Biological O2 Research Articles

Geodynamic oxidation of Archean terrestrial surfaces

One of the paradoxes of the Archean rock record is how evidence for oxidative continental weathering is plausible when the atmosphere lacked oxygen prior to the Great Oxidation Event ca. 2.5 billion years ago. We detected reactive oxygen species (ROS) production at various silicate-water interfaces in a simulated Archean atmosphere and calculated homolysis rates of silicon–oxygen bonds in these minerals. We find that ROS can be produced while silicates were eroded and transported by streams to depositional basins. The magnitude of ROS production increases from mafic- to felsic-silicates. During early continent formation, e.g., the Kaapvaal and Pilbara basins in the Paleoarchean, the emplacement of granitoids and their subsequent exposure to physical weathering might have annually provided at least 1.73 × 108 mol O2 before the biological O2 production. Importantly, this process could have oxidatively mobilized redox-sensitive elements to seawater, increasing the availability of bioessential nutrient elements for the Archean biosphere.

Spatiotemporal variations of the <i>δ</i>(O<sub>2</sub> ∕ N<sub>2</sub>), CO<sub>2</sub> and <i>δ</i>(APO) in the troposphere over the western North Pacific

Abstract. We analyzed air samples collected on board a C-130 cargo aircraft over the western North Pacific from May 2012 to March 2020 for atmospheric δ(O2 / N2) and CO2 amount fraction. Observations were corrected for significant artificial fractionation of O2 and N2 caused by thermal diffusion during the air sample collection using the simultaneously measured δ(Ar / N2). The observed seasonal cycles of the δ(O2 / N2) and atmospheric potential oxygen (δ(APO)) varied nearly in opposite phase to the cycle of the CO2 amount fraction at all latitudes and altitudes. Seasonal amplitudes of δ(APO) decreased with latitude from 34 to 25∘ N, as well as with increasing altitude from the surface to 6 km by 50 %–70 %, while those of the CO2 amount fraction decreased by less than 20 %. By comparing the observed values with the simulated δ(APO) and CO2 amount fraction values generated by an atmospheric transport model, we found that the seasonal δ(APO) cycle in the middle troposphere was modified significantly by a combination of the northern and southern hemispheric seasonal cycles due to the interhemispheric mixing of air. The simulated δ(APO) underestimated the observed interannual variation in δ(APO) significantly, probably due to the interannual variation in the annual mean air–sea O2 flux. Interannual variation in δ(APO) driven by the net marine biological activities, obtained by subtracting the assumed solubility-driven component of δ(APO) from the total variation, indicated a clear influence on annual net sea–air marine biological O2 flux during El Niño and net air–sea flux during La Niña. By analyzing the observed secular trends of δ(O2 / N2) and the CO2 amount fraction, global average terrestrial biospheric and oceanic CO2 uptakes for the period 2012–2019 were estimated to be (1.8±0.9) and (2.8±0.6) Pg a−1 (C equivalents), respectively.

Summer Oxygen Dynamics on a Southern Arabian Gulf Coral Reef

During the summer the Arabian Gulf is the world's warmest sea, also characterized by hypersalinity and extreme annual temperature fluctuations (12–35oC), making it marginal for coral growth. Yet extensive reefs occur in all eight nations bordering the Gulf. Here we present data demonstrating recurrent summer hypoxia events [oxygen concentration (O2) <2 mg l−1] at a reef in the southern Gulf. Currently these episodes are short enough (median 3 h, max 10 h) to preclude mass mortality. Will this always be the case? Predicting future Gulf hypoxia risk for coral reef ecosystems requires diagnosing the underlying causes driving the timing and magnitude of O2 swings. To this end, we compare our data with the output of a simple coupled 1-D water column/biogeochemical model of the reef environment. This allows us to give quantitative estimates of the O2 fluxes produced by photosynthesis both in the water column and within the coral framework, by respiration processes in the benthos, and from the atmosphere. We demonstrate the role of turbulent mixing, and in particular of tides, in shaping the temporal variability of the amplitude of the diel O2 cycle. We find that, in spite of significant turbulent mixing, which maintains the temperature vertically well-mixed, the biological O2 production and consumption is dominant over the atmospheric O2 flux, and is sufficient to generate vertical differences of 1 to 5 mg l−1 between the bottom and 1.5 m above it. While estimating future trends of hypoxia frequency will require further study, the present findings single out the relevant physical and biological processes (and their interplay) which deserve further scrutiny. The Gulf today experiences temperatures expected to occur across much of the tropics by the end of the century, and the observation of recurrent hypoxia events in the Gulf suggests that similar hypoxic phenomena may represent an important, but to date underappreciated, threat to the future of global coral reefs.

