O2 versus N2O respiration in a continuous microbial enrichment

Despite its ecological importance, essential aspects of microbial N₂O reduction—such as the effect of O₂ availability on the N₂O sink capacity of a community—remain unclear. We studied N₂O vs. aerobic respiration in a chemostat culture to explore (i) the extent to which simultaneous respiration of N₂O and O₂ can occur, (ii) the mechanism governing the competition for N₂O and O₂, and (iii) how the N₂O-reducing capacity of a community is affected by dynamic oxic/anoxic shifts such as those that may occur during nitrogen removal in wastewater treatment systems. Despite its prolonged growth and enrichment with N₂O as the sole electron acceptor, the culture readily switched to aerobic respiration upon exposure to O₂. When supplied simultaneously, N₂O reduction to N₂ was only detected when the O₂ concentration was limiting the respiration rate. The biomass yields per electron accepted during growth on N₂O are in agreement with our current knowledge of electron transport chain biochemistry in model denitrifiers like Paracoccus denitrificans. The culture’s affinity constant (K_S) for O₂ was found to be two orders of magnitude lower than the value for N₂O, explaining the preferential use of O₂ over N₂O under most environmentally relevant conditions.

     

Distribution of nitrous oxide and regulators of its production across a tropical rainforest catena in the Luquillo Experimental Forest, Puerto Rico

Understanding of N₂O fluxes to the atmosphere is complicated by interactions between chemical and physical controls on both production and movement of the gas. To better understand how N₂O production is controlled in the soil, we measured concentrations of N₂O and of the proximal controllers on its production in soil water and soil air in a field study in the Rio Icacos basin of the Luquillo Experimental Forest, Puerto Rico. A toposequence (ridge, slope-ridge break, slope, slope-riparian break, riparian, and streambank) was used that has been previously characterized for groundwater chemistry and surface N₂O fluxes. The proximal controls on N₂O production include NO₃ ^–, NH₄ ⁺, DOC, and O₂. Nitrous oxide and O₂ were measured in soil air and NO₃ ^–, NH₄ ⁺, and DO were measured in soil water. Nitrate and DOC disappeared from soil solution at the slope-riparian interface, where soil N₂O concentrations increased dramatically. Soil N₂O concentrations continued to increase through the flood plain and the streambank. Nitrous oxide concentrations were highest in soil air probes that had intermediate O₂ concentrations. Changes in N₂O concentrations in groundwater and soil air in different environments along the catena appear to be controlled by O₂ concentrations. In general, N processing in the unsaturated and saturated zones differs within each topographic position apparently due to differences in redox status.     

Amperometric method for determining nitrous oxide in denitrification and in nitrogenase-catalyzed nitrous oxide reduction

A conventional Clark-type O₂ probe was used to determine N₂O concentrations in suspensions. At a polarizing voltage of–0.95 V versus the reference Ag/AgCl electrode, the probe is almost half as sensitive for N₂O as for O₂, and the detection limit is less than 1 M N₂O. The probe can also be used to determine NO for which the suitable polarizing voltage is–0.7 V. The method was successfully applied for continuously recording dissimilatory formation or utilization of N₂O by intactAzospirillum brasilense Sp 7, NO production by extracts from this bacterium, and N₂O reduction catalyzed by nitrogenase in intactKlebsiella pneumoniae. It is concluded that the probe is useful for measuring N₂O or NO contents in bacterial suspensions when the O₂ level is zero or kept constant during the assays.     

Combined Oxygen and Nitrous Oxide Microsensor for Denitrification Studies

The construction of a microsensor which can be used to measure O₂ and N₂O simultaneously is described. The microsensor exhibited a linear response to both O₂ and N₂O, and the response to N₂O was independent of the O₂ concentration and vice versa. The N₂O detection limit of a microsensor with a tip diameter of 20 μm was around 1 μmol liter⁻¹. The signals for O₂ and N₂O were affected by hydrogen sulfide, but other interfering agents were not observed in the biofilms and sediments analyzed. Microprofiles of O₂ and N₂O were measured in a biofilm which was exposed to acetylene to block the N₂O reductase activity of denitrifying bacteria. O₂ penetrated about 0.5 mm into the biofilm and was not affected by acetylene, but the N₂O concentration at 1.4 mm depth increased from 32 to 411 μmol liter⁻¹ after the addition of the inhibitor. The shape of the N₂O profile after the addition of acetylene showed that denitrification (denitrifying activity) was detectable in all anoxic layers of the biofilm.     

