Photochemical and biochemical controls on reactive oxygen and iron speciation in the pelagic surface ocean

A time-dependent chemistry model is used to predict reactive oxygen species (ROS = H2O2 + O2−) and dissolved Fe (DFe) speciation in the surface ocean. A new feature of the model is inclusion of biological sources of superoxide. The model suggests that biochemistry mediated by phytoplankton cells is as important as photochemistry for the formation of ROS. Formation of stable organic Fe(III) complexes (FeL) maintains the concentration of DFe in seawater. Iron speciation in the model is also controlled by biochemical and photochemical processes, and is far from thermodynamic equilibrium. During light periods, photo-reduction of FeL produces dissolved inorganic iron much more than thermal decomposition and cell-surface reduction of FeL, thus facilitating phytoplankton uptake of iron in the ocean. During the nighttime, O2− produced by reductases on cell surfaces both reacts with FeL, producing Fe(II), and retards the oxidation of Fe(II) and subsequent formation of FeL; therefore significant levels of bio-available Fe is maintained through this period. Photo-reduction nearly balances the formation of FeL in the model, and may control bioavailability of dissolved iron. This suggests a possible extracellular mechanism of iron and light colimitation to primary productivity. A phytoplankton growth limitation by FeL photo-reduction depends on its rate coefficient for which we need extensive measurements in natural seawater.