External Oxygen Concentration Research Articles

Predicting physiologically-relevant oxygen concentrations in precision-cut liver slices using mathematical modelling.

Precision cut liver slices represent an encouraging ex vivo method to understand the pathogenesis of liver disease alongside drug induced liver injury. Despite being more physiologically relevant compared to in vitro models, precision cut liver slices are limited by the availability of healthy human tissue and experimental variability. Internal oxygen concentration and media composition govern the longevity and viability of the slices during the culture period and as such, a variety of approaches have been taken to maximise the appropriateness of the internal oxygen concentrations across the slice. The aim of this study was to predict whether it is possible to generate a physiologically relevant oxygen gradient of 35-65mmHg across a precision cut liver slice using mathematical modelling. Simulations explore how the internal oxygen concentration changes as a function of the diameter of the slice, the position inside the well and the external incubator oxygen concentration. The model predicts that the desired oxygen gradient may be achieved using a 5mm diameter slice at atmospheric oxygen concentrations, provided that the slice is positioned at a certain height within the well of a 12-well plate.

Numerical simulation on response characteristics of coal ignition under the disturbance of fluctuating heat

Owing to coal commonly existing in a fluctuating thermal environment, a two–step parallel reaction model for coal ignition was established to explore the development of coal ignition under fluctuating thermal convection, fluctuating thermal radiation, and convection–radiation coupling (CRC). To be specific, this included temperature evolution in a coal mass and fluctuating periodic characteristics. The nonlinear gain of coal spontaneous combustion caused by oscillatory heat flux conditions was researched. The results show that the thermal fluctuation at 0.1 Hz is conducive to coal ignition. The ignition delay time (IDT) exponentially decreases with the growth of both the thermal convection velocity and radiation heat flux. Under external thermal disturbance, the temperature, convection velocity, oxygen concentration, and depth of the sensitive area all vary periodically. Under different types of heat disturbance, the temperature and convection velocity fluctuate with an increasing trend, while the oxygen concentration fluctuates with a decreasing trend. There is a low phase difference between the temperature and the convection velocity while the phase difference between the temperature and oxygen concentration is about 90° Besides, the peak fluctuation of the oxidation rate of coal changes inversely with the depth of fluctuations of the sensitive area.

Numerical simulation of electrochemical mechanism of steel rebar corrosion in concrete under natural climate with time-varying temperature and humidity

In order to analyze and predict the corrosion ratio and rust layer distribution of steel rebar embedded in reinforced concrete under time-varying climatic conditions, a self-balancing steel corrosion model was established by connecting the environmental temperature, humidity, external chloride ions and oxygen concentration to the potential distribution and current density of the steel using finite element method. The model covered the whole corrosion process of steel rebar in concrete under natural climate, including the steel rebar depassivation, steel corrosion propagation and the rust layer expansion on the surface of steel rebar. In the proposed model, the microcell and macrocell corrosion were coupled by electrochemical polarization, and the cathode-to-anode area ratio was determined by the time-varying chloride distribution at the rebar surface. Taking the diffusion of chloride ions and oxygen in concrete into account, the distribution of corrosion potential of reinforcement, the change of corrosion current density of reinforcement, and the distribution of rust layer on the surface of reinforcement can be obtained based on the variation of natural temperature and humidity. The proposed model was validated utilizing the experimental data from the literatures. On this basis, a discussion about the influence of long-term climatic environments on the corrosion kinetics were conducted. The results suggest that environmental temperature and relative humidity affect the steel corrosion process mainly by affactting the chloride and oxygen diffusion coefficients, concrete resistivity, and concentration of dissolved oxygen in concrete pore solution. The corrosion current density of steel in concrete is sensitively influenced by the environmental temperature more than relative humidity. Although the initial exposure time has a significant impact on the accumulation process of chloride on steel surface within one climatic cycle, it has little influence on the final chloride distribution at the end of the period. However, the initial exposure time remarkably influences the evolution profile of rust layer thickness during the climatic cycle. The steel rebar initially exposed to a higher temperature suffers from more severe corrosion than that initially exposed to a lower temperature.

