Diffusive Oxygen Fluxes Research Articles

Isotopic Fractionation of Oxygen During Sediment Respiration

The apparent community respiration fractionation factor for deep-sea sediments has a predicted value of 9%0 (Bender, 1990). This value is based on a model of sediment oxygen consumption that satisfies the assumption of first order kinetics with respect to carbon (i.e. a pore water oxygen profile which decreases exponentially with depth below the sediment-water interface). This implies that the organic matter oxidation rate is carbon limited. Bender suggests the geometry of the sediments (a partially-closed system) prevents the expression of the full microbial respiration fractionation which has been determined to be 19.1%o (cz = 0.980) for freeliving bacteria (Kiddon et al., 1993). The apparent fractionation in sediments (9%0 or et = 0.991) has a square-root relation to the fractionation factor for bacterial cultures as a result of the assumption of an exponential oxygen decrease in the sediments. Brandes and Devol (1996) have presented oxygen isotope data for benthic chamber deployments in Puget Sound, Washington. Their results indicate an apparent sedimentary respiration fraction factor of 3%0 (~ = 0.997). The Puget Sound sediments are sites of rapid organic matter diagenesis and complete oxygen consumption within the top centimeter of the sediments. Brandes and Devol argued that the apparent fractionation is dramatically reduced from Bender's value of 9%0 (or from the free-living bacterial fractionation of 18%o) due to complete oxygen removal and due to the diffusive limitation present in a sedimentary system. Their model parameterizes intense diagenesis in discrete sediment micro-sites wherein oxygen is completely consumed. The apparent fractionation is small because oxygen concentrations go to zero at the micro-site. They suggest this small fractionation arises as a result of oxidant limitation. When the diffusive oxygen flux is insufficient to balance the respiratory demand all oxygen molecules are consumed and there is no distinction between the heavy and the light isotopes.

Calcite dissolution in sediments of the Ontong‐Java Plateau: In situ measurements of pore water O2 and pH

We present in situ electrode measurements of sediment resistivity, pore water oxygen, and pore water pH from three stations between 2300 and 3000 m depth on the Ontong‐Java Plateau in the western equatorial Pacific. One of these stations is also the site of a concurrent benthic chamber incubation experiment [Jahnke et al., 1994]. The pore water oxygen data and a steady state diffusion and reaction model constrain the depth‐dependent rate of oxic respiration in the sediments and imply a diffusive flux of oxygen to the sediments of 10–21 μmol cm−2 yr−1. Given these respiration rates, the pore water pH data cannot be explained without calcite dissolution driven by metabolically produced CO2. The dissolution necessary to explain the observations, quantified by a statistical approach, is 3.5–6 μmol cm−2 yr−1, which corresponds to at least 20–40% of the calcite rain to these sediments. Over 65% of the total dissolution is driven by metabolic CO2. Oxygen fluxes and net calcite dissolution constrained by the electrode data are compatible with the benthic chamber measurements of Jahnke et al. [1994]. The dissolution flux, while a significant part of the early diagenesis of calcite in these sediments, is less than would be predicted by earlier models of dissolution, and Jahnke et al. [1994] probably could not distinguish it from zero with the benthic chamber technique. The dissolution rates found in this study are lower than previous estimates because the respiration reaction is concentrated near the sediment‐water interface, and the calcite dissolution rate constants are very small. The statistical evaluation of the pore water pH data and model constrain the calcite dissolution rate constant to 0.005–0.16% d−1, following the general trend of lower values determined by in situ techniques rather than by laboratory methods.

