Iron reduction by photoproduced superoxide in seawater

The reaction of dissolved Fe(III) with photochemically produced Superoxide radical (O2−) was examined as a potentially important source of Fe(II) in sunlit seawater. The relative rates of Fe(III) reduction and Fe(II) oxidation by O2− were determined in the presence of 0.7 M Nad and high concentrations of O2−. At pH values above 5.5, most of the dissolved iron was present as Fe(II) after steady state was reached. [Colloidal Fe(III) oxyhydroxides did not react with Superoxide radical at appreciable rates.] The effect of organic and inorganic complexation on the relative rates of reactions of Cu(I) and Cu(II) with O2− also was examined. Using these results and previously published O2− flux measurements in sunlit open-ocean surface water, we calculate that, despite possible competition for O2− by copper, the steady-state concentration of O2− is high enough to result in significant concentrations of Fe(II). Our calculations indicate that, in the absence of organic complexation of Fe(III), 30–75% of the dissolved iron in the photic zone will be present as Fe(II) during daytime. Consistent with this hypothesis, illumination by simulated sunlight of open-ocean water samples (acidified to pH 7.3), to which 5 nM iron had been added, resulted in the conversion of approximately 60% of the dissolved iron into Fe(II) after 20 min.