Oxygen Concentration Profiles Research Articles

Influence of silver nanoparticles on benthic oxygen consumption of microbial communities in freshwater sediments determined by microelectrodes

The increased use of silver nanoparticles (AgNPs) will inevitably result in the release of these particles into aquatic environments, with sediments as a substantial sink. However, we do not know whether AgNPs present potential impacts in sediment functioning. In this study, a microcosm approach was constructed, and the potential impacts of AgNPs and PVP-coated AgNPs on oxygen consumption in freshwater sediments (collected from Taihu Lake) were determined using oxygen microelectrodes. To our knowledge, this is the first time that microelectrodes have been used to estimate the impacts of AgNPs in sediments. The steady-state oxygen microprofiles showed that environmental relevant concentration (1 mg/L nano-Ag) did not lead to an apparent change in the oxygen consumption rates of benthic microbial communities in sediment. The addition of 10 mg/L uncoated AgNPs resulted in remarkable differences in the oxygen concentration profiles within 4–5 h and significantly inhibited the oxygen consumption of benthic microbial communities in the upper sediment layer (∼1 mm) after 100 h. Simultaneously, an increase of oxygen consumption in sediment lower zones was observed. These results may suggest that aerobic microorganisms in the upper layer of the sediment reduced metabolic activity to avoid the toxic stress from AgNPs. Concomitantly, facultative aerobes below the metabolically active upper layer switched from fermentation or anaerobic respiration to aerobic respiration as oxygen bioavailability increased in the lower zones of the sediment. In addition, PVP coating reduced the nanotoxicity of AgNPs in benthic microorganisms due to the decreased dissolution of AgNPs in the filtered overlying water, a phenomenon that merits further investigation.

Modelling and simulation of passive feed direct methanol fuel cell

In this study, a mathematical model of the passive direct methanol fuel cell (DMFC) has been developed. This model considers the methanol and oxygen mass transport phenomena along with the electrochemical reactions. The model is validated with the experimental data. The model is used to predict the effect of methanol concentration on cell performance, variation of methanol crossover (MCO) with current density and methanol feed concentration, methanol and oxygen concentration profiles across the cell and the overpotentials. It was found that the MCO can be reduced by running the cell with low methanol concentration and at high current densities. Anode overpotential, compared to cathode overpotential, is found to be more responsible for the limiting current density. The outcome of the research will be useful for improving understanding of the transport phenomena in the passive DMFC and to optimise the cell design.

High spatial resolution analysis of the distribution of sulfate reduction and sulfide oxidation in hypoxic sediment in a eutrophic estuary.

Bottom hypoxia and consequential hydrogen sulfide (H2S) release from sediment in eutrophic estuaries is a major global environmental issue. We investigated dissolved oxygen, pH and H2S concentration profiles with microsensors and by sectioning sediment cores followed by colorimetric analysis. The results of these analyses were then compared with the physicochemical properties of the bottom water and sediment samples to determine their relationships with H2S production in sediment. High organic matter and fine particle composition of the sediment reduced the oxidation-reduction potential, stimulating H2S production. Use of a microsensor enabled measurement of H2S concentration profiles with submillimetre resolution, whereas the conventional sediment-sectioning method gave H2S measurements with a spatial resolution of 10 mm. Furthermore, microsensor measurements revealed H2S consumption occurring at the sediment surface in both the microbial mat and the sediment anoxic layer, which were not observed with sectioning. This H2S consumption prevented H2S release into the overlying water. However, the microsensor measurements had the potential to underestimate H2S concentrations. We propose that a combination of several techniques to measure microbial activity and determine its relationships with physicochemical properties of the sediment is essential to understanding the sulfur cycle under hypoxic conditions in eutrophic sediments.