Influence of O2 availability on NO and N2O release by nitrification and denitrification in soils

The availability of O₂ is believed to be one of the main factors regulating nitrification and denitrification and the release of NO and N₂O. The availability of O₂ in soil is controlled by the O₂ partial pressure in the gas phase and by the moisture content in the soil. Therefore, we investigated the influence of O₂ partial pressures and soil moisture contents on the NO and N₂O release in a sandy and a loamy silt and differentiated between nitrification and denitrification by selective inhibition of nitrification with 10 Pa acetylene. At 60% whc (maximum water holding capacity) NO and N₂O release by denitrification increased with decreasing O₂ partial pressure and reached a maximum under anoxic conditions. Under anoxic conditions NO and N₂O were only released by denitrification. NO and N₂O release by nitrification also increased with decreasing O₂ partial pressure, but reached a maximum at 0.1–0.5% O₂ and then decreased again. Nitrification was the main source of NO and N₂O at O₂ partial pressures higher than 0.1–0.5% O₂. At lower O₂ partial pressures denitrification was the main source of NO and N₂O. With decreasing O₂ partial pressure N₂O release increased more than NO release, indicating that the N₂O release was more sensitive against O₂ than the NO release. At ambient O₂ partial pressure (20.5% O₂) NO and N₂O release by denitrification increased with increasing soil moisture content. The maximum NO and N₂O release was observed at soil moisture contents of 65–80% whc and 100% whc, respectively. NO and N₂O release by nitrification also increased with increasing soil moisture content with a maximum at 45–55% whc and 90% whc, respectively. Nitrification was the main source of NO and N₂O at soil moisture contents lower than 90% whc and 80% whc, respectively. Higher soil moisture contents favoured NO and N₂O release by denitrification. Soil texture had also an effect on the release of NO and N₂O. The coarse-textured sandy silt released more NO than N₂O compared with the fine-textured loamy silt. At high soil moisture contents (80–100% whc) the fine-textured soil showed a higher N₂O release by denitrification than the coarse-textured soil. We assume that the fine-textured soil became anoxic at a lower soil moisture content than the coarse-textured soil. In conclusion, the effects of O₂ partial pressure, soil moisture and soil texture were consistent with the theory that denitrification increasingly contributes to the release of NO and in particular N₂O when conditions for soil microorganisms become increasingly anoxic.     

The role of interphase nitrogen transport in the dynamic measurement of the overall volumetric mass transfer cefficient in air-sparged systems

It has been shown both theoretically and experimentally that interphase nitrogen transport may have a significant influence on the rate of interphase oxygen transport, and thereby also on the value of the volumetric mass transfer coefficient of oxygen, k_la, determined in mechanically agitated bubble fermentors using the variants of dynamic method presented in the literature. The experiments were carried out in 1M KCI solution at five stirrer frequencies and two gas inlet levels. The gas interchanges were performed either without interrupting the aeration and agitation of the charge (A) or with the aeration and agitation of the charge turned on at the same time (B). The applied variants of the interchange were N₂→ O₂→, O₂→ N₂, N₂→ air, air→ N₂, O→ O₂, and O→ air. In the two last variants the oxygen dissolved in the charge was removed by reacting with sulfite ions. The k_la values calculated by allowing for the nitrogen transport for procedure A were approximately equal to the values obtained by disregarding the nitrogen transport, whereas those for procedure B were higher (up to 40%), than the values obtained disregarding the nitrogen transport.     

