Aquatic Oxygen Research Articles

High protein diets improve the growth of juvenile barramundi (Lates calcarifer) at elevated temperatures

Elevated temperatures and low aquatic oxygen levels are increasingly accompanying climate warming and can adversely affect the production and welfare of farmed fish. Both environmental factors and diet affect fish energetics. Still, little is known about the dietary effects on fish energetics at elevated temperatures and low aquatic oxygen levels and how that influences growth. Thus, a feeding trial was conducted to investigate the effect of two isoenergetic diets, high protein (HP, 57% crude protein and 9% total lipid) and high lipid (HL, 40% crude protein and 19% total lipid), on juvenile barramundi (Lates calcarifer, initial body mass: ∼ 3 g). Fish were acclimated for at least four weeks on the diets (three weeks at 35 °C) before measurements began (i.e.: measurements were taken between weeks four to six of the trial). Additionally, feed intake and maximum metabolic rate were tested under three levels of aquatic oxygen levels (normoxia >80, 60% and 30% air saturation). We found no significant dietary effect on feed intake, routine, maximum and postprandial peak metabolic rate. Gill ventilation rate was also unaffected by diet treatment. However, fish fed the HP diet had increased haematocrit and haemoglobin levels, improved growth rates, feed conversion ratio, and protein deposition relative to those fed the HL diet. Thus, this study demonstrated that a high protein diet enhanced key physiological traits such as oxygen-carrying capacity, potentially supporting improved juvenile barramundi growth at elevated temperatures.

Atlantic salmon Salmo salar do not prioritize digestion when energetic budgets are constrained by warming and hypoxia.

During summer, farmed Atlantic salmon (Salmo salar) can experience prolonged periods of warming and low aquatic oxygen levels due to climate change. This often results in a drop in feed intake; however, the physiological mechanism behind this behaviour is unclear. Digestion is a metabolically expensive process that can demand a high proportion of an animal's energy budget and might not be sustainable under future warming scenarios. We investigated the effects of elevated temperature and acute hypoxia on specific dynamic action (SDA; the energetic cost of digestion), and how much of the energy budget (i.e. aerobic scope, AS) was occupied by SDA in juvenile Atlantic salmon. AS was 9% lower in 21°C-acclimated fish compared to fish reared at their optimum temperature (15°C) and was reduced by ~50% by acute hypoxia (50% air saturation) at both temperatures. Furthermore, we observed an increase in peak oxygen uptakerate during digestion which occupied ~13% of the AS at 15°C and ~20% of AS at 21°C, and increased the total cost of digestion at 21°C. The minimum oxygen tolerance threshold in digesting fish was ~42% and ~53% at 15 and 21°C, respectively, and when digesting fish were exposed to acute hypoxia, gut transit was delayed. Thus, these stressors result in a greater proportion of the available energy budget being directed away from digestion. Moderate environmental hypoxia under both optimal and high temperatures severely impedes digestion and should be avoided to limit exacerbating temperature effects on fish growth.

Photodegradation of the Novel Herbicide Pyraquinate in Aqueous Solution: Kinetics, Photoproducts, Mechanisms, and Toxicity Assessment.

Pyraquinate, a newly developed 4-hydroxyphenylpyruvate dioxygenase class herbicide, has shown excellent control of resistant weeds in paddy fields. However, its environmental degradation products and corresponding ecotoxicological risks after field application remain ambiguous. In this study, we systematically investigate the photolytic behaviors of pyraquinate in aqueous solutions and in response to xenon lamp irradiation. The degradation follows first-order kinetics, and its rate depends on pH and the amount of organic matter. No vulnerability to light radiation is indicated. Ultrahigh-performance liquid chromatography with quadrupole-time-of-flight mass spectrometry and UNIFI software analysis reveals six photoproducts generated by methyl oxidation, demethylation, oxidative dechlorination, and ester hydrolysis. Gaussian calculation suggests that activities due to hydroxyl radicals or aquatic oxygen atoms caused these reactions on the premise of obeying thermodynamic criteria. Practical toxicity test results show that the toxicity of pyraquinate to zebrafish embryos is low but increases when the compound is combined with its photoproducts.