Emerging roles for thiol dioxygenases as oxygen sensors.

Cysteine dioxygenases, 3-mercaptopropionate dioxygenases and mercaptosuccinate dioxygenases are all thiol dioxygenases (TDOs) that catalyse oxidation of thiol molecules to sulphinates. They are Fe(II)-dependent dioxygenases with a cupin fold that supports a 3xHis metal-coordinating triad at the active site. They also have other, broadly common features including arginine residues involved in substrate carboxylate binding and a conserved trio of residues at the active site featuring a tyrosine important in substrate binding catalysis. Recently, N-terminal cysteinyl dioxygenase enzymes (NCOs) have been identified in plants (plant cysteine oxidases, PCOs), while human 2-aminoethanethiol dioxygenase (ADO) has been shown to act as both an NCO and a small molecule TDO. Although the cupin fold and 3xHis Fe(II)-binding triad seen in the small molecule TDOs are conserved in NCOs, other active site features and aspects of the overall protein architecture are quite different. Furthermore, the PCOs and ADO appear to act as biological O2 sensors, as shown by kinetic analyses and hypoxic regulation of the stability of their biological targets (N-terminal cysteine oxidation triggers protein degradation via the N-degron pathway). Here, we discuss the emergence of these two subclasses of TDO including structural features that could dictate their ability to bind small molecule or polypeptide substrates. These structural features may also underpin the O2 -sensing capability of the NCOs. Understanding how these enzymes interact with their substrates, including O2 , could reveal strategies to manipulate their activity, relevant to hypoxic disease states and plant adaptive responses to flooding.

ΔO2/N2′ as a New Tracer of Marine Net Community Production: Application and Evaluation in the Subarctic Northeast Pacific and Canadian Arctic Ocean

We compared field measurements of the biological O2 saturation anomalies, ΔO2/Ar and ΔO2/N2, from simultaneous oceanographic deployments of a membrane inlet mass spectrometer and optode/gas tension device (GTD). Data from the Subarctic Northeast Pacific and Canadian Arctic Ocean were used to evaluate ΔO2/N2 as an alternative to ΔO2/Ar for estimates of mixed layer net community production (NCP). We observed strong spatial coherence between ΔO2/Ar and ΔO2/N2, with small offsets resulting from differences in the solubility properties of Ar and N2 and their sensitivity to vertical mixing fluxes. Larger offsets between the two tracers were observed across hydrographic fronts and under elevated sea states, resulting from the differential time-response of the optode and GTD, and from bubble dissolution in the ship’s seawater lines. We used a simple numerical framework to correct for physical sources of divergence between N2 and Ar, deriving the tracer ΔO2/N2′. Over most of our survey regions, ΔO2/N2′ provided a better analog for ΔO2/Ar, and thus more accurate NCP estimates than ΔO2/N2. However, in coastal Arctic waters, ΔO2/N2 and ΔO2/N2′ performed equally well as NCP tracers. On average, mixed layer NCP estimated from ΔO2/Ar and ΔO2/N2′ agreed to within ∼2 mmol O2 m–2 d–1, with offsets typically smaller than other errors in NCP calculations. Our results demonstrate a significant potential to derive NCP from underway O2/N2 measurements across various oceanic regions. Optode/GTD systems could replace mass spectrometers for autonomous NCP derivation under many oceanographic conditions, thereby presenting opportunities to significantly expand global NCP coverage from various underway platforms.

Controlled Regulation of the Nitrile Activation of a Peroxocobalt(III) Complex with Redox-Inactive Lewis Acidic Metals.