Correction to: An analysis of structural phase transition and allied properties of cubic ReN and MoN compounds

The dependence of sensitivity of an explosive on its molecular structure may be mainly attributed to the molecular deformability, which can be expressed by some characteristic parameters, resonance energy for aromatic an explosive, strain energy for a strained-ring or strained-cage explosive, large π-π separation energy for a large π-π linked-explosive, bond rotational energy barriers of C–NO₂, N–NO₂, O–NO₂ for C–NO₂, N–NO₂, O–NO₂ bond-based explosives, and so on. Molecular polarizability of an explosive is also an important molecular deformability index, which can be effectively used to compare impact sensitivities of explosive’s isomers, isoelectronic species, and similar structures. Interestingly, comparing the molecular polarizabilities under external electric fields with different energy levels of isomeric N₂₀(Ih) and N₂₀(D_3d) clusters and the Mo₂N₂₀ and Re₂N₂₀ complex compounds, it is found that there are different energy thresholds of significant molecular expansion.

     

A simple and rapid GC/MS method for the simultaneous determination of gaseous metabolites

We modified and tuned a commercial model of a gas chromatography/mass spectrometry (GC/MS) instrument to develop a simple and rapid method for the simultaneous quantification of a variety of gas species. Using the developed method with the newly modified instrument, gas species such as H₂, N₂, O₂, CO, NO, CH₄, CO₂, and N₂O, which are common components of microbial metabolism, were accurately identified based on their retention times and/or mass-to-charge ratios (m/z) in less than 2.5 min. By examining the sensitivities and dynamic ranges for the detection of H₂, N₂, O₂, CH₄, CO₂, and N₂O, it was demonstrated that the method developed in this study was sufficient for accurately monitoring the production and the consumption of these gaseous species during microbial metabolism. The utility of the new method was demonstrated by a denitrification study with Pseudomonas aureofaciens ATCC 13985^T. This method will be suitable for a variety of applications requiring the identification of gaseous metabolites in microorganisms, microbial communities, and natural ecosystems.     

Simplified determination of lorazepam and oxazepam in biological fluids by gas chromatography—mass spectrometry

Lorazepam and oxazepam in plasma and urine were measued by gas chromatography—mass spectrometry. Oxazepam was used as an internal standard in the assay of lorazepam and vice versa. After removal of interfering substances with n-hexane, the drugs were extracted with benzene and converted to N₁,O₃-bistrimethylsilyl derivatives. Glucuronide forms of the drugs were extracted after hydrolysis with β-glucuronidase. A common fragment ion at m/e 429 was used to monitor the two drugs. The sensitivity was 2 ng/ml for both drugs, which was sufficient to determine plasma and urine concentrations after therapeutic doses to humans.     

Effects of elevated CO2 and O3 on N-cycling and N2O emissions: a short-term laboratory assessment

Background and aims

Elevated atmospheric CO₂ (eCO₂) and tropospheric O₃ (eO₃) can alter soil microbial processes, including those underlying N₂O emissions, as an indirect result of changes in plant inputs. In this study, effects of eCO₂ and eO₃ on sources of N₂O in a soybean (Glycine max (L.) Merr.) agroecosystem in Illinois (SoyFACE) were investigated. We hypothesized that increases in available C and anaerobic microhabitat under eCO₂ would stimulate N₂O emissions, with a proportionally larger increase in denitrification derived N₂O (N₂O_D) compared to nitrification plus nitrifier denitrification derived N₂O (N₂O_N+ND). We expected opposite effects under eO₃.

Methods

Isotopically labeled ¹⁵NH ₄ ¹⁴ NO₃ and ¹⁴NH ₄ ¹⁵ NO₃ were used to evaluate mineral N transformations, N₂O_D, and N₂O_N+ND in a 12-day incubation experiment.

Results

We observed minimal effects of eCO₂ and eO₃ on N₂O emissions, movement of ¹⁵?N through mineral N pools, soil moisture content and C availability. Possibly, altered C and N inputs by eCO₂ and eO₃ were small relative to the high soil organic C content and N-inputs via biological N₂-fixation, minimizing potential effects of eCO₂ and eO₃ on N-cycling.

Conclusion

We conclude that eCO₂ and eO₃ did not affect N₂O emissions in the short term. However, it remains to be tested whether N₂O emissions in SoyFACE will be unaltered by eCO₂ and eO₃ on a larger temporal scale under field conditions.     