Pore-water response on seasonal environmental changes in intertidal sediments of the Weser Estuary, Germany

In order to determine time-dependent changes in estuarine pore-water chemistry and flux variations across the sediment-water interface, sediment cores of an intertidal mud flat in the Weser Estuary were taken monthly over a one-year period. Sediment temperature, pH, Eh, Cl−, O2, NO 3 − , and SO 4 2− pore-water concentrations were measured and showed variations that relate to the changes of surface temperature and estuarine water composition. Fick's first law was applied to quantify diffusive fluxes from concentration gradients in the diffusive boundary layer and in the pore water. Total nitrate fluxes were calculated from flux chamber experiments. Diffusive oxygen fluxes increased from 5 mmol m−2 d−1 in winter to 18 mmol m−2 d−1 in early summer, while nitrate fluxes into the sediment increased from 3 mmol m−2 d−1 in winter to 60 mmol m−2 d−1 in early summer. Oxygen and nitrate fluxes into the sediment correlated linearly to sediment temperature. Sulfate fluxes increased from 0.5 mmol m−2 d−1 in winter to 10 mmol m−2 d−1 in August and September. Converted into carbon fluxes, the sum of these oxidants ranged from 10 mmol m−2 d−1 in winter to 80 mmol m−2 d−1 in summer. An estimation of the upper limit of the annual nitrate flux into the sediment showed that about 10% of the 250,000 t of nitrate discharged annually by the river may be decomposed within the inner Weser Estuary.

Early diagenesis of organic material in equatorial Pacific sediments: stpichiometry and kinetics

Benthic incubation chambers and sediment pore water profiles were used to study early diagenesis of organic matter in equatorial Pacific sediments. Replicate measurements with a flux chamber covering 720 cm 2 indicated that the spatial variability of oxygen, TCO 2, alkalinity, nitrate and silica fluxes at a single station did not exceed 10–35%. In contrast, diffusive fluxes of oxygen from replicate cores covering 70 cm 2 at a single station often showed greater variation. In January 1992, benthic oxygen consumption was fairly constant along the equator from 103°W to 140°W at 0.6-0.8 mmol m −2 day −1. In November 1992, consumption was roughly symmetrical across the equator along 140°W, with rates of 0.6-0.8 mmol m −2 day −1 between 2°S and 2°N, declining to rates of 0.1-0.2 mmol m −2 day −1 at 12°S and 9°N. Pore water oxygen profiles were fit with a reaction-diffusion model equation to evaluate reaction kinetics. Most profiles were adequately fit with a model that assumed reaction rates declined exponentially with depth, but at low latitudes better fits often were obtained with a model that assumed decomposing organic matter has two labile components and that each decays with first-order kinetics and decreases exponentially with depth. Results of both fits indicate that at least 70% of the organic matter degradation occurs within the upper 1–2 cm of sediment. At the low-latitude stations fit with the two-component model, 70–90% of the flux is attributable to the more labile component which has an average 1/e penetration depth of 0.4 ± 0.1 cm. The more refractory component at these stations has a penetration depth of 4.4 ± 0.4 cm. From estimates of sediment mixing rates, the mean life of all degrading organic matter at the higher latitude stations is 4–55 years, while at the stations fit with the two-component model, the lifetime of the more labile fraction is weeks to months, and the lifetime of the less labile component is 40–300 years. A third carbon fraction exists at all stations that is far more refractory. The O 2:CO 2 stoichiometry of remineralization is −1.45 ± 0.17, and the C:N ratio is 8 ± 1. Both ratios are in good agreement with those observed from sediment trap and hydrographic studies in the water column, and suggest that degrading organic matter has about 70% of its carbon in -CH 2O-groups and 30% in -CH 2-groups. The C:P atom ratios for benthic remineralization differ by a factor of 3 for the two cruises, showing substantial temporal variability and de-coupling from carbon, although the mean for the two cruises (170 ± 85) is not significantly different than remineralization ratios observed in the water column. The aerally-integrated benthic respiration rate for the equatorial Pacific upwelling region is at least 25% of the integrated respiration rate for the continental margin (slope + rise) areas of the Pacific, emphasizing the importance of the equatorial Pacific sediments as a major site of benthic carbon recycling. Benthic carbon remineralization rates determined during the past decade near the equator and 140°W have varied by a factor of 2, which is not surprising given the short lifetime of the majority of the carbon degrading. The temporal patterns of carbon remineralization rates resemble those of sea-surface temperature, suggesting that benthic carbon oxidation at this site may reflect water column productivity over relatively short timescales.

