FeLsb1, Fespprime{rcub}{lcub}cond{rcub}=1.2times10sp{lcub}13{rcub}{dollar}M{dollar}sp{lcub}-1{rcub}{dollar} and a weaker ligand class (L{dollar}sb2){dollar} with an average concentration of 1.5 nM with {dollar}Ksbsp{lcub}FeLsb2, Fespprime{rcub}{lcub}cond{rcub}=3.0times10sp{lcub}11{rcub}{dollar} M{dollar}sp{lcub}-1{rcub}.{dollar} The low concentration of dissolved iron in these waters, {dollar}sim{dollar}0.2 nM, combined with the excess of strong Fe(III)-binding chelators, results in extremely low equilibrium concentrations of dissolved inorganic iron, (Fe(III){dollar}spprime{dollar}) = 0.07 pM. The conditional stability constants of these natural ligands were comparable to those of the model Fe(III)-binding chelator Desferal{dollar}spcircler,{dollar} a siderophore, and a prosthetic heme compound, protoporphyrin-IX.; The chemical speciation of iron in the HNLC region of the equatorial Pacific, using the aforementioned newly developed electrochemical technique, was investigated as part of a mesoscale iron-enrichment study called Iron-Ex II. Samples were analyzed from waters outside of the iron-enriched areas in order to characterize the ambient conditions. The titration data for these surface water samples were modeled as consisting of two classes of ligands, a strong class (L{dollar}sb1){dollar} with an average surface-water concentration of 0.31 nM with a conditional stability constant {dollar}Ksbsp{lcub}FeLsb1,Fespprime{rcub}{lcub}cond{rcub}{dollar} = {dollar}4.7times10sp{lcub}12{rcub}{dollar} M{dollar}sp{lcub}-1{rcub}{dollar} and a weaker ligand class (L{dollar}sb2){dollar} with an average concentration of 0.19 nM with {dollar}Ksbsp{lcub}FeLsb2,Fespprime{rcub}{lcub}cond{rcub} = 6.5times 10sp{lcub}11{rcub}{dollar} M{dollar}sp{lcub}-1{rcub}.{dollar} Total dissolved iron concentrations, (Fe{dollar}sb{lcub}rm T{rcub}{dollar}), were measured using a new technique which combines UV-oxidation to destroy any natural Fe(III)-binding ligands, followed by adsorptive cathodic stripping voltammetry using salicylaldoxime as the added ligand. Ambient values of (Fe{dollar}sb{lcub}rm T{rcub} {dollar}) were found to be on the order of 20 pM in these surface waters.; Changes in ligand concentrations and iron speciation in response to mesoscale nanomolar levels of iron additions to the surface waters of the equatorial Pacific during the Iron-Ex II Experiment were also determined. The results from measurements made throughout the course of the iron-enrichment experiments suggested that the biota were the source of these Fe(III)-binding organic ligands and that they were produced rapid...