Redox-inactive metal ions play vital roles in biological O2 activation and oxidation reactions of various substrates. Recently, we showed a distinct reactivity of a peroxocobalt(III) complex bearing a tetradentate macrocyclic ligand, [CoIII(TBDAP)(O2)]+ (1) (TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane), toward nitriles that afforded a series of hydroximatocobalt(III) complexes, [CoIII(TBDAP)(R-C(═NO)O)]+ (R = Me (3), Et, and Ph). In this study, we report the effects of redox-inactive metal ions on nitrile activation of 1. In the presence of redox-inactive metal ions such as Zn2+, La3+, Lu3+, and Y3+, the reaction does not form the hydroximatocobalt(III) complex but instead gives peroxyimidatocobalt(III) complexes, [CoIII(TBDAP)(R-C(═NH)O2)]2+ (R = Me (2) and Ph (2Ph)). These new intermediates were characterized by various physicochemical methods including X-ray diffraction analysis. The rates of the formation of 2 are found to correlate with the Lewis acidity of the additive metal ions. Moreover, complex 2 was readily converted to 3 by the addition of a base. In the presence of Al3+, Sc3+, or H+, 1 is converted to [CoIII(TBDAP)(O2H)(MeCN)]2+ (4), and further reaction with nitriles did not occur. These results reveal that the reactivity of the peroxocobalt(III) complex 1 in nitrile activation can be regulated by the redox-inactive metal ions and their Lewis acidity. DFT calculations show that the redox-inactive metal ions stabilize the peroxo character of end-on Co-η1-O2 intermediate through the charge reorganization from a CoII-superoxo to a CoIII-peroxo intermediate. A complete mechanistic model explaining the role of the Lewis acid is presented.

Oxygenation, Life, and the Planetary System during Earth's Middle History: An Overview

The long history of life on Earth has unfolded as a cause-and-effect relationship with the evolving amount of oxygen (O2) in the oceans and atmosphere. Oxygen deficiency characterized our planet's first 2 billion years, yet evidence for biological O2 production and local enrichments in the surface ocean appear long before the first accumulations of O2 in the atmosphere roughly 2.4 to 2.3 billion years ago. Much has been written about this fundamental transition and the related balance between biological O2 production and sinks coupled to deep Earth processes that could buffer against the accumulation of biogenic O2. However, the relationship between complex life (eukaryotes, including animals) and later oxygenation is less clear. Some data suggest O2 was higher but still mostly low for another billion and a half years before increasing again around 800 million years ago, potentially setting a challenging course for complex life during its initial development and ecological expansion. The apparent rise in O2 around 800 million years ago is coincident with major developments in complex life. Multiple geochemical and paleontological records point to a major biogeochemical transition at that time, but whether rising and still dynamic biospheric oxygen triggered or merely followed from innovations in eukaryotic ecology, including the emergence of animals, is still debated. This paper focuses on the geochemical records of Earth's middle history, roughly 1.8 to 0.5 billion years ago, as a backdrop for exploring possible cause-and-effect relationships with biological evolution and the primary controls that may have set its pace, including solid Earth/tectonic processes, nutrient limitation, and their possible linkages. A richer mechanistic understanding of the interplay between coevolving life and Earth surface environments can provide a template for understanding and remotely searching for sustained habitability and even life on distant exoplanets.

Evaluation of Pollution Load: Heavy Metal Contents and Physiochemical Properties of the Great Kwa River, Calabar, Cross River State, Nigeria

Background and Objectives: Human activities can increase the amount of pollutants in the environment leading to water pollution. The contamination of surface water and sediments by heavy metals can result in adverse health conditions of humans, due to the bioaccumulation of metals. This study seeks to assess the pollution load of some heavy metals and physiochemical properties in Great Kwa River.
 Methods: Water samples were collected once a month from three stations within four selected months in wet and dry seasons. Samples were preserved by adding 10 ml of 6 N Nitric acid and stored at 5oC. Laboratory investigations were conducted and data analyzed statistically.
 Findings: The mean pH, dissolved O2, biological O2 demand, conductivity, NO3, NH4 and temperature were 7.44±1.24, 5.58±0.92, 0.957±0.002, 32.84±2.941, 0.1030±0.1701, 0.113±0.018 and 28.2±1.48, respectively during wet season. Mean concentration of Cu, Fe, Cr, Mn, Zn, Ni are 0.225±0.003, 0.489±0.009, 0.068±0.016, 0.102±0.006, 0.794±0.003, 0.031±0.001, respectively during the wet season. Mean concentration of metals in the river during the wet season were ranked in descending order as Zn > Fe > Cu > Mn > Cr > Ni. Iron (0.489±0.009 mg/l and 0.438±0.003 mg/l), chromium (0.068±0.016 mg/l and 0.055±0.008 mg/l), nickel (0.031±0.001 mg/l and 0.025±0.002 mg/l) were higher than Nigerian standard for drinking water in both seasons. The contamination factor of heavy metals in station 3 were 2.14, 1.56, 1.09, 0.89 for Fe, Cr, Ni, Zn, respectively, indicating moderate contamination. The pollution load index in stations 1, 2, 3 were 0.262, 0.537, 0.981, respectively.
 Conclusion: The present study provided base-line information on the pollution levels and physiochemical properties of the river. The river was not highly polluted by heavy metals except iron, chromium and nickel that were found above the recommended standards. However, drinking of water from the river over one’s lifetime is not advisable due to detrimental health hazards.