Oxygen pore water profiles in continental shelf sediments of the North Sea:turbulent versus molecular diffusion

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 145:63-75 (1996) - doi:10.3354/meps145063 Oxygen pore water profiles in continental shelf sediments of the North Sea: turbulent versus molecular diffusion Lohse L, Epping EHG, Helder W, van Raaphorst W Oxygen pore water profiles in North Sea sediments were measured with microelectrodes during 2 contrasting seasons. The measurements were conducted in a wide variety of sediments, including non-depositional areas on the southern shelf as well as depositional areas in the Skagerrak. All measurements were performed within minutes on board at in situ temperature. The curvature of oxygen profiles in sandy sediments on the southern shelf indicated the presence of a surface layer characterised by enhanced diffusion. The occurrence of enhanced diffusion was related to sedimentological and seasonal differences. Quantitative evaluation of the pore water profiles by a diffusion-reaction model indicated that the effective diffusion coefficients in a 0.2 to 16 mm subsurface layer were 1.5 to >100 times higher than the molecular diffusion coefficient. Highest effective diffusion coefficients were reported for non-depositional sediments characterised by low diffusive oxygen fluxes. Oxygen profiles in sediments of the depositional area of the Skagerrak indicated constant diffusivity throughout the sediment column. Diffusive fluxes calculated from profiles ranged from 5.2 to 8.9 mmol m-2 d-1 in August 1991, and were between 0.8 and 6.2 mmol m-2 d-1 in February 1992. Seasonal differences in sediments located in the Skagerrak area were minor. It is proposed that near-bottom tidal currents induce enhanced diffusion transport processes in the upper millimetres of the sandy sediments of the southern North Sea, while less energetic hydrodynamical conditions in the depositional area of the Skagerrak favour sediment-water exchange based on molecular diffusion only. Biogeochemical implications of the enhanced diffusivity close to the sediment-water interface are discussed. Oxygen · Sediment-water exchange · Microelectrode · Molecular diffusion · Turbulent diffusion · Enhanced transport · Bottom shear stress Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 145. Publication date: December 31, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Oxygen, nutrients, carbon and water exchange in the Skagerrak Basin

The mean oxygen depletion rate at 600 m depth in the Skagerrak basin water was 0.04 ml l −1 per month between 1955 and 1991 and in the mean oxygen minimum concentration 5.6 ml l −1. No indication of long-term increase in oxygen depletion rate or decrease in oxygen minimum concentration was observed. The average period between major inflows to the Skagerrak Basin was about 25 months. The extra low oxygen minimum level observed in 1990 (4.8 ml l −1) was explained by the prolonged residence time (40 months) of the basin water between 1987 and 1991. The volume mean oxygen consumption rate ( CONS) was 0.055 ml l −1 per month during the stagnation period 1987–1991 and the volume mean oxygen depletion rate ( DEPL) contributed about 75% to CONS. The vertical diffusive flux of oxygen ( DIFF) into the basin water then reduced the oxygen depletion rate with about 25%. The total supply of particulate carbon below the 350 m level was estimated to about 430 000 ton y −1 (43 ton km −2 y −1), the total nitrogen supply about 80 000 ton y −1 and phosphorus 11 000 ton y −1. Assuming that the organic matter is composed according to Redfield ratios, we also found that a substantial denitrification process must take place in the bottom sediments (30–50%). The main source of organic matter to the Skagerrak Basin Water seems to be the North Sea.

