Critical Oxygen Partial Pressure Research Articles

Adjusting metabolic rates and critical oxygen tension in planktonic copepods under increasing hypoxia in highly productive coastal upwelling zones

AbstractOngoing ocean deoxygenation is threatening marine organisms globally. In eastern boundary upwelling systems, planktonic copepods dominate the epipelagic zooplankton, being crucial in the marine food web. Yet, they must cope with severe hypoxia caused by shoaling of the oxygen minimum zone. Based on laboratory experiments during 2021, we found differential responses in the metabolic rate (MR) and critical oxygen partial pressure of three abundant copepods. Calanoides patagoniensis doubled its MR during the upwelling season, so better exploiting the spring phytoplankton bloom for feeding and reproduction while maintaining their critical oxygen partial pressure unchanged between seasons. Contrastingly, Paracalanus cf. indicus and Acartia tonsa, maintained their MRs throughout seasons, but significantly increased their critical oxygen partial pressure during the upwelling period, becoming less tolerant to hypoxia. Field observations showed that oxygen levels equal to or lower than the critical oxygen partial pressure is a common condition (70% of occurrence) that copepods encounter during the year in the upper 50 m layer. These findings suggest a species‐dependent trade‐off between the MR and the critical oxygen partial pressure, where some species can maintain the latter despite fluctuations in their MR (improved hypoxia tolerance) or maintain their MR at the expenses of a larger critical oxygen partial pressure (improved energy expenditures). These adaptive responses, under oxygen levels equal to or lower than the critical oxygen partial pressure, suggest that exacerbated hypoxia, driven by ocean deoxygenation and increased upwelling, will alter copepod distribution and cause higher copepod mortality, with potentially drastic consequences for marine food webs.

[Three-dimensional analysis of posttraumatic tibial shaft malunion and correction based on the healthy, contralateral leg].

Three-dimensional (3D) analysis and implementation with patient-specific cutting and repositioning blocks enables correction of complex tibial malunions. Correction can be planned using the contralateral side or astatistical model. Patient-specific 3D-printed cutting guide blocks enable aprecise osteotomy and reduction guide blocks help to achieve anatomical reduction. Depending on the type and extent of correction, fibula osteotomy may need to be considered to achieve the desired reduction. a)Poor soft tissue (flap surgery, adherent skin in field of operation); b)infection; c)peripheral artery disease (stageIII andIV classified according to Fontaine, critical transcutaneous oxygen partial pressure, TcPO2); d)general contraindication to surgery. Before surgery, a3D model of both lower legs is created based on computed tomography (CT) scans. Analysis of the deformity based on the contralateral side in a3D computer model (CASPA) and planning of the osteotomy. If the contralateral side also has adeformity, astatistical model can be used. Printing of patient-specific guides made of nylon (PA2200) for the osteotomy and reduction. Surgery is performed in supine position, antibiotic prophylaxis, thigh tourniquet, which is used as needed. Ventrolateral approach to the tibia. Attachment of the patient-specific osteotomy guide, performance of the osteotomy. Reduction using the guide. Fibula osteotomy through alateral approach is performed if the reduction of the tibia is hindered by the fibula. This can be performed freehand or with patient-specific guides. Wound closure. Compartment monitoring. Passive mobilization of the ankle in the cast as soon as the wound healing has progressed. Partial weightbearing in alower leg cast for at least 6-12weeks, depending on the routinely performed radiographic assessment 6weeks postoperatively. Thromboprophylaxis with low molecular weight heparin until cast removal. Patient-specific correction of malunions are generally good. This could be confirmed for distal tibial corrections. For tibial shaft deformities, the final results are still pending. Preliminary results, however, show good feasibility with apseudarthrosis rate of 10% without postoperative infection.

Caribbean scleractinian corals exhibit highly variable tolerances to acute hypoxia