Euphotic Zone Metabolism in the North Pacific Subtropical Gyre Based on Oxygen Dynamics

AbstractWe report in situ rates of gross oxygen production (GOP), community respiration (R), and net community production (NCP) in the North Pacific Subtropical Gyre derived from mixed layer O2/Ar measurements. The measurements were conducted between November 2013 and January 2019 at the site of the Hawaii Ocean Time‐series program. Biological O2 concentration anomalies in the mixed layer showed a consistent diel variation, with values increasing during daytime due to net community production and decreasing during nighttime due to respiration. In situ mixed layer GOP and R, determined from these variations, covaried but showed no clear seasonal pattern, averaging 0.9 and 0.8 mmol O2 m−3 d−1, respectively. In situ rates of NCP determined from mixed layer O2/Ar ranged between −1.2 and 16.6 mmol O2 m−2 d−1. Our analyses indicate that at certain times of the year the diapycnal flux of O2 across the base of the mixed layer may be non‐negligible and, therefore, a fraction of O2/Ar‐derived NCP may form below the mixed layer. The seasonal climatology of NCP below the mixed layer (down to 150 m) was also estimated using near‐monthly changes in dissolved O2 concentrations. These calculations allowed us to estimate NCP for the entire euphotic zone (0–150 m), which shows pronounced seasonality, with a maximum in April‐May and a minimum in December, when the ecosystem becomes temporarily net heterotrophic. Annual NCP was estimated to be 2.1 ± 0.6 mol O2 m−2 yr−1, approximately 1.7 times the export of C through sinking particles captured in sediment traps at 150 m.

Negative cooperativity upon hydrogen bond-stabilized O2 adsorption in a redox-active metal\u2013organic framework

The design of stable adsorbents capable of selectively capturing dioxygen with a high reversible capacity is a crucial goal in functional materials development. Drawing inspiration from biological O2 carriers, we demonstrate that coupling metal-based electron transfer with secondary coordination sphere effects in the metal–organic framework Co2(OH)2(bbta) (H2bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole) leads to strong and reversible adsorption of O2. In particular, moderate-strength hydrogen bonding stabilizes a cobalt(III)-superoxo species formed upon O2 adsorption. Notably, O2-binding in this material weakens as a function of loading, as a result of negative cooperativity arising from electronic effects within the extended framework lattice. This unprecedented behavior extends the tunable properties that can be used to design metal–organic frameworks for adsorption-based applications.

Coral reef survival under accelerating ocean deoxygenation

Global warming and local eutrophication simultaneously lower oxygen (O2) saturation and increase biological O2 demands to cause deoxygenation. Tropical shallow waters, and their coral reefs, are particularly vulnerable to extreme low O2 (hypoxia) events. These events can drive mass mortality of reef biota; however, they currently remain unaccounted for when considering coral reef persistence under local environmental alterations and global climatic change. In this Perspective, we integrate existing biological, ecological and geochemical evidence to consider how O2 availability and hypoxia affect reef biota, with particular focus on the ecosystem architects, reef-building corals, that operate as both O2 consumers and producers. We pinpoint fundamental knowledge gaps and highlight the need to understand sub-lethal hypoxia effects that are likely already in play. Climate change and other human activities are decreasing ocean oxygen content. This Perspective considers the limited data on effects to coral reefs, including oxygen thresholds for lethal and sub-lethal effects in coral reef taxa, and proposes key research questions to address this critical issue.

