Relating the Photochemical Production of Superoxide and Hydrogen Peroxide to the Photooxidation of Dissolved Organic Carbon in the Ocean

Newly published ratios between the photochemical production rates of carbon dioxide (CO2) and hydrogen peroxide (H2O2) from chromophoric dissolved organic carbon (CDOM) in marine waters provide better constraints on the significance of photochemical carbon oxidation in global ocean cycles; with yearly rates ~4 times greater than previous estimates, amounting to the remineralization of ~2.7 PgC yr^-1. Interestingly, our ratio approach, which suggests mechanistic similarities between H2O2 and CO2 production, predicts that direct photoproduction measurements of the precursor for H2O2, superoxide (O2−), may provide the best estimate of the maximum potential for photochemical carbon oxidation via CO₂ formation in the marine environment. This work presents laboratory and model results from the photochemical investigation of these reactive oxygen species and CO2 production in river, shelf, and offshore samples from the coast of Georgia. We have determined production rates and spectral quantum efficiency curves for irradiations with varied enzymatic, physical and chemical sample treatments to further constrain the stoichiometric relationships (eg. O2−:HOOH ratio range = 1.7 to > 4) between HOOH, O2−, and CO2 that will prove useful in quantifying and expanding our current knowledge of the role of photochemistry in marine carbon and redox cycling.