IntroductionClimate change, and the increase in sea surface temperature, is exacerbating ocean deoxygenation because of the inherent property of seawater to sequester less dissolved gas, such as oxygen, at warmer temperatures. While most coral reef studies focus on the effects of thermal stress and ocean acidification, few studies acknowledge the threat of hypoxia. Hypoxia is traditionally defined as 6.3 kPa (2 mg L-1 O2), however, a universal hypoxia threshold is not useful given the vast range of responses among marine organisms. The range of metabolic responses and tolerances to hypoxia are unknown for Caribbean coral species and their algal symbionts.ObjectiveHere, we quantified the spectrum of acute hypoxia tolerances and the range of metabolic responses of six ecologically and structurally important Caribbean coral species (Acropora cervicornis, Siderastrea radians, Siderastrea siderea, Porites astreoides, Porites porites, and Orbicella faveolata) and their algal symbionts (Symbiodinium, Breviolum, and Durusdinium spp.).MethodsA total of 24 coral fragments (4 individuals per species) were exposed to 10 distinct oxygen concentrations ranging from normoxia (20.38 kPa) to severe hypoxia (3.3 kPa). We used intermittent flow respirometry to measure coral host respiration in the dark and algal symbiont photosynthesis in the light at each oxygen level. We determined a line of best fit for the metabolic rate vs. PO2 data and calculated the critical oxygen partial pressure (PO2 crit), a method that has not been tested on symbiotic species.ResultsCoral species and their algal symbionts measured here displayed a wide range of hypoxia tolerances. For the coral hosts, PO2 crit values differed roughly two-fold ranging from 5.74 kPa to 16.93 kPa, and for the algal symbionts, PO2 crit values differed roughly three-fold ranging from 3.9 kPa to 11.3 kPa.DiscussionThese results should be regarded as a first step to characterizing the metabolic response and acute tolerance of multiple coral hosts and algal symbionts to a wide range of oxygen concentrations. Given that some PO2 crit values were above the generally accepted hypoxia threshold, these results have implications for the community composition of reefs under a rapidly changing climate and can guide purposeful reef restoration.

Oxidation thermodynamics and oxidation kinetics analysis of Au-20Sn solder

In this paper, the oxidation thermodynamics and oxidation kinetics of Au-20Sn solder at various temperatures were analyzed. The results indicated that the critical oxygen partial pressure to form SnO and SnO2 in Au-20Sn solder could be 1 × 10−44 Pascal (Pa) and 1 × 10−91 Pa at the temperature of 298 K, respectively. The oxidation kinetics analysis results indicated that the oxide on the surface of Au-20Sn solder consists mainly SnO2, while the major constituent of internal oxide is SnO. The reliability test results under salt spray environment have showed that the hermeticity of the package device gradually decreased with the increasing oxygen content in Au-20Sn solder. Meanwhile, the measured leakage rate of the packaging device could be lowered than 5 × 10−9 Pa·m–3·s–1 when the oxygen content in Au-20Sn solder is less than 28 ppm after high temperature aging.

Bathyal octopus, Muusoctopus leioderma, living in a world of acid: First recordings of routine metabolic rate and critical oxygen partial pressures of a deep water species under elevated pCO2.

Elevated atmospheric CO2 as a result of human activity is dissolving into the world's oceans, driving a drop in pH, and making them more acidic. Here we present the first data on the impacts of ocean acidification on a bathyal species of octopus Muusoctopus leioderma. A recent discovery of a shallow living population in the Salish Sea, Washington United States allowed collection via SCUBA and maintenance in the lab. We exposed individual Muusoctopus leioderma to elevated CO2 pressure (pCO2) for 1day and 7days, measuring their routine metabolic rate (RMR), critical partial pressure (P crit ), and oxygen supply capacity (α). At the time of this writing, we believe this is the first aerobic metabolic data recorded for a member of Muusoctopus. Our results showed that there was no change in either RMR, P crit or α at 1800µatm compared to the 1,000µatm of the habitat where this population was collected. The ability to maintain aerobic physiology at these relatively high levels is discussed and considered against phylogeny and life history.

