THE PHOTOLYSIS OF COLLOIDAL IRON IN THE OCEANS

THE extent to which iron limits primary production in open ocean waters depends not only on the aeolian supply 1-3, but also on factors that control its availability for biological uptake. Although the marine chemistry of iron is poorly understood, much of it occurs in refractory particulate 1,2 and colloidal 4 states-forms unavailable for direct assimilation by phytoplankton 5,6. But iron availability depends on its chemical lability 5, or ease of dissolution; hence processes that alter the lability of particulate and colloidal iron in sea water govern their availability to phytoplankton. Here we report that light increases the lability of colloidal iron in sea water of pH 8, with a photon-normalized spectral dependence that generally increases with decreasing wavelength from 400-300 nm. From optical modelling we predict that the incident solar spectrum, combined with the preferential attenuation of shorter ultraviolet wavelengths in sea water, will lead to a maximum depth-integrated photoreaction near 380-400 nm. Our results show that the photolysis of forms of solid iron may occur deep into the ocean's euphotic zone, and hence that the availability of iron to phytoplankton in the ocean may be much greater than previously thought.