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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. Non‐denitrifier nitrous oxide reductases dominate marine biomes

   https://doi.org/10.1111/1758-2229.12879  

   Bertagnolli, Anthony_D; Konstantinidis, Konstantinos_T; Stewart, Frank_J (September 2020, Environmental Microbiology Reports)

   Summary Microbial enzymes often occur as distinct variants that share the same substrate but differ in substrate affinity, sensitivity to environmental conditions, or phylogenetic ancestry. Determining where variants occur in the environment helps identify thresholds that constrain microbial cycling of key chemicals, including the greenhouse gas nitrous oxide (N₂O). To understand the enzymatic basis of N₂O cycling in the ocean, we mined metagenomes to characterize genes encoding bacterial nitrous oxide reductase (NosZ) catalyzing N₂O reduction to N₂. We examined data sets from diverse biomes but focused primarily on those from oxygen minimum zones where N₂O levels are often elevated. With few exceptions, marinenosZdata sets were dominated by ‘atypical’ clade II gene variants. AtypicalnosZhas been associated with low oxygen, enhanced N₂O affinity, and organisms lacking enzymes for complete denitrification, i.e., non‐denitrifiers. AtypicalnosZ often occurred in metagenome‐assembled genomes (MAGs) with nitrate or nitrite respiration genes, although MAGs with genes for complete denitrification were rare. We identified atypicalnosZ in several taxa not previously associated with N₂O consumption, in addition to known N₂O‐associated groups. The data suggest that marine environments generally select for high N₂O‐scavenging ability across diverse taxa and have implications for how N₂O concentration may affect N₂O removal rates. 

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3. 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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4. Global reconstruction reduces the uncertainty of oceanic nitrous oxide emissions and reveals a vigorous seasonal cycle

   https://doi.org/10.1073/pnas.1921914117  

   Yang, Simon; Chang, Bonnie X.; Warner, Mark J.; Weber, Thomas S.; Bourbonnais, Annie M.; Santoro, Alyson E.; Kock, Annette; Sonnerup, Rolf E.; Bullister, John L.; Wilson, Samuel T.; et al (May 2020, Proceedings of the National Academy of Sciences)

   Assessment of the global budget of the greenhouse gas nitrous oxide ( ${\mathrm{N}}_2$O) is limited by poor knowledge of the oceanic ${\mathrm{N}}_2$O flux to the atmosphere, of which the magnitude, spatial distribution, and temporal variability remain highly uncertain. Here, we reconstruct climatological ${\mathrm{N}}_2$O emissions from the ocean by training a supervised learning algorithm with over 158,000 ${\mathrm{N}}_2$O measurements from the surface ocean—the largest synthesis to date. The reconstruction captures observed latitudinal gradients and coastal hot spots of ${\mathrm{N}}_2$O flux and reveals a vigorous global seasonal cycle. We estimate an annual mean ${\mathrm{N}}_2$O flux of 4.2 ± 1.0 Tg N $\cdot {\mathrm{y}}^{-1}$, 64% of which occurs in the tropics, and 20% in coastal upwelling systems that occupy less than 3% of the ocean area. This ${\mathrm{N}}_2$O flux ranges from a low of 3.3 ± 1.3 Tg N $\cdot {\mathrm{y}}^{-1}$in the boreal spring to a high of 5.5 ± 2.0 Tg N $\cdot {\mathrm{y}}^{-1}$in the boreal summer. Much of the seasonal variations in global ${\mathrm{N}}_2$O emissions can be traced to seasonal upwelling in the tropical ocean and winter mixing in the Southern Ocean. The dominant contribution to seasonality by productive, low-oxygen tropical upwelling systems (>75%) suggests a sensitivity of the global ${\mathrm{N}}_2$O flux to El Niño–Southern Oscillation and anthropogenic stratification of the low latitude ocean. This ocean flux estimate is consistent with the range adopted by the Intergovernmental Panel on Climate Change, but reduces its uncertainty by more than fivefold, enabling more precise determination of other terms in the atmospheric ${\mathrm{N}}_2$O budget. 

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5. 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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