Coming up for air

The recent paper in Journal of Experimental Biology by Seibel et al. (2021) claims that a new term, the oxygen supply capacity (α), breathes new life into critical oxygen partial pressure (Pcrit). Despite the authors' good intentions, their idea does not resuscitate the 88 year old concept of Pcrit (Tang, 1933). It would be wonderful if the α-line were prescriptive and biologically meaningful across species and environmental conditions, but we suspect that it is not. We argue for what the data have told us for years: the relationship between any metabolic rate (MR) and PO2 is not adequately described by straight lines.We raise several problems with their overly simple linear model: maximum metabolic rate (MMR)=PO2×α. This linear line is anchored at the origin, must pass through any critical PO2 (Pcrit), has a single α value and passes through MMR.While the authors claim the α-line can ‘predict the maximum achievable MR at any PO2’, this claim is misleading for three reasons. Foremost, the indefinitely long line cannot predict the maximum aerobic MR (i.e. O2,max) of the animal without knowing its metabolic scope. Second, the line is based on only one experimental data point (Pcrit) that has associated error. If Pcrit is based on standard metabolic rate (SMR) data, the point lies near the origin and therefore has a large leverage effect on the slope of the line. Small absolute errors in its position can create large absolute errors in MMR, in proportion to the animal's metabolic scope. Finally, real data for several species raise doubts that the line is straight.Fry and Hart (1948) had a clear conceptual framework for investigating hypoxia tolerance in fish: ‘… the simplest and most direct methods of obtaining ecologically significant values of oxygen uptake are to measure the maximum rate over a series of varying oxygen tensions down to the asphyxia level, and to measure the standard rate over a range of tensions at which oxygen would not be the limiting factor even for the maximum rate’. This empirical approach is what Fred Fry envisaged for his ‘Limiting Oxygen Concentration’ (LOC) curve (Fry, 1947). In fact, reliable empirical data for MMR versus PO2 with groups of fish exist (Claireaux and Lagardère, 1999; Claireaux et al., 2000) and these LOC curves, based on best-fit statistical relationships, were non-linear. Still needed are empirical data using single fish.Unfortunately, the ‘oxygen supply’ concept (α0) is not new, other than its name. Its units are identical to those used for ‘conductance’, a term defined and long-used by respiratory physiologists (Piiper et al., 1971) to represent the effectiveness of gas transport in each step down the oxygen cascade. ‘Oxygen supply capacity’ (α) would be the maximum conductance of the entire system from the environment to the mitochondrion, but its use places all levels of oxygen cascade into a black box, where we lose sight of what determines conductance at each level. Maximum conductance is influenced by individual conductances at all levels of the oxygen cascade, from ventilation of the gas exchange surface, to diffusion through the surface, oxygen binding to respiratory proteins and dissolved in plasma, convection by the cardiovascular system and diffusion through tissues to mitochondria where oxygen is finally consumed. Fundamentally, most of these conductances are non-linear with respect to PO2. Oxygen binding in blood, for example, is sigmoidal or hyperbolic, and mitochondrial kinetics are hyperbolic. Furthermore, the tissues supported at SMR (oxygen-sensitive internal organs) are different from those supported at MMR (aerobic red muscle), so the ‘anatomy’ of the oxygen cascade changes between rest and activity. Thus, we are not convinced that the mechanisms in the oxygen cascade remain identical at Pcrit-SMR and Pcrit-MMR, an apparent requirement of the theory.Sadly, Seibel et al. (2021) do not address the very reasonable criticisms of Pcrit theory and methods (e.g. Wood, 2018; Cobbs and Alexander, 2018; Marshall et al., 2013). Also, they discourage examining data that depart from their model: ‘Using the α-method, MR measurements below Pcrit-SMR are not diagnostic and, thus, are not relevant’. They dismiss attempts of other approaches to understand responses as ‘purely descriptive’ and imply that approaches to understand the entire MR versus PO2 response curve ‘may not have any functional relationship to available oxygen’. In particular, the regulation index (RI), a metric that evaluates the empirical ability of animals to regulate MR in relation to PO2 (Mueller and Seymour, 2011), is dismissed as being devoid of underlying physiological mechanisms. Having discarded a need to consider the entire responses, they fail to recognize that their measure of Pcrit-SMR may not actually relate to SMR. For example, some species show a progressive MR decrease at high PO2 (e.g. Fig. 2B of Seibel et al., 2021, showing data from Mueller and Seymour, 2011), so the MR at Pcrit-SMR would appear lower than SMR. The whole response could represent metabolic suppression in response to hypoxia. In fact, well laid out and rigorously tested methodological and analytical procedures demonstrate the real possibility of metabolic suppression and show that Pcrit-SMR can be interpolated from empirical data for MR versus PO2 (Chabot et al., 2016, 2021; Claireaux and Chabot, 2016) rather than by selecting three points with the highest α values. Absent from Seibel et al. (2021) is the crucial validation that estimated Pcrit values do indeed match those measured using reliable SMR determinations.In conclusion, we feel Seibel et al. (2021) presented a theory trying to enforce itself onto real data. They cite Fry and Hart (1948) (‘the worth of such data to the ecologist must ultimately depend on proof that they have real significance as values limiting the activity of the organism in nature’) to justify their theory, but the equation of Seibel et al. (2021) lacks empirical realism. Indeed, forcing an equation relating MMR and PO2 through the origin does not represent empirical data (Claireaux and Lagardère, 1999; Claireaux et al., 2000) in a biologically meaningful manner. Still, you can always draw a straight line through two data points, in this case a biologically meaningless intercept at the origin and a potentially subjective Pcrit-SMR. Besides, MMR cannot be derived from such a line without prior knowledge of metabolic scope in normoxia.