Impaired Synthesis of Acetylcholine by Mild Hypoxic Hypoxia or Nitrous Oxide

The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N₂O), the effects of N²O were evaluated. N²O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N²O-anesthetized animals, measured with [U-¹⁴C]glucose and [1-²H₂,2-²H₂]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O₂ (partial pressure of oxygen, Pao₂= 120 mm Hg) were compared with rats exposed to 15% O₂ (Pao₂= 57 mm Hg) or 10% O₂ (Pao₂= 42 mm Hg). The synthesis of acetylcholine, measured with [U-¹⁴C]glucose, was decreased by 35 and 54% with 15% O₂ and 10% O₂ respectively; acetylcholine synthesis, measured with [1-²H₂,2-²H₂]choline, was decreased by 50 and 68% with 15% O₂ and 10% O₂ respectively. Animals breathing either 15% or 10% O₂ had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O₂. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated with hypoxia.     

In situ 15N-N2O site preference and O2 concentration dynamics disclose the complexity of N2O production processes in agricultural soil

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N₂O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N₂O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N₂O fluxes in an arable soil, combined with in situ high-frequency measurements of soil matrix oxygen (O₂) and N₂O concentrations, in situ ¹⁵N labeling, and N₂O ¹⁵N site preference (SP). The in situ O₂ concentration and further microcosm visualized spatiotemporal distribution of O₂ both suggested that O₂ dynamics were the proximal determining factor to matrix N₂O concentration and fluxes due to quick O₂ depletion after N fertilization. Further SP analysis and in situ ¹⁵N labeling experiment revealed that the main source for N₂O emissions was bacterial denitrification during the hot-wet summer with lower soil O₂ concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold-dry winter with higher soil O₂ concentration. The robust positive correlation between O₂ concentration and SP values underpinned that the O₂ dynamics were the key factor to differentiate the composite processes of N₂O production in in situ structured soil. Our findings deciphered the complexity of N₂O production processes in real field conditions, and suggest that O₂ dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N₂O production processes in undisturbed structure soils. Our results help to develop targeted N₂O mitigation measures and to improve process models for constraining global N₂O budget.     

Mechanism leading to N<Subscript>2</Subscript>O production in wastewater treating biofilm systems

Wastewater treatment plants are known to be important point sources for nitrous oxide (N₂O) in the anthropogenic N cycle. Biofilm based treatment systems have gained increasing popularity in the treatment of wastewater, but the mechanisms and controls of N₂O formation are not fully understood. Here, we review functional groups of microorganism involved in nitrogen (N) transformations during wastewater treatment, with emphasis on potential mechanism of N₂O production in biofilms. Biofilms used in wastewater treatment typically harbour aerobic and anaerobic zones, mediating close interactions between different groups of N transforming organisms. Current models of mass transfer and biomass interactions in biofilms are discussed to illustrate the complex regulation of N₂O production. Ammonia oxidizing bacteria (AOB) are the prime source for N₂O in aerobic zones, while heterotrophic denitrifiers dominate N₂O production in anoxic zones. Nitrosative stress ensuing from accumulation of NO₂ ^? during partial nitrification or denitrification seems to be one of the most critical factors for enhanced N₂O formation. In AOB, N₂O production is coupled to nitrifier denitrification triggered by nitrosative stress, low O₂ tension or low pH. Chemical N₂O production from AOB intermediates (NH₂OH, HNO, NO) released during high NH₃ turnover seems to be limited to surface-near AOB clusters, since diffusive mass transport resistance for O₂ slows down NH₃ oxidation rates in deeper biofilm layers. The proportion of N₂O among gaseous intermediates (NO, N₂O, N₂) in heterotrophic denitrification increases when NO or nitrous acid (HNO₂) accumulates because of increasing NO₂ ^?, or when transient oxygen intrusion impairs complete denitrification. Limited electron donor availability due to mass transport limitation of organic substrates into anoxic biofilm zones is another important factor supporting high N₂O/N₂ ratios in heterotrophic denitrifiers. Biofilms accommodating Anammox bacteria release less N₂O, because Anammox bacteria have no known N₂O producing metabolism and reduce NO₂ ^? to N₂, thereby lowering nitrosative stress to AOB and heterotrophs.     