An observation of primary production enhanced by coastal upwelling in the southwest East/Japan Sea

Abstract Coastal upwelling (CU) is an important process that causes changes in physical and chemical properties, resulting in variation of biological processes in a coastal area. In the southwestern part of the East/Japan Sea (SWES), CU has been alleged as one of the mechanisms responsible for higher net primary production (NPP) than the rest of the East/Japan Sea. We provide, for the first time, high spatial resolution underway observations of sea surface temperature, salinity, chlorophyll-a fluorescence and Δ(O2/Ar), revealing the physical and biological characteristics of the upwelled waters in the SWES. The cold, upwelled waters retained high fluorescence and Δ(O2/Ar), consistent with enhanced phytoplankton biomass and biological O2 production by CU. The net community production (NCP) in the ambient and upwelled waters were 33 ± 19 and 77 ± 41 mmol O2 m−2 d−1, respectively. The latter should be considered as a lower bound, containing a significant fraction of low O2 waters from subsurface. Satellite observation indicated that NPP in the upwelled waters were higher than the ambient waters by 51%. This implies that up to half of higher NPP in the SWES than the rest of the East/Japan Sea can be ascribed to CU events.

Mechanisms of Low‐Frequency Oxygen Variability in the North Pacific

This study investigates the mechanisms of interannual and decadal variability of dissolved oxygen (O2) in the North Pacific using historical observations and a hindcast simulation using the Community Earth System Model. The simulated variability of upper ocean (200 m) O2 is moderately correlated with observations where sampling density is relatively high. The dominant mode of O2 variability explains 24.8% of the variance and is significantly correlated with the Pacific Decadal Oscillation (PDO) index (r = 0.68). Two primary mechanisms are hypothesized by which the PDO controls upper ocean O2 variability. Vertical movement of isopycnals (“heave”) drives O2 variations in the deep tropics; isopycnal surfaces are depressed in the eastern tropics under the positive (El Niño‐like) phase of PDO, leading to O2 increases in the upper water column. In contrast to the tropics, changes in subduction are the primary control on extratropical O2 variability. These hypotheses are tested by contrasting O2 anomalies with the heave‐induced component of variability calculated from potential density anomalies. Isopycnal heave is the leading control on O2 variability in the tropics, but heave alone cannot fully explain the amplitude of tropical O2 variability, likely indicating reinforcing changes from the biological O2 consumption. Midlatitude O2 variability indeed reflects ocean ventilation downstream of the subduction region where O2 anomalies are correlated with the depth of winter mixed layer. These mechanisms, synchronized with the PDO, yield a basin‐scale pattern of O2 variability that are comparable in magnitude to the projected rates of ocean deoxygenation in this century under “unchecked” emission scenario.

Volcanically modulated pyrite burial and ocean–atmosphere oxidation

It is widely assumed that changes in the rate of biological O2 supply to the atmosphere played little, if any, role in driving the Great Oxidation Event (GOE) because extensive biological O2 production via oxygenic photosynthesis significantly predates atmospheric oxygenation on Earth. Moreover, the C isotope record seemingly precludes a dramatic increase in O2-liberating burial of organic C in the late Archean. However, organic C burial is not the only way in which the biosphere may oxidize the Earth's atmosphere. Biologically mediated burial of reduced mineral phases may also yield net oxidation. For example, reducing power can be transferred from organic matter to H2S via anaerobic respiration and ultimately stored in sedimentary pyrite (FeS2). Changes in pyrite burial may thus impact Earth's oxidation trajectory. Here, we revisit the earliest recognized episode of sulfide-rich (euxinic), pyrite-precipitating conditions in Earth's ocean, ∼200–300 million years prior to the GOE, as preserved in the ∼2.7–2.6 billion-year-old (Ga) Roy Hill Shale of the Jeerinah Formation. Applying Fe speciation and trace metal proxies to drillcore samples from two sites, we characterize both spatial and temporal heterogeneity in the accumulation of H2S in the otherwise Fe-rich Hamersley Basin. We argue that extensive pyrite burial in these locally euxinic environments was the consequence of enhanced volcanic emanations of SO2 to the atmosphere rather than oxidative weathering of crustal sulfides. Because SO2 reduction and burial as pyrite is chemically analogous to CO2 reduction and burial as organic matter, we further posit that this alternative oxidation pathway contributed to Earth's protracted oxygenation during the GOE. Our results thus highlight the need for continued study of the interplay between solid-Earth, volcanic, and biological processes and their joint modulation of oceanic and atmospheric composition.