Oxygen supply capacity breathes new life into critical oxygen partial pressure (Pcrit).

The critical oxygen partial pressure (Pcrit), typically defined as the PO2below which an animal's metabolic rate (MR) is unsustainable, is widely interpreted as a measure of hypoxia tolerance. Here, Pcrit is defined as the PO2at which physiological oxygen supply (α0) reaches its maximum capacity (α; µmol O2 g-1h-1kPa-1). α is a species- and temperature-specific constant describing the oxygen dependency of the maximum metabolic rate (MMR=PO2×α) or, equivalently, the MR dependence of Pcrit (Pcrit=MR/α). We describe the α-method, in which the MR is monitored as oxygen declines and, for each measurement period, is divided by the corresponding PO2to provide the concurrent oxygen supply (α0=MR/PO2). The highest α0 value (or, more conservatively, the mean of the three highest values) is designated as α. The same value of α is reached at Pcrit for any MR regardless of previous or subsequent metabolic activity. The MR need not be constant (regulated), standardized or exhibit a clear breakpoint at Pcrit for accurate determination of α. The α-method has several advantages over Pcrit determination and non-linear analyses, including: (1) less ambiguity and greater accuracy, (2) fewer constraints in respirometry methodology and analysis, and (3) greater predictive power and ecological and physiological insight. Across the species evaluated here, α values are correlated with MR, but not Pcrit. Rather than an index of hypoxia tolerance, Pcrit is a reflection of α, which evolves to support maximum energy demands and aerobic scope at the prevailing temperature and oxygen level.

Effects of Different Amounts of Nb Doping on Electrical, Optical and Structural Properties in Sputtered TiO2−x Films

Highly conductive TiO2 films with different Nb doping levels (up to 5 at%) were prepared by reactive DC magnetron sputtering under precise control of the oxygen partial pressure. They were deposited on unheated substrates, covered with a protective Si3N4 layer, and subsequently annealed at 300 °C. The doping efficiency of Nb is greater than 90%. Conductivity is a maximum for a partly oxidized target in the transition range. The best films exhibit a resistivity of 630 µΩ cm and a mobility of 7.6 cm2/Vs combined with a high transparency above 70%. Comparing the behavior of undoped and Nb-containing films, intrinsic limits of the conductivity in the TiO2−x:Nb system could be observed, and a consistent model explaining these findings is presented. The conductivity is limited—by decreasing electron density due to Nb oxidation—by increasing incorporation formation of Nb2O5 clusters as scattering centers with increasing oxygen partial pressure and Nb concentration, by a transition from the crystalline to the amorphous state of the films below a critical oxygen partial pressure.

Understanding of Solid Oxide Electrolysis Cell Degradation: The Role of the Electrode Overpotential

Solid oxide electrolysis cells (SOECs) can produce hydrogen, an increasingly important energy carrier, with considerably higher efficiency than other electrolysis technologies. A critical hurdle for the market penetration of SOECs is to ensure that degradation rates are sufficiently low to allow long-term operation. Degradation of the oxygen electrode has been shown to increase with increasing current density; however, the effects of other parameters such as temperature, electrode overpotential, and electrode composition have not been fully quantified. Moreover, significant degradation within the electrolyte as well as fuel electrode is observed during electrolysis operation, but the mechanisms are still not fully understood. Here, we report an experimental life test study of both full and symmetric cells with YSZ electrolytes along with various oxygen electrode materials including Sr(Ti0.3Fe0.7)O3 (STF), Sr(Ti0.3Fe0.63Co0.07)O3, (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF), and Ni-YSZ fuel electrodes. The observed dependences on temperature, current density, and electrode composition indicate that the electrode overpotential is the factor determining the electrode/electrolyte degradation and eventual cell failure. A theory that determines the electric potentials and oxygen partial pressure across the electrolyte is used to model the results, and can determine the condition for fracture or oxygen bubble formation. For example, fracture at the oxygen-electrode/electrolyte interface is observed when the electrode overpotential exceeds ~200 mV, corresponding to a critical oxygen partial pressure of 7410 atm; the model shows that the pressure is near its peak at this interface, and is sufficient to cause fracture because of the relatively low fracture toughness of the perovskite electrode.

Ocean acidification does not limit squid metabolism via blood oxygen supply.