Disturbance of primary producer communities disrupts the thermal limits of the associated aquatic fauna

Environmental fluctuation forms a framework of variability within which species have evolved. Environmental fluctuation includes predictability, such as diel cycles of aquatic oxygen fluctuation driven by primary producers. Oxygen availability and fluctuation shape the physiological responses of aquatic animals to warming, so that, in theory, oxygen fluctuation could influence their thermal ecology. We describe annual oxygen variability in agricultural drainage channels and show that disruption of oxygen fluctuation through dredging of plants reduces the thermal tolerance of freshwater animals. We compared the temperature responses of snails, amphipods, leeches and mussels exposed to either natural oxygen fluctuation or constant oxygen in situ under different acclimation periods. Oxygen saturation in channel water ranged from c. 0 % saturation at night to >300 % during the day. Temperature showed normal seasonal variation and was almost synchronous with daily oxygen fluctuation. A dredging event in 2020 dramatically reduced dissolved oxygen variability and the correlation between oxygen and temperature was lost. The tolerance of invertebrates to thermal stress was significantly lower when natural fluctuation in oxygen availability was reduced and decoupled from temperature. This highlights the importance of natural cycles of variability and the need to include finer scale effects, including indirect biological effects, in modelling the ecosystem-level consequences of climate change. Furthermore, restoration and management of primary producers in aquatic habitats could be important to improve the thermal protection of aquatic invertebrates and their resistance to environmental variation imposed by climate change.

Solar‐Driven Water Splitting in Photovoltaic Electrolysis Systems Using Copper Terpyridine Complexes as Oxygen Evolution Catalysts

Copper complexes, as kinds of molecular catalysts bioinspired from the copper O2‐activating enzymes, are attractive targets for water oxidation, which can undergo facile and reversible OO bond formation and cleavage under mild conditions. Herein, three copper(II) complexes of Cu(tpy), Cu(ctpy), and Cu(pctpy) are investigated as oxygen evolution catalysts, which are designed and synthesized using α,α′,α″‐terpyridyl(tpy) derivatives as ligands. In the electrochemical measurements, the carboxylic‐functionalized catalysts of Cu(ctpy) and Cu(pctpy) exhibit high water oxidation turnover frequencies of 1027 and 3363 s−1, respectively. The astonishing rates of O2 production exceed the oxygen evolution complex in natural photosystem II. Moreover, the photovoltaic electrolysis (PVE) system is constructed by connecting Si solar panels with the copper complex‐decorated electrodes. Under the irradiation of AM1.5G‐simulated sunlight, a high photocurrent density of 5.47 mA cm−2 and solar‐to‐hydrogen efficiency of 6.81% are achieved for solar water splitting without external bias. This is the first report PVE system using copper complex as a oxygen evolution catalyst molecular catalyst, which is of great significance to simulate the catalytic water oxidation process of natural enzymes and build an artificial photosynthesis system to achieve green, efficient, and stable decomposition of aquatic oxygen.

Ultrasensitive electrochemiluminescence immunosensor based on Co-doped MoOx as co-reactant generator for the detection of amyloid β-protein

Herein, a novel electrochemiluminescence (ECL) immunosensor was constructed to detect amyloid β-protein (Aβ) by using Co2+ doped MoOx (Co@MoOx) as both sensing substrate and coreactant generator, Au nanoclusters and isoluminol (Au NCs-ABEI) as secondary antibody label. Co@MoOx showed good electrolysis performance of aquatic oxygen, and the dissolved oxygen just acted as a coreactant of the ABEI emitter in this system. Au NCs was synthesized from reduced glutathione, which had ultrasmall size and good biocompatibility. Bioconjugate Au NCs-ABEI was constructed on Au NCs surface by cross-linking ABEI with amino group on ABEI surface. Through modifying Co@MoOx and Au NCs-ABEI on the electrode successively, the ECL signal intensity of immunosensor increased, which showed high sensitivity for the detection of Aβ, with the detection range from 1.00 fg·mL−1 to 500 pg·mL−1 and the detection limit as low as 0.330 fg·mL−1. This strategy will provide some reference for the early diagnosis and prevention of Alzheimer's disease (AD) and detection of other disease markers.