Oxygen concentration and mobility in conserved archaeological wood

Probes for measuring oxygen concentration were installed inside large samples of conserved wood from archaeological shipwrecks to record oxygen profiles as a function of depth under the wood surface. The concentration of oxygen was lower inside the wood than outside indicating that the supply of oxygen is limited and that oxygen is consumed within the material. This was confirmed by oxygen consumption rates averaging 1.3 ± 0.42 µg O2/g wood/day. The diffusion properties were estimated by placing the wood samples in a nitrogen atmosphere and measuring nitrogen breakthrough time to the oxygen probes at different depths. A high level of variation was observed between the breakthrough times measured at different depths of the samples. This indicated that the access of oxygen is irregular, possibly due to wood heterogeneity and uneven distribution of conservation agent. By recording oxygen concentrations inside archaeological wood under stable environmental conditions, it was shown that oxygen concentration profiles are almost constant over a time span of seven years. This indicates that oxygen consumption, and thus oxidative degradation, is constant for an extended period of time. It also shows that the oxygen concentration inside conserved archaeological wood is a steady state value controlled by the local balance between oxygen supply and oxygen consumption.

Phenomenological modelling and simulation of cell clusters in 3D cultures

Cell clustering and aggregation are fundamental processes in the development of several tissues and the progression of many diseases. The formation of these aggregates also has a direct impact on the oxygen concentration in their surroundings due to cellular respiration and poor oxygen diffusion through clusters. In this work, we propose a mathematical model that is capable of simulating cell cluster formation in 3D cultures through combining a particle-based and a finite element approach to recreate complex experimental conditions. Cells are modelled considering cell proliferation, cell death and cell-cell mechanical interactions. Additionally, the oxygen concentration profile is calculated through finite element analysis using a reaction-diffusion model that considers cell oxygen consumption and diffusion through the extracellular matrix and the cell clusters. In our model, the local oxygen concentration in the medium determines both cell proliferation and cell death. Numerical predictions are also compared with experimental data from the literature. The simulation results indicate that our model can predict cell clustering, cluster growth and oxygen distribution in 3D cultures. We conclude that the initial cell distribution, cell death and cell proliferation dynamics determine the size and density of clusters. Moreover, these phenomena are directly affected by the oxygen transport in the 3D culture.

Non-animal testing for skin sensitization: Replacement or mere supplement?

Tissue engineering in cardiovascular regenerative therapy requires the development of an efficient oxygen supply system for cell cultures. However, there are few studies which have examined human cardiomyocytes in terms of oxygen consumption and metabolism in culture. We developed an oxygen measurement system equipped with an oxygen microelectrode sensor and estimated the oxygen consumption rates (OCRs) by using the oxygen concentration profiles in culture medium. The heart is largely made up of cardiomyocytes, cardiac fibroblasts, and cardiac endothelial cells. Therefore, we measured the oxygen consumption of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs), cardiac fibroblasts, human cardiac microvascular endothelial cell and aortic smooth muscle cells. Then we made correlations with their metabolisms. In hiPSC-CMs, the value of the OCR was 0.71 ± 0.38 pmol/h/cell, whereas the glucose consumption rate and lactate production rate were 0.77 ± 0.32 pmol/h/cell and 1.61 ± 0.70 pmol/h/cell, respectively. These values differed significantly from those of the other cells in human heart. The metabolism of the cells that constitute human heart showed the molar ratio of lactate production to glucose consumption (L/G ratio) that ranged between 1.97 and 2.2. Although the energy metabolism in adult heart in vivo is reported to be aerobic, our data demonstrated a dominance of anaerobic glycolysis in an in vitro environment. With our measuring system, we clearly showed the differences in the metabolism of cells between in vivo and in vitro monolayer culture. Our results regarding cell OCRs and metabolism may be useful for future tissue engineering of human heart.