Ocean acidification is hypothesized to limit the performance of squid owing to their exceptional oxygen demand and pH sensitivity of blood-oxygen binding, which may reduce oxygen supply in acidified waters. The critical oxygen partial pressure (Pcrit), the PO2 below which oxygen supply cannot match basal demand, is a commonly reported index of hypoxia tolerance. Any CO2-induced reduction in oxygen supply should be apparent as an increase in Pcrit In this study, we assessed the effects of CO2 (46-143 Pa; 455-1410 μatm) on the metabolic rate and Pcrit of two squid species - Dosidicus gigas and Doryteuthis pealeii - through manipulative experiments. We also developed a model, with inputs for hemocyanin pH sensitivity, blood PCO2 and buffering capacity, that simulates blood oxygen supply under varying seawater CO2 partial pressures. We compare model outputs with measured Pcrit in squid. Using blood-O2 parameters from the literature for model inputs, we estimated that, in the absence of blood acid-base regulation, an increase in seawater PCO2 to 100 Pa (≈1000 μatm) would result in a maximum drop in arterial hemocyanin-O2 saturation by 1.6% at normoxia and a Pcrit increase of ≈0.5 kPa. Our live-animal experiments support this supposition, as CO2 had no effect on measured metabolic rate or Pcrit in either squid species.

Cold tolerance is unaffected by oxygen availability despite changes in anaerobic metabolism

Insect cold tolerance depends on their ability to withstand or repair perturbations in cellular homeostasis caused by low temperature stress. Decreased oxygen availability (hypoxia) can interact with low temperature tolerance, often improving insect survival. One mechanism proposed for such responses is that whole-animal cold tolerance is set by a transition to anaerobic metabolism. Here, we provide a test of this hypothesis in an insect model system (Thaumatotibia leucotreta) by experimental manipulation of oxygen availability while measuring metabolic rate, critical thermal minimum (CTmin), supercooling point and changes in 43 metabolites in moth larvae at three key timepoints (before, during and after chill coma). Furthermore, we determined the critical oxygen partial pressure below which metabolic rate was suppressed (c. 4.5 kPa). Results showed that altering oxygen availability did not affect (non-lethal) CTmin nor (lethal) supercooling point. Metabolomic profiling revealed the upregulation of anaerobic metabolites and alterations in concentrations of citric acid cycle intermediates during and after chill coma exposure. Hypoxia exacerbated the anaerobic metabolite responses induced by low temperatures. These results suggest that cold tolerance of T. leucotreta larvae is not set by oxygen limitation, and that anaerobic metabolism in these larvae may contribute to their ability to survive in necrotic fruit.

Hypoxia Tolerance and Metabolic Suppression in Oxygen Minimum Zone Euphausiids: Implications for Ocean Deoxygenation and Biogeochemical Cycles.

The effects of regional variations in oxygen and temperature levels with depth were assessed for the metabolism and hypoxia tolerance of dominant euphausiid species. The physiological strategies employed by these species facilitate prediction of changing vertical distributions with expanding oxygen minimum zones and inform estimates of the contribution of vertically migrating species to biogeochemical cycles. The migrating species from the Eastern Tropical Pacific (ETP), Euphausia eximia and Nematoscelis gracilis, tolerate a Partial Pressure (PO2) of 0.8 kPa at 10 °C (∼15 µM O2) for at least 12 h without mortality, while the California Current species, Nematoscelis difficilis, is incapable of surviving even 2.4 kPa PO2 (∼32 µM O2) for more than 3 h at that temperature. Euphausia diomedeae from the Red Sea migrates into an intermediate oxygen minimum zone, but one in which the temperature at depth remains near 22 °C. Euphausia diomedeae survived 1.6 kPa PO2 (∼22 µM O2) at 22 °C for the duration of six hour respiration experiments. Critical oxygen partial pressures were estimated for each species, and, for E. eximia, measured via oxygen consumption (2.1 kPa, 10 °C, n = 2) and lactate accumulation (1.1 kPa, 10 °C). A primary mechanism facilitating low oxygen tolerance is an ability to dramatically reduce energy expenditure during daytime forays into low oxygen waters. The ETP and Red Sea species reduced aerobic metabolism by more than 50% during exposure to hypoxia. Anaerobic glycolytic energy production, as indicated by whole-animal lactate accumulation, contributed only modestly to the energy deficit. Thus, the total metabolic rate was suppressed by ∼49-64%. Metabolic suppression during diel migrations to depth reduces the metabolic contribution of these species to vertical carbon and nitrogen flux (i.e., the biological pump) by an equivalent amount. Growing evidence suggests that metabolic suppression is a widespread strategy among migrating zooplankton in oxygen minimum zones and may have important implications for the economy and ecology of the oceans. The interacting effects of oxygen and temperature on the metabolism of oceanic species facilitate predictions of changing vertical distribution with climate change.