Microscale Biosensor for Measurement of Volatile Fatty Acids in Anoxic Environments

A microscale biosensor for acetate, propionate, isobutyrate, and lactate is described. The sensor is based on the bacterial respiration of low-molecular-weight, negatively charged species with a concomitant reduction of NO₃⁻ to N₂O. A culture of denitrifying bacteria deficient in N₂O reductase was immobilized in front of the tip of an electrochemical N₂O microsensor. The bacteria were separated from the outside environment by an ion-permeable membrane and supplied with nutrients (except for electron donors) from a medium reservoir behind the N₂O sensor. The signal of the sensor, which corresponded to the rate of N₂O production, was proportional to the supply of the electron donor to the bacterial mass. The selectivity for volatile fatty acids compared to other organic compounds was increased by selectively enhancing the transport of negatively charged compounds into the sensor by electrophoretic migration (electrophoretic sensitivity control). The sensor was susceptible to interference from O₂, N₂O, NO₂⁻, H₂S, and NO₃⁻. Interference from NO₃⁻ was low and could be quantified and accounted for. The detection limit was equivalent to about 1 μM acetate, and the 90% response time was 30 to 90 s. The response of the sensor was not affected by changes in pH between 5.5 and 9 and was also unaffected by changes in salinity in the range of 2 to 32‰. The functioning of the sensor over a temperature span of 7 to 30°C was investigated. The concentration range for a linear response was increased five times by increasing the temperature from 7 to 19.5°C. The life span of the biosensor varied between 1 and 3 weeks after manufacturing.     

N<Subscript>2</Subscript>O production by denitrification in an urban river: evidence from isotopes,functional genes,and dissolved organic matter

Rivers are important sources of N₂O emissions into the atmosphere. Nevertheless, N₂O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N₂O, NH₄ ⁺, NO₂ ^?, and NO₃ ^? in surface water to identify the microbial processes of N₂O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers (amoA-bacteria, nirK, nirS, nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N₂O production processes. The observed nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) of N₂O were within the expected isotopic range of N₂O produced by nitrate reduction, indicating that N₂O was dominantly produced by denitrification. The positive significant correlation between N₂O_Net concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N₂O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N₂O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N₂O. Measurement of isotope ratios of N₂O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N₂O production along the Tama River.     

Influence of carbon availability on the production of NO, N2O, N2 and CO2 by soil cores during anaerobic incubation

Net productions of permanent soil atmosphere gases (N₂, CO₂, O₂) and temporary gases (N₂O, NO) were monitored in soil cores using a non-interfering, fully automated measuring technique allowing highly time resolved measurements over prolonged periods. The influence of changes in available organic carbon on CO₂, N₂O, NO and N₂ production was studied by changing the soil carbon content through aerobic preincubations of different length, up to 21 days.The aerobic preincubation caused an increase in NO₃ ^- concentration and a decrease in available carbon content. Available carbon content dominated both CO₂ and total N gas (N₂+N₂O+NO) production during anaerobiosis. Both CO₂ and total N gas production rates decreased with increasing length of the previous aerobic preincubation, this in spite of the higher initial NO₃ ^- concentration.Total denitrification rates were closely related to the anaerobic CO₂ production rates. No relation was found between water soluble carbon content and total denitrification. The N₂O/N₂ ratio could be explained by an interaction of carbon availability, NO₃ ^- concentration and enzyme status. Net N₂O consumption was monitored. The balance between cumulative total N gas production and NO₃ ^- consumption varied according to the different treatments. Cumulative N₂O production exceeded cumulative N₂ production for 0 up to 5 days.     