Legitimacy and limitations of valuing the oxygen production of ecosystems

Oxygen production is an ecosystem service essential to life on Earth. However how it should be valued is controversial and depends on several factors. Here, we commented on how valuation might be applicable to the stock or flow of oxygen, whether additional oxygen produced at the micro or macro scale provides additional human wellbeing, and whether double counting may occur if oxygen production and carbon sequestration are both valued independently and added. We concluded that the flow of oxygen produced by ecosystems should be valued when: (1) high levels of atmospheric oxygen at specific micro-scale areas (e.g., a park) provides additional benefits to local human health and additional attraction to tourists; (2) micro-scale aquatic oxygen production (e.g., in a pond or aquafarm) avoids potential loss of aquatic products; and (3) macro-scale aquatic oxygen production (e.g., in global oceans) maintains marine contributions to humans (e.g., fishery resources). However, whether macro-scale atmospheric oxygen production should be valued is uncertian, because the effects of declining global atmospheric oxygen, especially in the short term, remain unclear. This needs further research. We also concluded that the values of oxygen production and carbon sequestration can be aggregated without double counting, given that the values are not duplicated in multiple ecosystem service categories. For example, oxygen production is best considered as contributing to gas regulation while carbon sequestration contributes to climate regulation. But one should not count and add both carbon sequestration and oxygen production as contributing to both gas and climate regulation. Techniques for valuing oxygen production may include the willingness to pay for additional health benefits of breathing extra high levels of atmospheric oxygen, the market price of industrial oxygen, the travel cost to natural ‘oxygen bars’, the avoided cost of losing aquatic resources, and the replacement cost of using artificial techniques to produce oxygen.

Integrative Approaches to Understanding Organismal Responses to Aquatic Deoxygenation.

AbstractOxygen bioavailability is declining in aquatic systems worldwide as a result of climate change and other anthropogenic stressors. For aquatic organisms, the consequences are poorly known but are likely to reflect both direct effects of declining oxygen bioavailability and interactions between oxygen and other stressors, including two-warming and acidification-that have received substantial attention in recent decades and that typically accompany oxygen changes. Drawing on the collected papers in this symposium volume ("An Oxygen Perspective on Climate Change"), we outline the causes and consequences of declining oxygen bioavailability. First, we discuss the scope of natural and predicted anthropogenic changes in aquatic oxygen levels. Although modern organisms are the result of long evolutionary histories during which they were exposed to natural oxygen regimes, anthropogenic change is now exposing them to more extreme conditions and novel combinations of low oxygen with other stressors. Second, we identify behavioral and physiological mechanisms that underlie the interactive effects of oxygen with other stressors, and we assess the range of potential organismal responses to oxygen limitation that occur across levels of biological organization and over multiple timescales. We argue that metabolism and energetics provide a powerful and unifying framework for understanding organism-oxygen interactions. Third, we conclude by outlining a set of approaches for maximizing the effectiveness of future work, including focusing on long-term experiments using biologically realistic variation in experimental factors and taking truly cross-disciplinary and integrative approaches to understanding and predicting future effects.

