Methane is a potent greenhouse gas commonly supersaturated in the oxic surfaces waters of oceans and lakes, yet canonical microbial methanogens are obligate anaerobes. One proposed methane production pathway involves microbial degradation of methylphosphonate (MPn), which can proceed in the presence of oxygen. Directly tracing dissolved methane to its source in oxic waters, however, remains a challenge. To address this knowledge gap, we quantified the carbon isotopic fractionation between substrate MPn and product methane (1.3‰) in lab experiments, which was 1 to 2 orders of magnitude smaller than canonical pathways of microbial methanogenesis (20 to 100‰). Together, these results indicated that microbial catabolism of MPn is a source of methane in surface oceans and lake waters, but to differentiate sources of MPn in nature a further accounting of all sources is necessary. Methane from this pathway must be considered in constraining the marine carbon cycle and methane budget.

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Marine microbes produce extracellular reactive oxygen species (ROS) such as superoxide and hydrogen peroxide (H 2 O 2 ) 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 H 2 O 2 in the marine water column. In this study, we use isotope-labeling techniques to determine the redox fate of H 2 O 2 in a range of marine environments off the West Coast of California. We find that H 2 O 2 reduction is greater than or equal to H 2 O 2 oxidation at most sampled depths, with notable exceptions in some surface and intermediate water depths. The observation that H 2 O 2 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.2 and H 2 O 2 have been observed in both freshwater and seawater, and have been found to result directly