Model for attrition-enhanced char combustion in fluidized beds

Attrition in fluidized beds can be important due to particle collisions removing fines from the surface of a large char particle. A model for the attrition enhanced combustion of a char particle is presented. When oxygen penetrates the surface of a particle and reacts with the solid, the surface layer becomes more fragile. Fine particles are generated due to percolation and attrition contributing to the mass loss from the mother particle in addition to burning. A shrinking particle model including the effects of attrition and gasification has been developed. The oxidation is assumed to take place at the particle external surface in the classical shrinking particle model i.e. in a layer with zero thickness. The basis for the shrinking particle model is explained by studying the diffusion and reaction processes taking place in a narrow layer on the particle surface in more detail. The Arrhenius-type chemical reaction kinetics parameters (pre-exponential coefficient, activation energy, reaction order) are not purely of chemical nature but they are apparent in the classical shrinking particle model. A more realistic model, where oxidation takes place in a thin layer instead of taking place at the surface and gasification takes place in the core region, is discussed. The model predicts the thickness of this layer and oxygen concentration and char conversion (or density) profiles in this layer. The classical shrinking particle model is, however, applicable with modifications. Pore diffusivity, intrinsic reaction rate and attrition all contribute to the apparent parameters for kinetics calculated with respect to outer surface.

Diffusion of oxygen in bulk GaN crystals at high temperature and at high pressure

Experimental studies of diffusion of oxygen in bulk wurtzite-type GaN crystals grown by Halide Vapor Phase Epitaxy (HVPE) are reported. Oxygen concentration profiles were studied in as-grown GaN crystals and also after annealing of crystals at temperatures up to 3400K and pressures up to 9GPa. Investigated crystals contained large conical defects i.e. pinholes of significantly higher oxygen concentration (NO=(2–4)×1019cm−3) than that in the bulk matrix (NO<1×1017cm−3). The pinholes were revealed by a photo-etching method in as-grown and annealed GaN samples. Confocal micro-Raman spectroscopy was applied to measure the profiles of free electron concentration, which directly corresponds to the concentration of oxygen impurity. Lateral scanning across the interfaces between pinholes and matrix in the as-grown HVPE GaN crystals showed sharp step-like carrier concentration profiles. Annealing at high temperature and high pressure resulted in the diffusion blurring of the profiles. Analysis of obtained data allowed for the first time for estimation of oxygen diffusion coefficients DO(T, P). The obtained values of DO(T, P) are anomalously small similarly to the values obtained by Harafuji et al. by molecular dynamic calculations for self-diffusion of nitrogen. Whereas oxygen and nitrogen are on the same sublattice it could explain the similarity of their diffusion coefficients.

A model of long-term oxidation and leaching processes in pyritic coal cleaning wastes

This study presents a numerical model to simulate a long-term pyrite oxidation process and subsequent multi-component reactive transportation of the oxidation productions in an abandoned coal waste pile in Northeast of Iran. For this purpose, 240 solid samples of wastes were obtained from different depths of 10 vertical trenches excavated in the pile. Then, geochemical analyses, including determination of pyrite content remaining in the waste particles, paste pH tests, sulfate concentration, were conducted on the collected samples. In addition, oxygen concentration profile associated with each trench was determined. The results show that the oxidation zone is limited to the shallower depths of the pile where oxygen was readily available. Furthermore, governing mathematical equations describing oxygen transport, pyrite oxidation, and transport of oxidation products were formulated based on the fact that the oxidation process follows a shrinking core concept, and gaseous diffusion is the main process for oxygen supply within the waste materials. The transport equation incorporates physical processes of advection and dispersion. Eventually, an existing one-dimensional numerical finite volume model was slightly modified, and the governing equations were simultaneously solved using a commercial computational fluid dynamics software called PHOENICS by providing extra codes for those terms of equations which are not included in PHOENICS governing equation. The results of simulation for 15 years of abandoned of the pile showed close agreement with the geochemical data. The simulations were further performed to predict the pyrite oxidation and subsequent transportation processes for years 5, 10, and 20 from now. The results revealed that more load of the oxidation products would be expected than those are generated now, and the products would reach the pile base. It will result in environmental problems for soil, surface, and groundwater of the study area and nearby lands in future.

