THE PHOTOCONVERSION OF COLLOIDAL IRON OXYHYDROXIDES IN SEAWATER

The marine photochemistry of colloidal Fe oxyhydroxides was studied in order to determine its role in modifying the availability of Fe for phytoplankton. Using a new technique that measures a labile portion of total Fe in seawater, the lability of synthetic crystalline and noncrystalline colloidal Fe oxyhydroxides was found to increase in pH 8 seawater upon irradiation with UV light, and simulated and natural full spectrum sunlight. The increased lability appeared to result from an organic-dependent Fe photoreduction followed by very rapid Fe(II) oxidation and Fe(III) reprecipitation as ferrihydrite. Aging in the dark caused significant reductions in the lability of the photoproduced ferrihydrites, with both synthetic iron oxides and natural iron from sunlit surface seawater. The results suggest that the lability of Fe in surface seawater varies in response to competing processes of photoredox and aging of Fe on oxyhydroxide surfaces. Because the lability of colloidal Fe correlates positively with its availability to marine algae, the photolysis of colloidal Fe has direct implications for phytoplankton.