Modeling the oxygen inhibition in microalgae: An experimental approach based on photorespirometry

Microalgae cultivation has been the object of relevant interest for many industrial applications. Where high purity of the biomass/product is required, closed photobioreactors (PBRs) appear to be the best technological solution. However, as well as cost, the major drawback of closed systems is oxygen accumulation, which is well known to be responsible for growth inhibition. Only a few quantitative approaches have attempted to describe and model oxygen inhibition, which is the result of different biological mechanisms. Here, we have applied a photorespirometric protocol to assess and quantify the effect of high oxygen concentration on photosynthetic production rate. In particular, the effects of light intensity and biomass concentration were assessed, resulting in different maximum inhibitory oxygen concentrations. Literature models available were found not to fully represent experimental data as a function of concentration and light. Accordingly, a new formulation was proposed and validated to describe the photosynthetic rate as a function of external oxygen concentration.

Every Breath You Take: Non-invasive Real-Time Oxygen Biosensing in Two- and Three-Dimensional Microfluidic Cell Models.

Knowledge on the availability of dissolved oxygen inside microfluidic cell culture systems is vital for recreating physiological-relevant microenvironments and for providing reliable and reproducible measurement conditions. It is important to highlight that in vivo cells experience a diverse range of oxygen tensions depending on the resident tissue type, which can also be recreated in vitro using specialized cell culture instruments that regulate external oxygen concentrations. While cell-culture conditions can be readily adjusted using state-of-the-art incubators, the control of physiological-relevant microenvironments within the microfluidic chip, however, requires the integration of oxygen sensors. Although several sensing approaches have been reported to monitor oxygen levels in the presence of cell monolayers, oxygen demands of microfluidic three-dimensional (3D)-cell cultures and spatio-temporal variations of oxygen concentrations inside two-dimensional (2D) and 3D cell culture systems are still largely unknown. To gain a better understanding on available oxygen levels inside organ-on-a-chip systems, we have therefore developed two different microfluidic devices containing embedded sensor arrays to monitor local oxygen levels to investigate (i) oxygen consumption rates of 2D and 3D hydrogel-based cell cultures, (ii) the establishment of oxygen gradients within cell culture chambers, and (iii) influence of microfluidic material (e.g., gas tight vs. gas permeable), surface coatings, cell densities, and medium flow rate on the respiratory activities of four different cell types. We demonstrate how dynamic control of cyclic normoxic-hypoxic cell microenvironments can be readily accomplished using programmable flow profiles employing both gas-impermeable and gas-permeable microfluidic biochips.

Denitrification in Soil Aggregate Analogues-Effect of Aggregate Size and Oxygen Diffusion

Soil-borne nitrous oxide (N$_2$O) emissions have a high spatial and temporal variability which is commonly attributed to the occurrence of hotspots and hot moments for microbial activity in aggregated soil. Yet there is only limited information about the biophysical processes that regulate the production and consumption of N$_2$O on microscopic scales in undisturbed soil. In this study, we introduce an experimental framework relying on simplified porous media that circumvents some of the complexities occuring in natural soils while fully accounting for physical constraints believed to control microbial activity in general and denitrification in particular. We used this framework to explore the impact of aggregate size and external oxygen concentration on the kinetics of O$_2$ consumption, as well as CO$_2$ and N$_2$O production. Model aggregates of different sizes (3.5 vs. 7\,mm diameter) composed of porous, sintered glass were saturated with a defined growth medium containing roughly 10$^9$ cells ml$^{-1}$ of the facultative anaerobic, \textsl{nosZ}-deficient denitrifier \textsl{Agrobacterium tumefaciens} with N$_2$O as final denitrification product and incubated at five different oxygen levels (0-13\,vol-$\%$). We demonstrate that the onset of denitrification depends on the amount of external oxygen and the size of aggregates. Smaller aggregates were better supplied with oxygen due to a larger surface-to-volume ratio, which resulted in faster growth and an earlier onset of denitrification. In larger aggregates, the onset of denitrification was more gradual, but with comparably higher N$_2$O production rates once the anoxic aggregate centers were fully developed. The normalized electron flow from the reduced carbon substrate to N-oxyanions (e$^{-}_{\rm denit}$/e$^{-}_{\rm total}$ ratio) could be solely described as a function of initial oxygen concentration in the headspace with a simple, hyperbolic model, for which the two empirical parameters changed with aggregate size in a consistent way. These findings confirm the important role of soil structure on N$_2$O emissions from denitrification by shaping the spatial patterns of microbial activity and anoxia in aggregated soil. Our dataset may serve as a benchmark for constraining or validating spatially explicit, biophysical models of denitrification in aggregated soil.