Arctic sediments (Svalbard): consumption and microdistribution of oxygen

Total sediment oxygen consumption rates (TSOC or J tot), measured during sediment-water incubations, and sediment oxygen microdistributions were studied at 16 stations in the Arctic Ocean (Svalbard area). The oxygen consumption rates ranged between 1.85 and 11.2 mmol m −2 d −1, and oxygen penetrated from 5.0 to ⩾ 59 mm into the investigated sediments. Measured TSOC exceeded the calculated diffusive oxygen fluxes ( J diff) by 1.1–4.8 times. Diffusive fluxes across the sediment-water interface were calculated using the whole measured microprofiles, rather than the linear oxygen gradient in the top sediment layer. The lack of a significant correlation between found abundances of bioirrigating meiofauna and high J tot J diff ratios as well as minor discrepancies in measured TSOC between replicate sediment cores, suggest molecular diffusion, not bioirrigation, to be the most important transport mechanism for oxygen across the sediment-water interface and within these sediments. The high ratios of J tot J diff obtained for some stations were therefore suggested to be caused by topographic factors, i.e. underestimation of the actual sediment surface area when one-dimensional diffusive fluxes were calculated, or sampling artifacts during core recovery from great water depths. Measured TSOC correlated to water depth raised to the −0.4 to −0.5 power (TSOC = water depth −0.4 to −0.5) for all investigated stations, but they could be divided into two groups representing different geographical areas with different sediment oxygen consumption characteristics. The differences in TSOC between the two areas were suggested to reflect hydrographic factors (such as ice coverage and import/production of reactive particulate organic material) related to the dominating water mass (Atlantic or polar) in each of the two areas. The good correlation between TSOC and water depth −0.4 to −0.5 rules out any of the stations investigated to be topographic depressions with pronounced enhanced sediment oxygen consumption.

Oxygen exchange in the isolated, arrested guinea pig heart: theoretical and experimental observations

A model of oxygen transport in perfused myocardial tissue is presented. Steady-state conditions are assumed in order to mimic the metabolic rate of the arrested heart. The model incorporates Michaelis-Menten dependence of mitochondrial oxygen consumption, oxymyoglobin saturation and oxyhemoglobin saturation on oxygen partial pressure (PO2). The transport equations model both the advective supply of oxygen via the coronary circulation and the diffusive exchange of oxygen between tissues and environment across the epicardial and endocardial surfaces. The left ventricle is approximated by an axisymmetric prolate spheroid and the transport equations solved numerically using finite element techniques. Solution yields the PO2 profile across the heart wall. Integration of this profile yields the simulated rate of metabolic oxygen uptake determined according to the Fick principle. Correction for the diffusive flux of oxygen across the surfaces yields the simulated true metabolic rate of oxygen consumption. Simulated values of oxygen uptake are compared with those measured experimentally according to the Fick principle, using saline-perfused, Langendorff-circulated, K(+)-arrested, guinea pig hearts. Four perfusion variables were manipulated: arterial PO2, environmental PO2, coronary flow and perfusion pressure. In each case agreement between simulated and experimentally determined rates of oxygen consumption gives confidence that the model adequately describes the advective and diffusive transport of oxygen in the isolated, arrested, saline-perfused heart.

Relation between sediment-water fluxes of oxygen and silicate and faunal abundance at continental shelf, slope and deep-water stations in the northwest Mediterranean