Aerobic scope and oxygen regulation of New Zealand geoduck (Panopea zelandica) in response to progressive hypoxia

Recent efforts to cultivate subtidal New Zealand geoducks (Panopea zelandica Quoy and Gaimard, 1835) in intertidal areas have been challenged due to daily hypoxia. Hence, further understanding of physiological responses of geoducks of different sizes under different oxygen partial pressures (PO2) is required to optimize their husbandry. Without this knowledge, the risk of increasing cultivation costs due to growth depression and mortality may increase in both magnitude and frequency. Hence, this study investigated the respiration, aerobic scope, critical oxygen partial pressure (PcO2), and oxygen regulation capacity from normoxia to hypoxia in two size classes of fed and starved P. zelandica. The PcO2 was determined to be ~4kPa for all geoduck groups. The respiration rates of fed small geoducks decreased significantly from normoxia (16.7–~21kPa) to mild hypoxia (PcO2–16.7kPa). Conversely, the respiration rates of starved geoducks from both size classes, and large fed geoducks were maintained at a constant level when exposed to the same change in oxygen concentration. However, all geoducks experienced decreased respiration rates during severe hypoxia (0kPa–PcO2). In addition, overall oxyregulatory capacity, assessed using a regulation index, was affected by size rather than by nutritional stress. Large geoducks maintain oxygen consumption across an oxygen gradient more effectively than small geoducks. Also, the aerobic scope of small geoducks decreased significantly with declining PO2, while large geoducks maintained their aerobic scope under hypoxia. Hence, to avoid stress and maximize growth potential of cultivated geoducks, smaller individuals should be grown in areas that remain normoxic, whereas larger individuals may be transferred to on-growing sites that experience periodic hypoxia, potentially including tidal emersion. Statement of relevanceThe New Zealand geoduck clam (Panopea zelandica) has been identified as an emerging aquaculture species. However, efforts to cultivate geoducks intertidally have been challenged due to daily hypoxia. Hence, this study reports on physiological responses (respiration, aerobic scope, critical oxygen partial pressure, and oxygen regulation capacity) of geoducks of different sizes under different oxygen partial pressures to optimize their husbandry.

Oxygen safety margins set thermal limits in an insect model system.

A mismatch between oxygen availability and metabolic demand may constrain thermal tolerance. While considerable support for this idea has been found in marine organisms, results from insects are equivocal and raise the possibility that mode of gas exchange, oxygen safety margins and the physico-chemical properties of the gas medium influence heat tolerance estimates. Here, we examined critical thermal maximum (CTmax) and aerobic scope under altered oxygen supply and in two life stages that varied in metabolic demand in Bombyx mori (Lepidoptera: Bombycidae). We also systematically examined the influence of changes in gas properties on CTmax. Larvae have a lower oxygen safety margin (higher critical oxygen partial pressure at which metabolism is suppressed relative to metabolic demand) and significantly higher CTmax under normoxia than pupae (53°C vs 50°C). Larvae, but not pupae, were oxygen limited with hypoxia (2.5 kPa) decreasing CTmax significantly from 53 to 51°C. Humidifying hypoxic air relieved the oxygen limitation effect on CTmax in larvae, whereas variation in other gas properties did not affect CTmax. Our data suggest that oxygen safety margins set thermal limits in air-breathing invertebrates and the magnitude of this effect potentially reconciles differences in oxygen limitation effects on thermal tolerance found among diverse taxa to date.

Environmental Physiology of the Jumbo Squid,Dosidicus gigas(d'Orbigny, 1835) (Cephalopoda: Ommastrephidae): Implications for Changing Climate*