Synthesis,characterization, and electrocatalytic behaviour of hydrothermally grown nanostructured La2O3 and La2O3/K-complex

Rare earth metals play a conspicuous role in magnetic resonance imaging (MRI) for detecting cancerous cells. The alkali metal potassium is a neurotransmitter in the sodium–potassium pump in biomedical sciences. This unique property of rare earth metals and potassium drew our attention to carry forward this study. Therefore, in this work, previously synthesized potassium (K) complexes formed by the reflux of 4-N,N-dimethylaminobenzoic acid (DBA) and potassium hydroxide in methanol, and named [(μ2–4-N,N-dimethylaminobenzoate-κO)(μ2–4-N,N-dimethylaminobenzoic acid-κO)(4-N,N-dimethylaminobenzoic acid-κO) potassium(I) coordination polymer)] were treated hydrothermally with La₂O₃ nanomaterials to obtain a nanohybrid La₂O₃/K-complex. After that, the K-complex was analyzed using single-crystal X-ray diffraction and ¹H and ¹³C NMR spectroscopy. In addition, the structural and morphological properties of the as-prepared nanostructured La₂O₃/K-complex were also characterized, which involved an investigation using X-ray diffraction (XRD)spectroscopy, Fourier transform infrared (FTIR) spectroscopy, atomic force spectroscopy (AFM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) analysis. After this, the electrochemical redox behaviour of the synthesized nanohybrid material was studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Therefore, the results from these studies revealed that the as-prepared material was a La₂O₃/K-complex that has a promising future role in sensing various analytes, as it showed effective electrocatalytic behaviour.     

Synthesis and structure of μ-3,3′-[ 1,2-ethanediyl-bis(nitrilomethylidyne)-bis(2-hydroxybenzoato)] aquadicopper(II) hydrate

The complex μ-3,3′-[1,2-ethanediyl-bis(nitrilome- thylidyne)-bis(2-hydroxybenzoato)] aquadicopper(II) hydrate, C₁₈H₁₆N₂O₈Cu₂, was isolated from an attempted preparation of a copper lanthanum binuclear complex. The dark purple crystals are monoclinic, space group P2₁/n, with 4 molecules per unit cell; dimensions a = 13.961(5), b = tl.787(3), c = 11.622(3) Å and β = 113.09(2)°. The final R was 0.046 for the 2062 reflections used in the analysis. The Cu atom in the N₂O₂ cavity is five coordinate with CuN distances of 1.879 and 1.880 Å and CuO distances of 1.898 and 1.900 Å. A water molecule at 2.557 Å completes the square pyramidal arrangement. The second Cu in the O₄ cavity is square planar, with CuO distances to the bridging oxygens of 1.914 and 1.909 Å and to the carboxy oxygens of 1.871 and 1.882 Å. A survey of copper complexes in a square planar N₂O₂ arrangement has led to the equation δCu from the N₂O₂ plane = 0.822 – 0.275 (CuO axial distance) with a correlation coefficient of 0.98 for the 12 structures in which the Cu atom is bonded to a fifth oxygen atom. A model for the transition from square planar to square pyramidal geometry is proposed.     

Microbial N2O consumption in and above marine N2O production hotspots

The ocean is a net source of N₂O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N₂O via microbial N₂O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N₂O 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, O₂ tolerance, and community composition of N₂O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N₂O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N₂O cycling. Microbes from the oxic layer consume N₂O 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 N₂O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O₂ and N₂O gradients right above the ODZ is a previously ignored potential gatekeeper of N₂O and should be accounted for in the marine N₂O budget.Subject terms: Water microbiology, Biogeochemistry, Microbial ecology     

Electrochemical and spectroelectrochemical studies of complexes of 1,10-phenanthroline-5,6-dione

Electrochemical and spectroelectrochemical (UV-Vis, IR, EPR) of pd (pd = 1,10-phenanthroline-5,6-dione), Pt(N,N′-pd)Cl2, Pd(N,N′-pd)Cl₂, [Ru(bpy)₂(N,N′-pd)]Cl₂ (bpy = 2,2′-bipyridine) and Pt(O,O′-pd)(PPh₃)₂, where N,N′ and O,O′ refers to coordination of pd to the metal centre via N and O atoms, respectively, reveals that the electron transfer processes between +0.5 and −1.25 V all occur at the pd ligand in agreement with DFT calculations. The two CO groups carry a significant amount of the negative charge in mono-reduced pd¹⁻. The mode of coordination of pd has a greater influence on its redox chemistry than the metal centre or the ancillary ligands.