Flame Spread on Inclined Wood Surfaces: Influence of External Heat Flux and Ambient Oxygen Concentration

ABSTRACTFire development is influenced by many factors, such as oxygen concentration, external radiation, and the inclination angle of the combustible material. This article examines the effect of these factors on flame spread over a charring solid using a series of experimental studies. The results show that the flame spread can be divided into three regimes based on oxygen concentration: an unsustained flame spread regime (Yo ≤ 0.8), a stable combustion regime (0.8 < Yo < 1.3), and an accelerating flame spread regime (Yo ≥ 1.3), where Yo is the oxygen concentration ratio in mole fraction. The critical angle for the transition from the stable combustion regime to the accelerating combustion regime is between 10° and 20°, measured from the horizontal. When the fuel is exposed to external radiation, the flame propagation velocity generally increases. At reduced oxygen concentrations, the increase in flame spread rate is greater with external radiation, whereas at a higher oxygen concentration level, the oxygen concentration, rather than the external radiation, dominates the increased flame spread rate.

Effects of Oxygen Concentration on Radiation-Aided and Self-sustained Smoldering Combustion of Radiata Pine

Smoldering combustion is an important form of combustion in wildfires and hazard reduction burning because it plays vital roles in pollutant emission, fire re-ignition, and ecological impact. Smoldering combustion can be classified as either radiation-aided or self-sustained, depending on the nature of the reactions. The latter is often considered a more hazardous type of smoldering combustion, because it can persist for a long period of time and can transition into flaming combustion. However, there is a lack of understanding of the differences between radiation-aided and self-sustained smoldering combustion processes, especially regarding characterization. The aim of this study is to investigate and quantify the differences between radiation-aided and self-sustained smoldering combustion in biomass. Experiments were conducted using an infrared heat lamp to heat pulverize fuel samples in a reactor. The external energy input and oxygen concentration were controlled in order to achieve radiation-aided and ...

Short-Term Molecular Acclimation Processes of Legume Nodules to Increased External Oxygen Concentration

Nitrogenase is an oxygen labile enzyme. Microaerobic conditions within the infected zone of nodules are maintained primarily by an oxygen diffusion barrier (ODB) located in the nodule cortex. Flexibility of the ODB is important for the acclimation processes of nodules in response to changes in external oxygen concentration. The hypothesis of the present study was that there are additional molecular mechanisms involved. Nodule activity of Medicago truncatula plants were continuously monitored during a change from 21 to 25 or 30% oxygen around root nodules by measuring nodule H2 evolution. Within about 2 min of the increase in oxygen concentration, a steep decline in nitrogenase activity occurred. A quick recovery commenced about 8 min later. A qPCR-based analysis of the expression of genes for nitrogenase components showed a tendency toward upregulation during the recovery. The recovery resulted in a new constant activity after about 30 min, corresponding to approximately 90% of the pre-treatment level. An RNAseq-based comparative transcriptome profiling of nodules at that point in time revealed that genes for nodule-specific cysteine-rich (NCR) peptides, defensins, leghaemoglobin and chalcone and stilbene synthase were significantly upregulated when considered as a gene family. A gene for a nicotianamine synthase-like protein (Medtr1g084050) showed a strong increase in count number. The gene appears to be of importance for nodule functioning, as evidenced by its consistently high expression in nodules and a strong reaction to various environmental cues that influence nodule activity. A Tnt1-mutant that carries an insert in the coding sequence (cds) of that gene showed reduced nitrogen fixation and less efficient acclimation to an increased external oxygen concentration. It was concluded that sudden increases in oxygen concentration around nodules destroy nitrogenase, which is quickly counteracted by an increased neoformation of the enzyme. This reaction might be induced by increased formation of NCR peptides and necessitates an efficient iron supply to the bacteroid, which is probably mediated by nicotianamine.The paper is dedicated to the 85th birthday of Prof. Dr. Günther Schilling, University of Halle/Wittenberg, Germany, https://de.wikipedia.org/wiki/Günther_Schilling