Dosidicus gigas (d'Orbigny, 1835) is a large, active squid that undergoes a diel vertical migration in the Eastern Tropical and Temperate Pacific. It is a voracious predator on zooplankton and micronekton and supports a large fishery. It is further preyed upon by large vertebrate predators, including whales. Its horizontal distribution is closely tied to productive upwelling regions that are characterized by strong oxygen minimum zones (OMZs). The apparent association with extreme hypoxia is surprising given its large size and high oxygen demand. As part of its daily vertical migration, D. gigas experiences daily temperature changes of 15–20°C, oxygen partial pressures ranging from near anoxia (< 0.8 kPa) to air-saturation (21 kPa) and pH changes from ∼8.1 to < 7.6 at depth. Oxygen minimum zones are believed to be expanding due to climate change, with minimum oxygen levels in the core of the OMZ declining and the low oxygen horizon shoaling. Simultaneously, surface waters are becoming more acidic and temperatures are rising. Here I review the extensive studies of this species that have been conducted over the past decade. D. gigas has a high affinity respiratory protein in the blood that supports a low critical oxygen partial pressure (3.8 kPa at 20 °C) and aerobic survival at night in the upper 200 meters of the water column. A pronounced pH- and temperature sensitivity of oxygen binding promotes oxygen transport across a depth range and in support of high rates of oxygen utilization but may impose constraints on high-temperature and CO2 tolerance. At its deeper, colder daytime habitat depth, D. gigas undergoes a pronounced metabolic suppression. Reduced activity levels and an apparent suspension of transcription and translation contribute to a ∼80% reduction in oxygen demand under 1% oxygen (0.8 kPa at 10 °C). Anaerobic metabolic pathways contribute some energy under these conditions. This metabolic suppression likely limits feeding at depth. Sub-critical oxygen levels, rather than temperature, predator avoidance or prey availability, appear to set the daytime depth distribution. Thus, expanding oxygen minimum zones will alter the daytime depth of peak abundance for these squids while ocean acidification and warming may impose a shallow ceiling above which squid performance is limited. The role of climate change in setting the vertical and horizontal distribution of the species is discussed.

Ecophysiology of native and alien-invasive clams in an ocean warming context

Both climate change and biological invasions are among the most serious global environmental threats. Yet mechanisms underlying these eventual interactions remain unclear. The aim of this study was to undertake a comprehensive examination of the physiological and biochemical responses of native (Ruditapes decussatus) and alien-invasive (Ruditapes philippinarum) clams to environmental warming. We evaluated thermal tolerance limits (CTMax), routine metabolic rates (RMRs) and respective thermal sensitivity (Q10 values), critical oxygen partial pressure (Pcrit), heat shock response (HSP70/HSC70 levels), lipid peroxidation (MDA build-up) and antioxidant enzyme [glutathione-S-transferase (GST), catalase (CAT) and superoxide dismutase (SOD)] activities. Contrary to most studies that show that invasive species have a higher thermal tolerance than native congeners, here we revealed that the alien-invasive and native species had similar CTMax values. However, warming had a stronger effect on metabolism and oxidative status of the native R. decussatus, as indicated by the higher RMRs and HSP70/HSC70 and MDA levels, as well as GST, CAT and SOD activities. Moreover, we argue that the alien-invasive clams, instead of up-regulating energetically expensive cellular responses, have evolved a less demanding strategy to cope with short-term environmental (oxidative) stress-pervasive valve closure. Although efficient during stressful short-term periods to ensure isolation and guarantee longer survival, such adaptive behavioural strategy entails metabolic arrest (and the enhancement of anaerobic pathways), which to some extent will not be advantageous under the chronically warming conditions predicted in the future.

The jumbo squid, Dosidicus gigas (Ommastrephidae), living in oxygen minimum zones I: Oxygen consumption rates and critical oxygen partial pressures

Dosidicus gigas is a large, metabolically active, epipelagic squid known to undertake diel vertical migrations across a large temperature and oxygen gradient in the Eastern Pacific. Hypoxia is known to cause metabolic suppression in D. gigas. However, the precise oxygen level at which metabolic suppression sets in is unknown. Here we describe a novel ship-board swim tunnel respirometer that was used to measure metabolic rates and critical oxygen partial pressures (Pcrit) for adult squids (2–7kg). Metabolic rate measurements were validated by comparison to the activity of the Krebs cycle enzyme, citrate synthase, in mantle muscle tissue (2–17kg). We recorded a mean routine metabolic rate of 5.91μmolg−1h−1 at 10°C and 12.62μmolg−1h−1 at 20°C. A temperature coefficient, Q10, of 2.1 was calculated. D. gigas had Pcrits of 1.6 and 3.8kPa at 10 and 20°C, respectively. Oxygen consumption rate (MO2) varied with body mass (M) according to MO2=11.57M−0.12±0.03 at 10°C. Citrate synthase activity varied with body mass according to Y=9.32M−0.19±0.02.