Electrons from Solid Electrode Alter Phototrophic Mat Morphology and Metabolic Activity

Phototrophic microbial mats are structured microbial communities composed of highly interactive species that have complete energy and elements within them. Aside from being an excellent ecological model that can be used to investigate how microbial populations associate and interact, microbial mats have the potential for energy capture and conversion applications. To use mats for such purpose, it is critical to understand the energy transfer and activity of microbial populations under the existence of external electron donor or acceptor sources. One method to accomplish this is by introducing a solid electrode with a controlled electrochemical potential to act as an electron donor or acceptor to the mat systems. We hypothesized that varying the electrode potential would, in turn, change the structure, activity, and community composition of the mat via regulation of electron transfer processes. In this work, the electrodes were placed under mats to establish a mat electrochemical system for testing our hypothesis. The phototrophic microbial mat was harvested from Hot Lake, a hypersaline, epsomitic lake located near Oroville (Washington, USA). The electrodes in contact with the base of the mat were polarized at -0.7 VAg/AgCl (hereafter referred to as the “cathodic mat)”. Electrodes that served as an electron sink were polarized at +0.3 VAg/AgCl (hereafter referred as the “anodic mat”). We quantified the diel variations of electron transfer rates between the electrode and the cathodic or anodic mats. We observed variations in current over a diel light cycle in which the lowest and highest magnitudes of anodic current occurred simultaneously with those of cathodic current. To understand the factors affecting these variations we examined the influences of temperature, light intensity, and substrate additions, separately, to electron transfer rate. We also used microelectrodes to measure the depth profiles of sulfide and oxygen concentrations and compared them with the variations of electron transfer rates over a diel cycle. To understand how anodic or cathodic electron transfer processes impacted mat structure and activity, we used nuclear magnetic resonance (NMR) imaging for morphology and NMR metabolite analysis to quantify the activity. The results showed that the quantitative porosity and diffusion coefficients were higher in the cathodic mat than in the anodic mat. In addition, cathodic mats produced significantly higher concentrations of osmo-protectants such as betaine and trehalose. The anodic mat had metabolic profiles similar to the control (non-polarized) mat. Finally, we employed amplicon sequencing across the V4 region of the 16S gene to determine the effect of the electrochemical regime upon mat community structure. The community structure data were consistent with variations in metabolite profiles and suggested greater changes in microbial community structure and function in the cathodic vs. the anodic and control mats. Although further work is needed to determine whether the electron transfer to and from the electrode to the mat was mediated by microbial metabolism of selectively-enriched community on the electrode or was caused by abiotic reactions on electrode, our data strongly suggest that donation of electrons via a solid electrode is capable of altering phototrophic mat morphology, community composition, and metabolic activity. It is possible that the addition of electrons from electrode to the mat alleviated the energy limitations allowing the mat to maintain large carbon pools of osmolytes. In addition, some microorganisms in the mat might use electrons from the electrode to drive carbon fixations which resulted in selectively-enriched community on the electrode and the increase of the total carbon available to the mat.

Numerical simulations of soil physicochemistry and aeration influences on the external corrosion and cathodic protection design of buried pipeline steels

Corrosion of oil and gas transmission pipelines is a serious industrial problem with potentially catastrophic environmental and financial consequences. A finite element model of the external corrosion of buried steel pipelines at coating failures is developed here to better predict degradation in different soil and cathodic protection (CP) environments. Synergistic interactions between steady-state temperature, potential, and oxygen concentration profiles in the soil surrounding the pipeline structure are quantified and discussed. Conductivity and oxygen diffusivity of soil conditions are represented as functions of soil matter, air porosity, and volumetric wetness. Theoretical formulations are uniquely merged with corrosion experiments conducted on actual pipeline steel samples, greatly improving simulation results. Overall, drier sand and clay soil structures cause the most corrosion, whereas wetter conditions impede oxygen diffusion and significantly augment hydrogen evolution. Geometric location of the coating breakdown site relative to the ground surface and the CP anode has a particular influence on oxygen concentration profiles and pipeline corrosion. Model convergence is tested with a mesh sensitivity study, and the model’s ability in evaluating practical design changes in the CP system is demonstrated.