Extracellular matrix production by nucleus pulposus and bone marrow stem cells in response to altered oxygen and glucose microenvironments.

Bone marrow (BM) stem cells may be an ideal source of cells for intervertebral disc (IVD) regeneration. However, the harsh biochemical microenvironment of the IVD may significantly influence the biological and metabolic vitality of injected stem cells and impair their repair potential. This study investigated the viability and production of key matrix proteins by nucleus pulposus (NP) and BM stem cells cultured in the typical biochemical microenvironment of the IVD consisting of altered oxygen and glucose concentrations. Culture-expanded NP cells and BM stem cells were encapsulated in 1.5% alginate and ionically crosslinked to form cylindrical hydrogel constructs. Hydrogel constructs were maintained under different glucose concentrations (1, 5 and 25 mM) and external oxygen concentrations (5 and 20%). Cell viability was measured using the Live/Dead® assay and the production of sulphated glycosaminoglycans (sGAG), and collagen was quantified biochemically and histologically. For BM stem cells, IVD-like micro-environmental conditions (5 mM glucose and 5% oxygen) increased the accumulation of sGAG and collagen. In contrast, low glucose conditions (1 mM glucose) combined with 5% external oxygen concentration promoted cell death, inhibiting proliferation and the accumulation of sGAG and collagen. NP-encapsulated alginate constructs were relatively insensitive to oxygen concentration or glucose condition in that they accumulated similar amounts of sGAG under all conditions. Under IVD-like microenvironmental conditions, NP cells were found to have a lower glucose consumption rate compared with BM cells and may in fact be more suitable to adapt and sustain the harsh microenvironmental conditions. Considering the highly specialised microenvironment of the central NP, these results indicate that IVD-like concentrations of low glucose and low oxygen are critical and influential for the survival and biological behaviour of stem cells. Such findings may promote and accelerate the translational research of stem cells for the treatment of IVD degeneration.

A Minimal Nitrogen Fixation Gene Cluster from Paenibacillus sp. WLY78 Enables Expression of Active Nitrogenase in Escherichia coli

Most biological nitrogen fixation is catalyzed by molybdenum-dependent nitrogenase, an enzyme complex comprising two component proteins that contains three different metalloclusters. Diazotrophs contain a common core of nitrogen fixation nif genes that encode the structural subunits of the enzyme and components required to synthesize the metalloclusters. However, the complement of nif genes required to enable diazotrophic growth varies significantly amongst nitrogen fixing bacteria and archaea. In this study, we identified a minimal nif gene cluster consisting of nine nif genes in the genome of Paenibacillus sp. WLY78, a gram-positive, facultative anaerobe isolated from the rhizosphere of bamboo. We demonstrate that the nif genes in this organism are organized as an operon comprising nifB, nifH, nifD, nifK, nifE, nifN, nifX, hesA and nifV and that the nif cluster is under the control of a σ70 (σA)-dependent promoter located upstream of nifB. To investigate genetic requirements for diazotrophy, we transferred the Paenibacillus nif cluster to Escherichia coli. The minimal nif gene cluster enables synthesis of catalytically active nitrogenase in this host, when expressed either from the native nifB promoter or from the T7 promoter. Deletion analysis indicates that in addition to the core nif genes, hesA plays an important role in nitrogen fixation and is responsive to the availability of molybdenum. Whereas nif transcription in Paenibacillus is regulated in response to nitrogen availability and by the external oxygen concentration, transcription from the nifB promoter is constitutive in E. coli, indicating that negative regulation of nif transcription is bypassed in the heterologous host. This study demonstrates the potential for engineering nitrogen fixation in a non-nitrogen fixing organism with a minimum set of nine nif genes.

