Light-Induced Redox Cycling of Iron: Roles for CO2 Uptake and Release by Aquatic Ecosystems

Abstract

The light-induced redox cycling of iron plays an important role for the formation of iron species that are available to phytoplankton. In aquatic ecosystems, siderophore-type ligands are likely to be involved in the formation of bioavailable iron via photolysis of both solution and surface Fe(III) complexes. Atmospheric waters are important “reactors” for the formation of potentially bioavailable iron. Atmospheric iron input is a significant external iron source to many oceanographic regions. Since iron is an essential micronutrient for phytoplankton, the presence of bioavailable iron in the euphotic zone of oceans is a prerequisite for the removal of atmospheric CO2 via the biological pump, particularly in high-nitrate, low-chlorophyll regions. Light-induced reduction of Fe(III) is accompanied by the oxidation of dissolved organic matter (DOM). DOM photooxidation results in the formation of CO2 and to a smaller extent CO. Also the photooxidation of colored dissolved organic matter (CDOM) may be catalyzed by iron and results in CDOM bleaching. CDOM photobleaching is particularly pronounced in stratified aquatic systems and negatively affects the biological pump since CDOM protects phytoplankton from the damaging solar UV-B radiation. Changes in continental hydrology due to climate and land-use change are expected to enhance the flux of terrigenous DOM and iron from terrestrial to aquatic ecosystems, possibly increasing the rate of CO2 formation via iron-catalyzed photooxidation of DOM. Hence, the light-induced redox cycling of iron may play an important role in climate-carbon cycle feedbacks.