Ratiometric Oxygen Imaging to Predict Oxygen Diffusivity in Oak Wood During Red Wine Barrel Aging

In this work, we present a high-resolution oxygen imaging approach to study the two-dimensional oxygen distribution inside an oak stave in contact with wine and that applies the series resistance model to explain the dynamic evolution of oak wood oxygen transfer rate (OTR). Oxygen flux throughout the oak stave has been studied by considering the wood as a permeable membrane with moisture content (MC) in a decreasing gradient from the wine-contacting side of the oak stave to the side in contact with atmospheric air in cellar conditions. The presence of different levels of liquid across the thickness of the wet stave modifies the oxygen diffusion flux, as the diffusion coefficient of oxygen in water is four orders of magnitude lower than in air. The stave resembles a multilayered membrane, where wood with an MC over the fiber saturation point represents a distinct layer. To that end, three simultaneous measurements were made, namely the MC profile of the wood within the thickness of the stave at different liquid-wood contact times, the OTR of the stave at those times, and finally the oxygen concentration profile within the thickness of the stave using planar optical sensors, a color camera, and ratiometric image analysis. The results show heat flux and oxygen flux that is analogous to that in a multilayer.

Real-time quantitation of internal metabolic activity of three-dimensional engineered tissues using an oxygen microelectrode and optical coherence tomography.

Recent progress in tissue engineering technology has enabled us to develop thick tissue constructs that can then be transplanted in regenerative therapies. In clinical situations, it is vital that the engineered tissues to be implanted are safe and functional before use. However, there is currently a limited number of studies on real-time quality evaluation of thick living tissue constructs. Here we developed a system for quantifying the internal activities of engineered tissues, from which we can evaluate its quality in real-time. The evaluation was achieved by measuring oxygen concentration profiles made along the vertical axis and the thickness of the tissues estimated from cross-sectional images obtained noninvasively by an optical coherence tomography system. Using our novel system, we obtained (i) oxygen concentration just above the tissues, (ii) gradient of oxygen along vertical axis formed above the tissues within culture medium, and (iii) gradient of oxygen formed within the tissues in real-time. Investigating whether these three parameters could be used to evaluate engineered tissues during culturing, we found that only the third parameter was a good candidate. This implies that the activity of living engineered tissues can be monitored in real-time by measuring the oxygen gradient within the tissues. The proposed measuring strategy can be applied to developing more efficient culturing methods to support the fabrication of engineered thick tissues, as well as providing methods to confirm the quality in real-time. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 855-864, 2017.

Investigation of Low-Temperature Hydrogen Plasma-Etching Processes for Silicon Wafer Solar Cell Surface Passivation in an Industrial Inductively Coupled Plasma Deposition Tool

A hydrofluoric-acid (HF)-free hydrogen plasma dry etching process prior to the deposition of intrinsic amorphous silicon onto thin n-type planar Czochralski silicon wafers is developed. The influence of substrate temperature, hydrogen flow rate, and power density on the passivation quality is investigated. Advanced characterization using spectroscopic ellipsometry and transmission electron microscopy shows the impact of the etching conditions, especially the temperature and gas flow rates, on the surface quality and interface properties. It is found that the native oxide can only be removed effectively when wafers are subjected to higher temperature or lower hydrogen flow rate. The hydrogen, oxygen, and carbon concentration profiles at the a -Si/ c -Si interface of the plasma-etched samples are studied and compared with the traditionally HF cleaned interface to gain a better understanding of the reasons for the superior passivation quality.

Microscale profiling of photosynthesis-related variables in a highly productive biofilm photobioreactor.

