IRON REDUCTION BY PHOTOPRODUCED SUPEROXIDE IN SEAWATER

The reaction of dissolved Fe(III) with photochemically produced supero xide radical (O-2(-)) was examined as a potentially important source o f Fe(II) in sunlit seawater. The relative rates of Fe(III) reduction a nd Fe(II) oxidation by O-2(-) were determined in the presence of 0.7 M NaCl and high concentrations of O-2(-). At pH values above 5.5, most of the dissolved iron was present as Fe(II) after steady state was rea ched. [Colloidal Fe(III) oxyhydroxides did not react with superoxide r adical at appreciable rates.] The effect of organic and inorganic comp lexation on the relative rates of reactions of Cu(I) and Cu(II) with O -2(-) also was examined. Using these results and previously published O-2(-) flux measurements in sunlit open-ocean surface water, we calcul ate that, despite possible competition for O-2(-) by copper, the stead y-state concentration of O-2(-) is high enough to result in significan t concentrations of Fe(II). Our calculations indicate that, in the abs ence of organic complexation of Fe(III), 30-75% of the dissolved iron in the photic zone will be present as Fe(II) during daytime. Consisten t with this hypothesis, illumination by simulated sunlight of open-oce an water samples (acidified to pH 7.3), to which 5 nM iron had been ad ded, resulted in the conversion of approximately 60% of the dissolved iron into Fe(II) after 20 min.