Body size is not critical for criticalPO2 in scarabaeid and tenebrionid beetles

Constraints on oxygen delivery potentially limit animal body size. Because diffusion rates are highly distance dependent, and because tracheal length increases with size, gas exchange was traditionally thought to be more difficult for larger insects. As yet the effect of body size on critical oxygen partial pressure (P(crit)) has not been measured for any clade of insect species for which there are interspecific data on tracheal scaling. We addressed this deficiency by measuring P(crit) over a 4150-fold mass range (ratio of largest to smallest species mean) of two families of Coleoptera (Tenebrionidae and Scarabaeidae). We exposed adult beetles to progressively lower oxygen levels and measured their ability to maintain CO(2) release rates. Absolute metabolic rates increased hypometrically with beetle body mass (M) at both normoxic (M(0.748)) and hypoxic (M(0.846)) conditions. P(crit), however, was independent of body size. Maximum overall conductances for oxygen from air to mitochondria (G(O(2),max)) matched metabolic rates as insects became larger, likely enabling the similar P(crit) values observed in large and small beetles. These data suggest that current atmospheric oxygen levels do not limit body size of insects because of limitations on gas exchange. However, increasing relative investment in the tracheal system in larger insects may produce trade-offs or meet spatial limits that constrain insect size.

Low O2 avoidance is associated with physiological perturbation but not exhaustion in the snapper (Pagrus auratus: Sparidae)

It is already known that the New Zealand snapper (Pagrus auratus, Sparidae) does not avoid hypoxia until reaching an oxygen partial pressure (PO2) of 3.1±1.2kPa at 18°C. Avoidance at this level of PO2 and temperature is below the critical oxygen partial pressure of the species (Pcrit=5.8±0.6kPa, 43.5±4.5mmHg) and is therefore expected to result in major physiological stress. Results from the current study showed that avoidance was associated with numerous physiological perturbations, including a significant endocrine response, haematological changes, osmoregulatory disturbance and metabolic adjustments in the heart, liver and muscle. Snapper clearly experienced physiological stress at the point of avoidance but they were not however in a state of physiological exhaustion since some fuel reserves were still available. In addition to avoidance, snapper also showed a subtle reduction in swimming speed — this energy-saving response may have helped snapper minimise the physiological challenge of low O2 residence. It is therefore concluded that snapper can reside in water below their Pcrit threshold for brief periods of time and, without any evidence of physiological exhaustion at the point of avoidance, fish should recover quickly once normoxia is selected. Lastly, with signs of anaerobic metabolism in cardiac tissue at the point of avoidance, we tentatively suggest that snapper may leave hypoxia to protect heart function.

Ventilation rates and activity levels of juvenile jumbo squids under metabolic suppression in the oxygen minimum zones

The Humboldt (jumbo) squid, Dosidicus gigas, is a part-time resident of the permanent oxygen minimum zone (OMZ) in the Eastern Tropical Pacific and, thereby, it encounters oxygen levels below its critical oxygen partial pressure. To better understand the ventilatory mechanisms that accompany the process of metabolic suppression in these top oceanic predators, we exposed juvenile D. gigas to the oxygen levels found in the OMZ (1% O(2), 1 kPa, 10 °C) and measured metabolic rate, activity cycling patterns, swimming mode, escape jet (burst) frequency, mantle contraction frequency and strength, stroke volume and oxygen extraction efficiency. In normoxia, metabolic rate varied between 14 and 29 μmol O(2) g(-1) wet mass h(-1), depending on the level of activity. The mantle contraction frequency and strength were linearly correlated and increased significantly with activity level. Additionally, an increase in stroke volume and ventilatory volume per minute was observed, followed by a mantle hyperinflation process during high activity periods. Squid metabolic rate dropped more than 75% during exposure to hypoxia. Maximum metabolic rate was not achieved under such conditions and the metabolic scope was significantly decreased. Hypoxia changed the relationship between mantle contraction strength and frequency from linear to polynomial with increasing activity, indicating that, under hypoxic conditions, the jumbo squid primarily increases the strength of mantle contraction and does not regulate its frequency. Under hypoxia, jumbo squid also showed a larger inflation period (reduced contraction frequency) and decreased relaxed mantle diameter (shortened diffusion pathway), which optimize oxygen extraction efficiency (up to 82%/34%, without/with consideration of 60% potential skin respiration). Additionally, they breathe 'deeply', with more powerful contractions and enhanced stroke volume. This deep-breathing behavior allows them to display a stable ventilatory volume per minute, and explains the maintenance of the squid's cycling activity under such O(2) conditions. During hypoxia, the respiratory cycles were shorter in length but increased in frequency. This was accompanied by an increase in the number of escape jets during active periods and a faster switch between swimming modes. In late hypoxia (onset ~170 ± 10 min), all the ventilatory processes were significantly reduced and followed by a lethargic state, a behavior that seems closely associated with the process of metabolic suppression and enables the squid to extend its residence time in the OMZ.