Use of Culture Geometry to Control Hypoxia-Induced Vascular Endothelial Growth Factor Secretion from Adipose-Derived Stem Cells: Optimizing a Cell-Based Approach to Drive Vascular Growth

Adipose-derived stem cells (ADSCs) possess potent angiogenic properties and represent a source for cell-based approaches to delivery of bioactive factors to drive vascularization of tissues. Hypoxic signaling appears to be largely responsible for triggering release of these angiogenic cytokines, including vascular endothelial growth factor (VEGF). Three-dimensional (3D) culture may promote activation of hypoxia-induced pathways, and has furthermore been shown to enhance cell survival by promoting cell-cell interactions while increasing angiogenic potential. However, the development of hypoxia within ADSC spheroids is difficult to characterize. In the present study, we investigated the impact of spheroid size on hypoxia-inducible transcription factor (HIF)-1 activity in spheroid cultures under atmospheric and physiological oxygen conditions using a fluorescent marker. Hypoxia could be induced and modulated by controlling the size of the spheroid; HIF-1 activity increased with spheroid size and with decreasing external oxygen concentration. Furthermore, VEGF secretion was impacted by the hypoxic status of the culture, increasing with elevated HIF-1 activity, up to the point at which viability was compromised. Together, these results suggest the ability to use 3D culture geometry as a means to control output of angiogenic factors from ADSCs, and imply that at a particular environmental oxygen concentration an optimal culture size for cytokine production exists. Consideration of culture geometry and microenvironmental conditions at the implantation site will be important for successful realization of ADSCs as a pro-angiogenic therapy.

Radiation enhanced B-Number

Radiative heat transfer plays a crucial role in most fire hazard related combustion processes. The existing classic and modified mass transfer B-Numbers have deficiencies for some burning cases of fires because they do not involve external radiation and/or assume flame transparent to thermal radiation. This study extended the concept of the mass transfer B-Number using the one-dimensional steady-state diffusion flame model. An analytical expression of the universal mass transfer B-Number BR was derived, which applies to semi-transparent flames with external radiation. Compared with the classic mass transfer B-Number, in the numerator denoting the net heat generated and held by the combustion system, BR has an additional term that represents radiative heat which is being held and transferred within the semi-transparent flame; as well as in the denominator denoting the convective heat needing for maintaining the combustion, the contribution from the radiative heat is deducted from the total heat required for vaporizing a unit mass of fuel at the fuel surface. BR increases as external radiation and/or ambient oxygen concentration raise. In order to examine its physical significance, new parameters were defined and illustrated, such as the mass transfer equivalent absorption coefficient, specific radiant heat feedback and radiation factor, that demonstrate interactions of heat and mass transfers and radiative absorption and emission within inhomogeneous flames. Effects of the external radiation and ambient oxygen concentration on these parameters were also evaluated. Increases in the radiation factor as the ambient oxygen concentration rises theoretically validated enhanced radiative heat transfer in rich oxygen combustion.

The effect of pressure and oxygen concentration on the combustion of PMMA

An experimental study of the flammability properties of PMMA at low pressures and oxygen concentrations was performed. The work was motivated by the importance of these effects on fire safety in the aviation industry. Measurements were obtained in a mass loss calorimeter inside a large 10m3 pressure vessel capable of reaching pressures as low as 0.1atm. The PMMA flammability was characterized by measuring the burning rate and the time to ignition of small test samples. These were ignited and burned under different external heat fluxes, total pressures and oxygen concentrations. The combined effects of pressure and oxygen concentration on the burning rate, combustion flow field, and ignition were evaluated. Results showed that at low pressure, the burning rate was less intense with a decrease in the mass loss rate. However, the reduction in pressure caused a shortened ignition delay time. Experimental measurements were compared with a simple analytical model showing good agreement. The results also show how pressure and oxygen concentration contributed to the heat transfer from the flame. The model revealed that a single function in oxygen and pressure could account for both flame radiative and convective effects. As a result, a power law fit was obtained for the relation of the combined pressure and oxygen effect on the burning rate. This correlation shows a good agreement with the measurements and predicts the burning rate behavior for the full range of pressure and oxygen tests.