Measurements of sediment oxygen demand (SOD), silicate fluxes, benthic macroand meiofauna and phytopigments were made on continental shelf, slope and deep-water sediments in the northwest Meaiterranean in November-December 1991. The mean SOD (f SD), measured by onboard incubations of sediment cores with overlying water, was 301 + 83 pm01 m-' h-' for the shelf stations, 146 + 53 pm01 m-2 h-' for the slope stations, and 44 f 10 pm01 m-2 h-' for the deep-water stations. The SOD values for the slope and deep-water stations are comparable to earlier reported data from open oceanic sites. Considering the limited data set that we present, substantial downslope transport of organic matter at these stations cannot be concluded. Mean shcate effluxes from the sediment, measured in the same incubation experiments, amounted to 52 ? 14, 48 f 20 and 16 pm01 m-' h-' for shelf, slope and deep-water stations, respectively. Diffusive oxygen and silicate fluxes were calculated from pore-water profiles with Fick's first law of diffusion The irngatlon flux was defined as the difference between incubation and diffusive flux. The oxygen irngation flux contributed on average 39 ? 10%. 17 f 15% and 5 + 5% to the oxygen incubation flux for the shelf. slope and deep-water stahons. respectively, and showed a strong positive relation with macrofaunal density, illustrating the importance of macrofaunal activities on sediment-water solute exchanges. In contrast to the oxygen irrigation fluxes, the silicate irrigation fluxes showed less distinction between shelf and slope stations and contributed on average 60 + 6 %, 61 + 9 % and 19 % to the silicate incubation fluxes for the shelf, slope and deep-water stations, respectively. The relation between silicate irrigation flux and macrofaunal density was not significant. Another indication for the lack of organic matter transport across the slope was the monotonic decrease of faunal density, biomass and phytopigments w t h increasing depth. Only at the upper slope station were traces of a past deposition event found. Below this depth phytopigments were below detection level. The absence of phaeopigments in the near-bottom water and their high concentration in the shelf sediments indicated little resuspension of sediment.

Increased capillary segment length in cerebral cortical microvessels of rats exposed to 3 weeks of hypobaric hypoxia.

The common mammalian physiological response to moderate hypoxia, such as experienced with exposure to altitude up to about 5000m, initially includes increased ventilation and increased cerebral blood flow, followed by elevated hemoglobin concentration (Dempsey and Forster, 1982). We have previously shown that rats follow this usual pattern (Shockley and LaManna, 1988; LaManna et al., 1984; LaManna et al., 1989; LaManna et al., 1992; LaManna, in press). Rats adapt to moderate hypoxia by increased ventilation, increased blood hemoglobin and failure to gain weight (LaManna et al., 1992). For the brain, the most dramatic response to continued hypoxia was an increase in capillary density, resulting in decreased intercapillary distance achieved through angiogenesis that allows brain tissue oxygen diffusion flux to remain adequate. This structural plasticity is accompanied by functional changes as indicated by the increased glucose transporter density observed in the hypoxic adapted rat brain (Harik et al., 1991). The purpose of this present study was to begin to characterize any changes in the geometry of the microvascular capillary network resulting from hypoxic induced angiogenesis.KeywordsCerebral Blood FlowSegment LengthCapillary DensityHypobaric HypoxiaHypoxic BrainThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Direct measurements of oxygen microprofiles and distribution of phospholipid-P in a two-phase soil—manure system

Concentrations of phospholipid-P (Plip-P) were used as an estimate of biomass in soil. Plip-P was correlated with biomass-C, as determined by chloroform fumigation-direct extraction, in soil from a fertilizer experiment, although a background of non-biomass Plip-P was suggested. The distribution of Plip-P was followed during three weeks in a two-phase model system designed to study microbial processes in and around a manure-saturated zone. Concentrations of Plip-P in the soil were unaffected by the presence of manure at ⩾ 4 mm distance from the soil-manure interface. A sharp gradient between manure and soil phases was maintained throughout the three weeks. Oxygen microelectrodes (i.d. ca. 5 μm, o.d. 150–200 μm) were used for direct measurements of oxygen penetration into the manure-saturated zone. No decrease in oxygen concentration was recorded in the soil phase. By Day 1 oxygen penetrated only 0.15 mm into the manure, increasing to 2-2.5 mm after 21 days. During this period the diffusive flux of oxygen decreased five-fold. Aerobic decomposition of dissolved C in the manure was apparently restricted to a thin layer of 0–2 mm thickness below the soil-manure interface.

Calculation of oxygen diffusion across the surface of isolated perfused hearts.

