Considering the role of photochemistry in controlling the effective redox state in the surface ocean

Photochemical reactions within dissolved organic matter (DOM) are a source of reactive oxygen species (ROS) everywhere the sun shines on seawater. The oxidation state of the surface ocean is a fundamental chemical parameter controlling all redox reactive compounds, and ROS participates directly in establishing this parameter. Superoxide (O₂⁻) is the primary ROS that controls hydrogen peroxide (H₂O₂) formation and the fate of O₂ in photooxidation. Together, O₂⁻ and H₂O₂ are involved in the degradation of organic pollutants, organic carbon cycles, redox cycling of Fe, Cu and Mn which effect marine phytoplankton distributions, and contribute to external oxidative stress in major taxa of marine organisms. Thus, quantitative data on the production, sinks, and concentrations of O₂⁻ and H₂O₂ in the ocean are fundamental to a full understanding of their role in important biogeochemical cycles. New data suggests that critical O₂⁻ redox cycles may have been severely underestimated by extrapolating mechanisms derived in freshwater solutions to seawater, and by assuming “dark” decay reactions are the only ones operative in sunlight. Direct measurements of O₂⁻ steady state, decay rates and H₂O₂accumulation in irradiated samples with various DOM sources show evidence for short-lived, photochemically produced reactive transients that suggest a ROS redox cycle “in the light” that has not been adequately defined. This presentation provides new ROS kinetic data and considers potential consequences to models of the effective redox chemistry in the surface ocean with the “lights on.”