A biomimetic theranostic O2-meter for cancer targeted photodynamic therapy and phosphorescence imaging

In this report, a biomimetic theranostic oxygen (O2)-meter (cancer cell membrane@Pt(II) porphyrinic-metal organic framework, designated as mPPt) was constructed for cancer targeted and phosphorescence image-guided photodynamic therapy (PDT). mPPt presents high photosensitizers (PSs) loading and evitable self-quenching behaviors for favorable biological O2 sensing and PDT. Besides, endowed by the surface functionalization of cancer cell membrane, the homotypic targeting and immune escape abilities of mPPt could dramatically enhance its cancer targeting ability. Importantly, the O2-dependent phosphorescence responsibility of mPPt could be employed to pre-evaluate the real time O2 level in situ and guide the PDT under light irradiation. A significant anticancer effect is observed after intravenous injection of mPPt and subsequent treatment with PDT with no obvious side effects. As a versatile platform for cell imaging, O2 fluctuation monitoring as well as PDT, this biomimetic O2-meter exhibits great potential for biological analysis and personalized cancer theranostics.

Effective intermediate-spin iron in O2-transporting heme proteins

Proteins carrying an iron-porphyrin (heme) cofactor are essential for biological O2 management. The nature of Fe-O2 bonding in hemoproteins is debated for decades. We used energy-sampling and rapid-scan X-ray Kβ emission and K-edge absorption spectroscopy as well as quantum chemistry to determine molecular and electronic structures of unligated (deoxy), CO-inhibited (carboxy), and O2-bound (oxy) hemes in myoglobin (MB) and hemoglobin (HB) solutions and in porphyrin compounds at 20-260 K. Similar metrical and spectral features revealed analogous heme sites in MB and HB and the absence of low-spin (LS) to high-spin (HS) conversion. Amplitudes of Kβ main-line emission spectra were directly related to the formal unpaired Fe(d) spin count, indicating HS Fe(II) in deoxy and LS Fe(II) in carboxy. For oxy, two unpaired Fe(d) spins and, thus by definition, an intermediate-spin iron center, were revealed by our static and kinetic X-ray data, as supported by (time-dependent) density functional theory and complete-active-space self-consistent-field calculations. The emerging Fe-O2 bonding situation includes in essence a ferrous iron center, minor superoxide character of the noninnocent ligand, significant double-bond properties of the interaction, and three-center electron delocalization as in ozone. It resolves the apparently contradictory classical models of Pauling, Weiss, and McClure/Goddard into a unifying view of O2 bonding, tuned toward reversible oxygen transport.

Cyanobacterial Diazotrophy and Earth's Delayed Oxygenation.

The redox landscape of Earth’s ocean-atmosphere system has changed dramatically throughout Earth history. Although Earth’s protracted oxygenation is undoubtedly the consequence of cyanobacterial oxygenic photosynthesis, the relationship between biological O2 production and Earth’s redox evolution remains poorly understood. Existing models for Earth’s oxygenation cannot adequately explain the nearly 2.5 billion years delay between the origin of oxygenic photosynthesis and the oxygenation of the deep ocean, in large part owing to major deficiencies in our understanding of the coevolution of O2 and Earth’s key biogeochemical cycles (e.g., the N cycle). For example, although possible links between O2 and N scarcity have been previously explored, the consequences of N2 limitation for net biological O2 production have not been examined thoroughly. Here, we revisit the prevailing view that N2 fixation has always been able to keep pace with P supply and discuss the possibility that bioavailable N, rather than P, limited export production for extended periods of Earth’s history. Based on the observation that diazotrophy occurs at the expense of oxygenesis in the modern ocean, we suggest that an N-limited biosphere may be inherently less oxygenic than a P-limited biosphere—and that cyanobacterial diazotrophy was a primary control on the timing and tempo of Earth’s oxygenation by modulating net biogenic O2 fluxes. We further hypothesize that negative feedbacks inhibit the transition between N and P limitation, with the implication that the pervasive accumulation of O2 in Earth’s ocean-atmosphere system may not have been an inevitable consequence of oxygenic photosynthesis by marine cyanobacteria.