Although exact mathematical descriptions of oxygen diffusion into unperfused isolated organs are known, no analytic solution is available for perfused organs. Here, we derive an equation for oxygen diffusion across the epicardial surface of perfused hearts. Our oxygen transport model incorporates oxygen delivery from the capillary perfusate, oxygen consumption, diffusion among adjacent capillary exchange regions, and diffusion across the epicardial surface. The wall of the heart is modeled by a slab of tissue. The derived equation fits experimental data on the diffusive oxygen flux across the surface of isolated saline-perfused guinea pig hearts, obtained by measuring oxygen uptake from the perfusate at various oxygen tensions in the surroundings of the isolated heart. The model predicts that in isolated arrested guinea pig hearts suspended in air, 25% of the oxygen taken up from the saline perfusate (PO2 approximately 680 mmHg) diffuses out of the heart across the surface. In beating isolated guinea pig hearts, 5% of the oxygen taken up from the perfusate diffuses across the surface.

Direct measurement of the diffusive sublayer at the deep sea floor using oxygen microelectrodes

THE diffusive sublayer is the region of fluid next to a solid surface, where turbulence is suppressed and molecular diffusion dominates transport of solutes. Diffusive impedance of solute exchange across the benthic sublayer in the deep sea can limit the rates of some diagenetic reactions in the sediment. We present the first direct in situ measurements of the thickness of the diffusive sublayer for dissolved oxygen in the deep sea. The positions of 17 oxygen microelectrode profiles relative to the visible sediment/water inter-face reveal that the sublayer is 0.5–1.5 mm thick, with measurements ranging to 3.5 mm. The sublayer reduces the diffusive flux of oxygen into the sediments by ∼10% in these environments. Also, the diffusive flux of isotopically light carbon through the sublayer should cause the 13C content at the interface of typical deep-sea sediments to be ∼0.1%> lighter than the bottom-water value, setting a limit on the precision of the record of past bottom-water given by the carbon isotope composition of the shells of benthic foraminifera.

Oxygen microprofiles of trickling filter biofilms

Oxygen microprofiles of photosynthetic and non-photosynthetic biofilms of trickling filters from a sewage treatment plant were studied with Clark type oxygen microelectrodes. The oxygen profiles in photosynthetic biofilms exhibited pronounced changes with changing light conditions, and the profiles of both types of biofilm were affected by addition of nutrients. The existence of a 100–500 μm thick diffusive boundary layer in the water just above the biofilm was evident from all recordings. The O 2 gradient in this boundary layer was used to calculate diffusive fluxes of oxygen exchanged between the biofilms and the overlying water phase. The calculated fluxes were compared to the overall oxygen consumption rates measured experimentally and to the photosynthetic activity as measured with the microelectrodes. Approximately 60–70% of the oxygen produced in the algal films during photosynthesis was consumed within the films. At least one third of this consumption may be due to photorespiration. Dark/light and light/dark shifts demonstrated a very dynamic nature of the O 2 status of the algal films, showing an increase from anaerobic conditions to 500% air saturation and vice versa within 25 min. In the same regime, pH profiles showed a similar dynamic change, whereby pH varied between 8.1 and 9.7 in the same period. The data obtained with the non-photosynthetic biofilms show that the oxygen respiration and oxygen penetration are mostly limited by diffusive oxygen transport through the boundary layer. The overall oxygen consumption of the heterotrophic biofilms equalled within ± 10% the estimates made from flux calculations.

