The Redox Fate Of Hydrogen Peroxide In The Marine Water Column

Marine microbes produce extracellular reactive oxygen species (ROS) such as superoxide and hydrogen peroxide (H2O2) as a result of regulated and nonregulated physiological and metabolic reactions. ROS production can be a sink and cryptic recycling flux of dissolved oxygen that may rival other key fluxes in the global oxygen cycle; however, the low abundance and high turnover rate of ROS makes this figure difficult to constrain. One key step in determining the disparity between the gross production of ROS and the net sink of dissolved oxygen lies in understanding the degradation pathways of H2O2 in the marine water column. In this study, we use isotope-labeling techniques to determine the redox fate of H2O2 in a range of marine environments off the West Coast of California. We find that H2O2 reduction is greater than or equal to H2O2 oxidation at most sampled depths, with notable exceptions in some surface and intermediate water depths. The observation that H2O2 oxidation can exceed reduction in the dark ocean indicates the presence of an oxidizing decay pathway that is not among the known suite of microbially mediated enzymatic pathways (i.e., catalase and peroxidase), pointing to an abiotic and/or a nonenzymatic decay pathway at intermediate water depths. These results highlight the complexity and heterogeneity of ROS decay pathways in natural waters and their unconstrained regulation of oxygen levels within the ocean.