HRE-Type Genes are Regulated by Growth-Related Changes in Internal Oxygen Concentrations During the Normal Development of Potato (Solanum tuberosum) Tubers

The occurrence of hypoxic conditions in plants not only represents a stress condition but is also associated with the normal development and growth of many organs, leading to adaptive changes in metabolism and growth to prevent internal anoxia. Internal oxygen concentrations decrease inside growing potato tubers, due to their active metabolism and increased resistance to gas diffusion as tubers grow. In the present work, we identified three hypoxia-responsive ERF (StHRE) genes whose expression is regulated by the gradual decrease in oxygen tensions that occur when potato tubers grow larger. Increasing the external oxygen concentration counteracted the modification of StHRE expression during tuber growth, supporting the idea that the actual oxygen levels inside the organs, rather than development itself, are responsible for the regulation of StHRE genes. We identified several sugar metabolism-related genes co-regulated with StHRE genes during tuber development and possibly involved in starch accumulation. All together, our data suggest a possible role for low oxygen in the regulation of sugar metabolism in the potato tuber, similar to what happens in storage tissues during seed development.

Investigating the steady state of multicellular spheroids by revisiting the two-fluid model

In this paper we examine the steady state of tumour spheroids considering a structure in which the central necrotic region contains an inner liquid core surrounded by dead cells that keep some mechanical integrity. This partition is a consequence of assuming that a finite delay is required for the degradation of dead cells into liquid. The phenomenological assumption of constant local volume fraction of cells is also made. The above structure is coupled with a simple mechanical model that views the cell component as a viscous fluid and the extracellular liquid as an inviscid fluid. By imposing the continuity of the normal stress throughout the whole spheroid, we show that a steady state can exist only if the forces on cells at the outer boundary (provided e.g. by a surface tension) are intense enough, and in such a case we can compute the stationary radius. By giving reasonable values to the parameters, the model predicts that the stationary radius decreases with the external oxygen concentration, as expected from experimental observations.

A Review of Models for the Charring Rate of Solid Wood, Nail- and Glue- Laminated Structural Members

Simple calculation models for charring rate of solid wood, nail- and glue-laminated structural elements for use in fire design of wooden structures have been compared with test results from the literature. The applicability to these structural elements, and the accuracy of the models, have been evaluated based on parameters influencing the charring rate, which include species, density, moisture content, char contraction factor, external heat flux, oxygen concentration and duration of the fire. The models evaluated in this article are applicable to one- or two-dimensional charring under standard fire exposure.

Transcript and metabolite profiling of the adaptive response to mild decreases in oxygen concentration in the roots of arabidopsis plants

Background and AimsOxygen can fall to low concentrations within plant tissues, either because of environmental factors that decrease the external oxygen concentration or because the movement of oxygen through the plant tissues cannot keep pace with the rate of oxygen consumption. Recent studies document that plants can decrease their oxygen consumption in response to relatively small changes in oxygen concentrations to avoid internal anoxia. The molecular mechanisms underlying this response have not been identified yet. The aim of this study was to use transcript and metabolite profiling to investigate the genomic response of arabidopsis roots to a mild decrease in oxygen concentrations.MethodsArabidopsis seedlings were grown on vertical agar plates at 21, 8, 4 and 1 % (v/v) external oxygen for 0·5, 2 and 48 h. Roots were analysed for changes in transcript levels using Affymetrix whole genome DNA microarrays, and for changes in metabolite levels using routine GC-MS based metabolite profiling. Root extension rates were monitored in parallel to investigate adaptive changes in growth.Key ResultsThe results show that root growth was inhibited and transcript and metabolite profiles were significantly altered in response to a moderate decrease in oxygen concentrations. Low oxygen leads to a preferential up-regulation of genes that might be important to trigger adaptive responses in the plant. A small but highly specific set of genes is induced very early in response to a moderate decrease in oxygen concentrations. Genes that were down-regulated mainly encoded proteins involved in energy-consuming processes. In line with this, root extension growth was significantly decreased which will ultimately save ATP and decrease oxygen consumption. This was accompanied by a differential regulation of metabolite levels at short- and long-term incubation at low oxygen.ConclusionsThe results show that there are adaptive changes in root extension involving large-scale reprogramming of gene expression and metabolism when oxygen concentration is decreased in a very narrow range.