Oxygen microprofiles measured in situ in deep ocean sediments

Diffusive fluxes of oxygen into sediments cannot be calculated reliably without accurate fine-scale concentration measurements very near an undisturbed sediment–water interface. For deep-sea sediments this is difficult, and thus few data on seafloor oxygen gradients have been reported1–4. We present here bottom-water/pore-water oxygen microprofiles that were measured in situ with microelectrodes and a free-vehicle microprofiling instrument at ocean depths of 1,275 and 3,790 m. The vertical distribution of oxygen in the upper few centimetres of sediment reflects variations in bottom-water oxygen concentration, oxygen demand and biologically produced small-scale (millimetre to centimetres) sediment heterogeneity and microtopography. Oxygen microprofiles measured on board ship in box-cored sediments from the same sites have features that are similar to the profiles measured in situ except within the first 2–4 mm below the sediment surface. This difference is probably caused by changes in boundary conditions and disturbance of the uppermost sediment during coring and sample retrieval.

Myoglobin diffusion in bovine heart muscle.

The rotational mobility of myoglobin in situ was determined by proton nuclear magnetic resonance line width measurements of a characteristic myoglobin resonance observed in bovine heart muscle spectra. The protein diffuses intracellularly at nearly half the rate observed in dilute solution. This high mobility allows the oxygenated form of myoglobin to contribute significantly to the overall diffusive flux of oxygen in respiring heart muscle.

SOME OBSERVATIONS ON OXYGEN TRANSPORT IN CONIFER SEEDLINGS

SummaryThe diffusive flux of oxygen from the roots of intact conifer seedlings was measured by a polarographic technique. With an anaerobic solution bathing the roots maintained at 23° C and with their aerial parts in air, flux rates were appreciable from eight of the twenty‐five seedlings tested. When the aerial parts were placed in oxygen, there was a large increase in flux from each pine seedling. While their general response was barely measurable, a significant but much smaller increase occurred from eight of the spruce. Oxygen transport through the plants appeared to be by diffusion chiefly in the gas phase and significant flux rates from the roots with aerial parts in air might indicate some capacity for wet soil tolerance in the seedlings concerned.When the external concentration of oxygen was increased five‐fold there was a disproportionate increase in oxygen flux from pine roots indicating that respiration in the plants might be affecting the flux rates. By cooling the bathing medium to 30 C a further demonstration of the respiratory effect‐a large increase in flux rate‐was obtained. With respiration reduced by cooling, the flux rates from the pine seedlings increased approximately five‐fold for a five‐fold increase in the external oxygen concentration. Cooling did not induce any pronounced response from the spruce seedlings.Internal diffusional resistance and oxygen leakage from the sub‐apical parts of the roots may account for the absence of high apical flux rates in the pines and the extremely low readings recorded for the spruce seedlings.

Analysis of oxygen diffusion to a measuring probe in animal tissue

A characterization of the diffusive flux of oxygen to a measuring device implanted in animal tissue is presented. Cardiovascular and metabolic activity in the tissue is accounted for by insertion of supply and consumption terms into the equations of change for the system. Reasonable values of the system parameters are used to determine expected responses. Considerable error in the determination of oxygen partial pressure with open and high permeability probes is predicted if supply and consumption are ignored. The effects of various parameters on the predicted probe response are discussed.

FACILITATED DIFFUSION OF OXYGEN THROUGH HAEMERYTHRIN SOLUTIONS.

HAEMOGLOBIN present in water solution facilitates the steady-state diffusion of oxygen through that solution1,2; the total oxygen flux is greater than the flux through an equivalent protein solution, for example, a solution of methaemoglobin. The haemoglobin-mediated oxygen flux may be considered to be additive to the simple diffusion flux of oxygen. Fig. 1, which although obtained with the protein haemerythrin is entirely typical of results obtained with haemoglobin, reveals that the flux mediated by an oxygen-binding protein is constant in amount for a particular protein concentration and independent of the oxygen partial pressure difference across the solution at oxygen partial pressures sufficient to saturate largely the carrier. This property has the interesting consequence that at the low oxygen partial pressure presumably existing in tissue the haemoglobin mediated flux would be expected to make up the largest part of the total oxygen flux. Quite possibly the facilitation of oxygen diffusion in the interior of muscle cells is a major function of myoglobin.