In the present study depth profiles of light, oxygen, pH and photosynthetic performance in an artificial biofilm of the green alga Halochlorella rubescens in a porous substrate photobioreactor (PSBR) were recorded with microsensors. Biofilms were exposed to different light intensities (50-1,000 μmol photons m(-2) s(-1) ) and CO2 levels (0.04-5% v/v in air). The distribution of photosynthetically active radiation showed almost identical trends for different surface irradiances, namely: a relatively fast drop to a depth of about 250 µm, (to 5% of the incident), followed by a slower decrease. Light penetrated into the biofilm deeper than the Lambert-Beer Law predicted, which may be attributed to forward scattering of light, thus improving the overall light availability. Oxygen concentration profiles showed maxima at a depth between 50 and 150 μm, depending on the incident light intensity. A very fast gas exchange was observed at the biofilm surface. The highest oxygen concentration of 3.2 mM was measured with 1,000 μmol photons m(-2) s(-1) and 5% supplementary CO2. Photosynthetic productivity increased with light intensity and/or CO2 concentration and was always highest at the biofilm surface; the stimulating effect of elevated CO2 concentration in the gas phase on photosynthesis was enhanced by higher light intensities. The dissolved inorganic carbon concentration profiles suggest that the availability of the dissolved free CO2 has the strongest impact on photosynthetic productivity. The results suggest that dark respiration could explain previously observed decrease in growth rate over cultivation time in this type of PSBR. Our results represent a basis for understanding the complex dynamics of environmental variables and metabolic processes in artificial phototrophic biofilms exposed to a gas phase and can be used to improve the design and operational parameters of PSBRs.

Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion–Clipperton Fracture Zone

The Clarion–Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is characterized by organic carbon-starved sediments and meter-scale oxygen penetration into the sediment. Furthermore, numerous seamounts occur throughout its deep-sea plain, which may serve as conduits for low-temperature hydrothermal seawater circulation through the oceanic crust. Recent studies in deep-sea environments of the Pacific and Atlantic Oceans have suggested and presented evidence of dissolved constituent exchange between the seawater flowing in the basaltic crust and the pore water of the overlying sediments. Through high-resolution pore-water oxygen and nutrient measurements, we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of Teddy Bare, a small seamount in the CCFZ. At three sites, located 1000, 700 and 400 m away from the foot of the seamount, we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m toward the basaltic basement. Nitrate (NO3−) concentrations mirror the oxygen concentration profiles, as they increase with sediment depth but decrease towards the basement. These profiles suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments and a downward diffusion of NO32− from sediment pore water into the basaltic crust. At one site, we determined that the 87Sr/86Sr ratios of the bottom water and of the deep sediment near the basaltic crust are similar, further supporting diffusive exchange between basaltic crust fluids and sediment pore water.Transport-reaction modeling performed at two of the study sites revealed that (1) the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders-of-magnitude lower rates than between the upper sediment and the overlying bottom water. These results further show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount.

Oxygen measurement in interstitially perfused cellularized constructs cultured in a miniaturized bioreactor.

The possibility of developing engineered tissue in vitro and maintaining the cell viability and functionality is primarily related to the possibility of controlling key culture parameters such as oxygen concentration and cell-specific oxygen consumption. We measured these parameters in a three-dimensional (3D) cellularized construct maintained under interstitially perfused culture in a miniaturized bioreactor. MG63 osteosarcoma cells were seeded at high density on a 3D polystyrene scaffold. The 3D scaffolds were sensorized with sensor foils made of a polymer, which fluoresce with intensity proportional to the local oxygen tension. Images of the sensor foil in contact with the cellularized construct were acquired with a video camera every four hours for six culture days and were elaborated with analytical imaging software to obtain oxygen concentration maps. The data collected indicate a globally decreasing oxygen concentration profile, with a total drop of 28% after six days of culture and an average drop of 10.5% between the inlet and outlet of the perfused construct. Moreover, by importing the measured oxygen concentration data and the cell counts in a model of mass transport, we calculated the cell-specific oxygen consumption over the whole culture period. The consumption increased with oxygen availability and ranged from 0.1 to 0.7 µmol/h/106 cells. The sensors used here allowed a non-invasive, contamination-free and non-destructive oxygen measurement over the whole culture period. This study is the basis for optimization of the culture parameters involved in oxygen supply, in order to guarantee maintenance of cell viability in our system.