Oxygen profiles in egg masses predicted from a diffusion–reaction model

We developed a novel diffusion-reaction model to describe spatial and temporal changes in oxygen concentrations in gelatinous egg masses containing live, respiring embryos. We used the model in two ways. First, we constructed artificial egg masses of known metabolic density using embryos of the Antarctic sea urchin Sterechnius neumayeri, measured radial oxygen profiles at two temperatures, and compared our measurements to simulated radial oxygen profiles generated by the model. We parameterized the model by measuring the radius of the artificial masses, metabolic densities (=embryo metabolic rate x embryo density) and oxygen diffusion coefficients at both ambient (-1.5 degrees C) or slightly warmer (+1.5-2 degrees C) temperatures. Simulated and measured radial oxygen profiles were similar, indicating that the model captured the major biological features determining oxygen distributions. Second, we used the model to analyze sources of error in step-change experiments for determining oxygen diffusion coefficients (D), and to determine the suitability of simpler, analytical equations for estimating D. Our analysis indicated that embryo metabolism can lead to large (several-fold) overestimates of D if the analytical equation is fitted to step-down-traces of central oxygen concentration (i.e. external oxygen concentration stepped from some high value to zero). However, good estimates of D were obtained from step-up-traces. We used these findings to estimate D in egg masses of three species of nudibranch molluscs: two Antarctic species (Tritonia challengeriana and Tritoniella belli; -1.5 and +2 degrees C) and one temperate Pacific species (Tritonia diomedea; 12 and 22 degrees C). D for all three species was approximately 8 x 10(-6) cm(2) s(-1), and there was no detectable effect of temperature on estimated D. For the Antarctic species, D in egg masses was 70-90% of its value in seawater of similar temperature.

Effect of oxygen concentration on intracellular pH, glucose‐6‐phosphate and NTP content in rice (Oryza sativa) and wheat (Triticum aestivum) root tips: in vivo 31P‐NMR study

Hypoxic pretreatment is known to induce anoxia tolerance in plant species sensitive to oxygen deprivation. However, we still do not have detailed information on changes in cytoplasmic and vacuolar pH (pHcyt and pHvac) in plants under low‐oxygen availability (hypoxia) and under anoxia. To investigate this, we have studied the influence of hypoxia and anoxia on pHcyt and pHvac, glucose‐6‐phosphate (Glc‐6‐P) and nucleotide triphosphate (NTP) contents in rice (Oryza sativa L.) root tips in comparison with those of wheat (Triticum aestivum L.) with in vivo 31P‐nuclear magnetic resonance. Both cereals responded to hypoxia similarly, by rapid cytoplasmic acidification (from pH 7.6–7.7 to 7.1), which was followed by slow partial recovery (0.3 units after 6 h). Anoxia led to a dramatic pHcyt drop in tissues of both species (from pH 7.6–7.7 to less than 7.0) and partial recovery took place in rice only. In wheat, the acidification continued to pH 6.8 after 6 h of exposure. In both plants, NTP content followed the dynamics of pHcyt. There was a strong correlation between NTP content and cytoplasmic H+ activity ([H+]cyt= 10−pHcyt) for both hypoxic and anoxic conditions. Glc‐6‐P content increased in rice under anoxia and hypoxia. In wheat, Glc‐6‐P was not detectable under anoxia but increased under hypoxia. In this study, rice root tips were shown to behave as anoxia tolerant tissues. Our results suggest that the initial cytoplasmic acidification and subsequent pHcyt are differently regulated in anoxia tolerant and intolerant plants and depend on the external oxygen concentration.