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1. Global Nitrous Oxide Production Determined by Oxygen Sensitivity of Nitrification and Denitrification

   https://doi.org/10.1029/2018GB005887  

   Ji, Qixing; Buitenhuis, Erik; Suntharalingam, Parvadha; Sarmiento, Jorge L.; Ward, Bess B. (December 2018, Global Biogeochemical Cycles)

   Abstract The ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N₂O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N₂O fluxes, we report new relationships between oxygen concentration and rates of N₂O production from nitrification and denitrification directly measured with¹⁵N tracers in the Eastern Tropical Pacific. Highest N₂O production rates occurred near the oxic‐anoxic interface, where there is strong potential for N₂O efflux to the atmosphere. The dominant N₂O source in oxygen minimum zones was nitrate reduction, the rates of which were 1 to 2 orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N₂O from both nitrification and denitrification. These experimental data provide new constraints to a multicomponent global ocean biogeochemical model, which yielded annual oceanic N₂O efflux of 1.7–4.4 Tg‐N (median 2.8 Tg‐N, 1 Tg = 10¹² g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N₂O production pathway in the marine environment. 

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2. Microbial N2O consumption in and above marine N2O production hotspots

   https://doi.org/10.1038/s41396-020-00861-2  

   Sun, Xin; Jayakumar, Amal; Tracey, John C.; Wallace, Elizabeth; Kelly, Colette L.; Casciotti, Karen L.; Ward, Bess B. (May 2021, The ISME Journal)

   null (Ed.)

   Abstract The ocean is a net source of N 2 O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N 2 O via microbial N 2 O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N 2 O consumption measurements from anoxic zones to the sharp oxygen gradient above them, and experimentally determine kinetic parameters in both oxic and anoxic seawater for the first time. We find that the substrate affinity, O 2 tolerance, and community composition of N 2 O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N 2 O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N 2 O cycling. Microbes from the oxic layer consume N 2 O under anoxic conditions at a much faster rate than microbes from anoxic zones. These experimental results are in keeping with model results which indicate that N 2 O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O 2 and N 2 O gradients right above the ODZ is a previously ignored potential gatekeeper of N 2 O and should be accounted for in the marine N 2 O budget. 

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3. A critical review of the ¹⁵ N ₂ tracer method to measure diazotrophic production in pelagic ecosystems

   https://doi.org/10.1002/lom3.10353  

   White, Angelicque E.; Granger, Julie; Selden, Corday; Gradoville, Mary R.; Potts, Lindsey; Bourbonnais, Annie; Fulweiler, Robinson W.; Knapp, Angela N.; Mohr, Wiebke; Moisander, Pia H.; et al (March 2020, Limnology and Oceanography: Methods)

   Abstract Dinitrogen (N₂) fixation is an important source of biologically reactive nitrogen (N) to the global ocean. The magnitude of this flux, however, remains uncertain, in part because N₂fixation rates have been estimated following divergent protocols and because associated levels of uncertainty are seldom reported—confounding comparison and extrapolation of rate measurements. A growing number of reports of relatively low but potentially significant rates of N₂fixation in regions such as oxygen minimum zones, the mesopelagic water column of the tropical and subtropical oceans, and polar waters further highlights the need for standardized methodological protocols for measurements of N₂fixation rates and for calculations of detection limits and propagated error terms. To this end, we examine current protocols of the¹⁵N₂tracer method used for estimating diazotrophic rates, present results of experiments testing the validity of specific practices, and describe established metrics for reporting detection limits. We put forth a set of recommendations for best practices to estimate N₂fixation rates using¹⁵N₂tracer, with the goal of fostering transparency in reporting sources of uncertainty in estimates, and to render N₂fixation rate estimates intercomparable among studies. 

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4. Nutrient management offsets the effect of deoxygenation and warming on nitrous oxide emissions in a large US estuary

   https://doi.org/10.1126/sciadv.adq5014  

   Tang, Weiyi; Da, Fei; Tracey, John C; Intrator, Naomi; Kunes, Moriah A; Lee, Jenna A; Wan, Xianhui Sean; Jayakumar, Amal; Friedrichs, Marjorie_A M; Ward, Bess B (December 2024, Science Advances)

   Many estuaries experience eutrophication, deoxygenation and warming, with potential impacts on greenhouse gas emissions. However, the response of N₂O production to these changes is poorly constrained. Here we applied nitrogen isotope tracer incubations to measure N₂O production under experimentally manipulated changes in oxygen and temperature in the Chesapeake Bay—the largest estuary in the United States. N₂O production more than doubled from nitrification and increased exponentially from denitrification when O₂was decreased from >20 to <5 micromolar. Raising temperature from 15° to 35°C increased N₂O production 2- to 10-fold. Developing a biogeochemical model by incorporating these responses, N₂O emissions from the Chesapeake Bay were estimated to decrease from 157 to 140 Mg N year⁻¹from 1986 to 2016 and further to 124 Mg N year⁻¹in 2050. Although deoxygenation and warming stimulate N₂O production, the modeled decrease in N₂O emissions, attributed to decreased nutrient inputs, indicates the importance of nutrient management in curbing greenhouse gas emissions, potentially mitigating climate change. 

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5. Dark biological superoxide production as a significant flux and sink of marine dissolved oxygen

   https://doi.org/10.1073/pnas.1912313117  

   Sutherland, Kevin M.; Wankel, Scott D.; Hansel, Colleen M. (January 2020, Proceedings of the National Academy of Sciences)

   The balance between sources and sinks of molecular oxygen in the oceans has greatly impacted the composition of Earth’s atmosphere since the evolution of oxygenic photosynthesis, thereby exerting key influence on Earth’s climate and the redox state of (sub)surface Earth. The canonical source and sink terms of the marine oxygen budget include photosynthesis, respiration, photorespiration, the Mehler reaction, and other smaller terms. However, recent advances in understanding cryptic oxygen cycling, namely the ubiquitous one-electron reduction of O 2 to superoxide by microorganisms outside the cell, remains unexplored as a potential player in global oxygen dynamics. Here we show that dark extracellular superoxide production by marine microbes represents a previously unconsidered global oxygen flux and sink comparable in magnitude to other key terms. We estimate that extracellular superoxide production represents a gross oxygen sink comprising about a third of marine gross oxygen production, and a net oxygen sink amounting to 15 to 50% of that. We further demonstrate that this total marine dark extracellular superoxide flux is consistent with concentrations of superoxide in marine environments. These findings underscore prolific marine sources of reactive oxygen species and a complex and dynamic oxygen cycle in which oxygen consumption and corresponding carbon oxidation are not necessarily confined to cell membranes or exclusively related to respiration. This revised model of the marine oxygen cycle will ultimately allow for greater reconciliation among estimates of primary production and respiration and a greater mechanistic understanding of redox cycling in the ocean. 

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