Stuck between a rock and a hard place: zooplankton vertical distribution and hypoxia in the Gulf of Finland, Baltic Sea

Zooplankton often migrate vertically to deeper dark water during the day to avoid visual predators such as fish, a process which can strengthen benthic–pelagic coupling. In the Gulf of Finland, Baltic Sea, a pronounced hypoxic layer develops when there is an inflow of anoxic bottom water from the Central Baltic Sea, which could be a barrier for vertical migrants. Here, we report an acoustic study of the distributions of crustacean zooplankton (mysid shrimp and the copepod Limnocalanus macrurus), gelatinous zooplankton (Aurelia aurita) and fish. Zooplankton trawl nets were used to ground-truth acoustic data. Vertical profiles of oxygen concentration were taken, and the physiological impact of hypoxia on mysids was investigated using biochemical assays. We hypothesised that the vertical distribution of zooplankton and fish would be significantly affected by vertical heterogeneity of oxygen concentrations because anoxia and hypoxia are known to affect physiology and swimming behaviour. In addition, we hypothesised that mysids present in areas with hypoxia would exhibit a preparatory antioxidant response, protecting them from oxidative damage during migrations. The acoustic data showed peaks of crustacean zooplankton biomass in hypoxic ( 75 m), whereas fish shoals and A. aurita medusae were found in normoxic (5–6 mL L−1) upper water layers (<40 m), with individual fish in deeper water excepting that rule. Mysid shrimp from areas with hypoxia had significantly enhanced antioxidant potential compared with conspecifics from areas with no hypoxia and had no significant indications of oxidative damage. We conclude that mysids can protect themselves from oxidative damage, enabling them to inhabit hypoxic water. Our data suggest that hypoxic and low oxygen zones (up to 4 mL L−1) may provide some zooplankton species with a refuge from visual predators such as fish.

In Situ Combustion of Light Oil: Stoichiometric, Kinetic, and Thermodynamic Analyses from the Flow Experiments

In situ combustion of light oil (37°API) was studied here through combustion tube experiment and mathematical modeling. The flue gas at the effluent provided a reaction stoichiometry, and indicated an aggregate molecular entity similar to bitumen as the burning fraction of the crude oil. A mathematical model based on 1D heat and mass transport with three phases in porous media effectively simulated the sustenance and progression of the combustion front, observed experimentally. The sensitivity of these aspects to the perturbations in rate constants, activation energy, and heat of reaction is discussed here. The extent of accumulation of oil and water at the downstream side of the combustion front was evaluated. The simulation results concerning cumulative collection of oil at the effluent was verified with experimental data. The width of the combustion zone, evident from the concentration profile of oxygen, is discussed in this article.

Effect of the stoichiometry of niobium oxide on the resistive switching of Nb2O5 based metal–insulator–metal stacks

The oxygen concentration profiles, which develop at the interfaces between niobium pentoxide and the Al or Pt electrode in a metal–insulator–metal stack, were investigated by means of the X-ray and electron energy loss spectroscopies in a scanning transmission electron microscope with high resolution. The contact between Al and Nb2O5 was found to facilitate diffusion of oxygen from Nb2O5 to the Al electrode and to support the formation of a thin aluminum oxide layer at the Nb2O5/Al interface. In contrast, almost no diffusion of oxygen from Nb2O5 was observed at the Nb2O5/Pt interface. Different extent of the oxygen diffusion correlates with the observed differences in the resistive switching of the Pt/Nb2O5/Al and Pt/Nb2O5/Pt stacks.