Biological and physical controls on N2, O2, and CO2 distributions in contrasting Southern Ocean surface waters

AbstractWe present measurements of pCO2, O2 concentration, biological oxygen saturation (ΔO2/Ar), and N2 saturation (ΔN2) in Southern Ocean surface waters during austral summer, 2010–2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chl a) concentrations in regions of frontal mixing and sea ice melt. pCO2 and ΔO2/Ar exhibited large spatial gradients (range 90 to 450 µatm and −10 to 60%, respectively) and covaried strongly with Chl a. However, the ratio of biological O2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time scales of O2 and CO2 air‐sea equilibration. We measured significant oceanic CO2 uptake, with a mean air‐sea flux (~ −10 mmol m−2 d−1) that significantly exceeded regional climatological values. N2 was mostly supersaturated in surface waters (mean ΔN2 of +2.5%), while physical processes resulted in both supersaturation and undersaturation of mixed layer O2 (mean ΔO2phys = 2.1%). Box model calculations were able to reproduce much of the spatial variability of ΔN2 and ΔO2phys along the cruise track, demonstrating significant effects of air‐sea exchange processes (e.g., atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment‐corrected surface ΔO2/Ar data, ranged from ~ −40 to > 300 mmol O2 m−2 d−1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and stratified regions of sea ice melt, reflecting physical controls on surface water light fields and nutrient availability.

STABILITY OF CO2ATMOSPHERES ON DESICCATED M DWARF EXOPLANETS

We investigate the chemical stability of CO2-dominated atmospheres of desiccated M dwarf terrestrial exoplanets using a 1-dimensional photochemical model. Around Sun-like stars, CO2 photolysis by Far-UV (FUV) radiation is balanced by recombination reactions that depend on water abundance. Planets orbiting M dwarf stars experience more FUV radiation, and could be depleted in water due to M dwarfs' prolonged, high-luminosity pre-main sequences (Luger & Barnes 2015). We show that, for water-depleted M dwarf terrestrial planets, a catalytic cycle relying on H2O2 photolysis can maintain a CO2 atmosphere. However, this cycle breaks down for atmospheric hydrogen mixing ratios <1 ppm, resulting in ~40% of the atmospheric CO2 being converted to CO and O2 on a time scale of 1 Myr. The increased O2 abundance leads to high O3 concentrations, the photolysis of which forms another CO2-regenerating catalytic cycle. For atmospheres with <0.1 ppm hydrogen, CO2 is produced directly from the recombination of CO and O. These catalytic cycles place an upper limit of ~50% on the amount of CO2 that can be destroyed via photolysis, which is enough to generate Earth-like abundances of (abiotic) O2 and O3. The conditions that lead to such high oxygen levels could be widespread on planets in the habitable zones of M dwarfs. Discrimination between biological and abiotic O2 and O3 in this case can perhaps be accomplished by noting the lack of water features in the reflectance and emission spectra of these planets, which necessitates observations at wavelengths longer than 0.95 microns.

Evaluating Southern Ocean biological production in two ocean biogeochemical models on daily to seasonal timescales using satellite chlorophyll and O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; / Ar observations

Abstract. We assess the ability of ocean biogeochemical models to represent seasonal structures in biomass and net community production (NCP) in the Southern Ocean. Two models are compared to observations on daily to seasonal timescales in four different sections of the region. We use daily satellite fields of chlorophyll (Chl) as a proxy for biomass and in situ observations of O2 and Ar supersaturation (ΔO2 / Ar) to estimate NCP. ΔO2 / Ar is converted to the flux of biologically generated O2 from sea to air (O2 bioflux). All data are aggregated to a climatological year with a daily resolution. To account for potential regional differences within the Southern Ocean, we conduct separate analyses of sections south of South Africa, around the Drake Passage, south of Australia, and south of New Zealand. We find that the models simulate the upper range of Chl concentrations well, underestimate spring levels significantly, and show differences in skill between early and late parts of the growing season. While there is a great deal of scatter in the bioflux observations in general, the four sectors each have distinct patterns that the models pick up. Neither model exhibits a significant distinction between the Australian and New Zealand sectors and between the Drake Passage and African sectors. South of 60° S, the models fail to predict the observed extent of biological O2 undersaturation. We suggest that this shortcoming may be due either to problems with the ecosystem dynamics or problems with the vertical transport of oxygen.