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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., Competing Interests: The authors declare no competing interest., (Copyright © 2020 the Author(s). Published by PNAS.)

Natural abundance nitrogen and oxygen isotopes of nitrate (δ 15 N NO3 and δ 18 O NO3 ) provide an important tool for evaluating sources and transformations of natural and contaminant nitrate (NO 3 - ) in the environment. Nevertheless, conventional interpretations of NO 3 - isotope distributions appear at odds with patterns emerging from studies of nitrifying and denitrifying bacterial cultures. To resolve this conundrum, we present results from a numerical model of NO 3 - isotope dynamics, demonstrating that deviations in δ 18 O NO3 vs. δ 15 N NO3 from a trajectory of 1 expected for denitrification are explained by isotopic over-printing from coincident NO 3 - production by nitrification and/or anammox. The analysis highlights two driving parameters: (i) the δ 18 O of ambient water and (ii) the relative flux of NO 3 - production under net denitrifying conditions, whether catalyzed aerobically or anaerobically. In agreement with existing analyses, dual isotopic trajectories >1, characteristic of marine denitrifying systems, arise predominantly under elevated rates of NO 2 - reoxidation relative to NO 3 - reduction (>50%) and in association with the elevated δ 18 O of seawater. This result specifically implicates aerobic nitrification as the dominant NO 3 - producing term in marine denitrifying systems, as stoichiometric constraints indicate anammox-based NO 3 - production cannot account for trajectories >1. In contrast, trajectories <1 comprise the majority of model solutions, with those representative of aquifer conditions requiring lower NO 2 - reoxidation fluxes (<15%) and the influence of the lower δ 18 O of freshwater. Accordingly, we suggest that widely observed δ 18 O NO3 vs. δ 15 N NO3 trends in freshwater systems (<1) must result from concurrent NO 3 - production by anammox in anoxic aquifers, a process that has been largely overlooked., Competing Interests: The authors declare no conflict of interest.

Archaeal membrane lipids known as glycerol dibiphytanyl glycerol tetraethers (GDGTs) are the basis of the TEX86 paleotemperature proxy. Because GDGTs preserved in marine sediments are thought to originate mainly from planktonic, ammonia-oxidizing Thaumarchaeota, the basis of the correlation between TEX86 and sea surface temperature (SST) remains unresolved: How does TEX86 predict surface temperatures, when maximum thaumarchaeal activity occurs below the surface mixed layer and TEX86 does not covary with in situ growth temperatures? Here we used isothermal studies of the model thaumarchaeon Nitrosopumilus maritimus SCM1 to investigate how GDGT composition changes in response to ammonia oxidation rate. We used continuous culture methods to avoid potential confounding variables that can be associated with experiments in batch cultures. The results show that the ring index scales inversely (R(2) = 0.82) with ammonia oxidation rate (ϕ), indicating that GDGT cyclization depends on available reducing power. Correspondingly, the TEX86 ratio decreases by an equivalent of 5.4 °C of calculated temperature over a 5.5 fmol·cell(-1)·d(-1) increase in ϕ. This finding reconciles other recent experiments that have identified growth stage and oxygen availability as variables affecting TEX86 Depth profiles from the marine water column show minimum TEX86 values at the depth of maximum nitrification rates, consistent with our chemostat results. Our findings suggest that the TEX86 signal exported from the water column is influenced by the dynamics of ammonia oxidation. Thus, the global TEX86-SST calibration potentially represents a composite of regional correlations based on nutrient dynamics and global correlations based on archaeal community composition and temperature.