Plasticity of thermal performance curves in a narrow range endemic water beetle

Thermal history can plastically alter the response of ectotherms to temperature, and thermal performance curves (TPCs) are powerful tools for exploring how organismal-level performance varies with temperature. Plasticity in TPCs may be favoured in thermally variable habitats, where it can result in fitness benefits. However, thermal physiology remains insufficiently studied for freshwater insects despite freshwater biodiversity being at great risk under global change. Here, we assess how acclimation at either summer or winter average temperatures changes TPCs for locomotion activity and metabolism in Enochrus jesusarribasi (Hydrophilidae), a water beetle endemic to shallow saline streams in SE Spain. This beetle is a bimodal gas exchanger and so we also assessed how aerial and aquatic gas exchange varied across temperatures for both acclimation treatments. Responses of locomotory TPCs to thermal acclimation were relatively weak, but high temperature acclimated beetles tended to exhibit higher maximum locomotor activity and reduced TPC breadth than those acclimated at lower temperature. High temperature acclimation increased the thermal sensitivity of metabolic rates, contrary to the response generally found in aquatic organisms. Higher metabolic rates upon high temperature acclimation were achieved by increasing aerial, rather than aquatic oxygen uptake. Such plastic respiratory behaviour likely contributed to enhanced locomotor performance at temperatures around the optimum and thermal plasticity could thus be an important component in the response of aquatic insects to climate change. However, high temperature acclimation appeared to be detrimental for locomotion in subsequent exposure at upper sublethal temperatures, suggesting that this narrow range endemic may be vulnerable to future climate warming. This study demonstrates that TPCs are context-specific, differing with performance metric as well as thermal history. Such context dependency must be considered when using TPCs to predict organismal responses to climate change.

The role of tidal acclimation on the physiological responses of the green shore crab, Carcinus maenas, to thermal stress

Animals inhabiting the intertidal zone are exposed to abrupt changes in environmental conditions associated with the rise and fall of the tide. For convenience, the majority of laboratory studies on intertidal organisms have acclimated individuals to permanently submerged conditions, and thus we currently know little about how acclimation to cyclic emersion-immersion influences the ability of intertidal organisms to cope with fluctuations in environmental conditions. In the present study, the green shore crab Carcinus maenas was acclimated to either a simulated tidal regime of continuous emersion-immersion (tidal) or to permanently submerged conditions (non-tidal). Aquatic oxygen consumption was assessed when crabs were exposed to cooling (15 °C - 5 °C) or warming (15 °C - 25 °C) regimes. Tidal crabs had lower levels of oxygen consumption over the 15 °C - 20 °C range. During acute thermal stress, the Q10 values of non-tidal crabs increased as the temperature declined to 5 °C, whereas tidal crabs maintained relatively consistent Q10 values across the temperature range (5 °C–25 C). These changes in oxygen consumption were paralleled by changes in haematological parameters. Tidal crabs had higher levels of haemocyanin which may have augmented oxygen delivery to the tissues and buffered acid-base imbalances. Surprisingly lactate levels were higher in tidal crabs, these appeared unconnected to anaerobic metabolism, but may have been an important metabolic fuel for tidal crabs. These results suggest that acclimation to a simulated tidal cycle may result in different physiological responses to environmental stressors and as such is an important consideration when assessing the responses of intertidal animals in a laboratory setting.

Parents living in water, embryos developing in air: respiratory adaptations to use both environments in the freshwater gastropod Pomacea figulina (Gastropoda, Ampullariidae)

ABSTRACT The ability to use oxygen from both air and water has been considered key to the colonization of terrestrial environments by invertebrates. The freshwater gastropod Pomacea figulina generally lives submerged, although females lay their eggs mainly above water during nighttime, on the stems of aquatic plants, and the embryos develop inside the air-exposed calcareous egg capsules. However, little is known about the physiological adaptations that the ovipositing females and the developing embryos have developed to breathe in air. In this study, we report sexual differences in the aerial and aquatic oxygen consumption of adults and embryonic respiration in aerially developing egg clusters. We found that the males and females of this species can obtain oxygen from both water and air, although uptake rates for both sexes were much lower in air than in water. Females in air consumed oxygen 42% faster than males, which may be related with the habit of egg laying above water level. Developing embryos inside calcareous eggs obtained oxygen faster when submersed under water than when exposed to air, although embryos near hatching were better able to use oxygen from both air and water than embryos at earlier stages of development. Our data suggest that for P. figulina, laying eggs in the terrestrial environment may be more a strategy for protecting embryos from predators than a respiratory adaptation.

An enigmatic mass mortality event of Blanding’s Turtles (Emydoidea blandingii) in a protected area

Mass mortality events (MMEs) can remove up to 90% of individuals in a population and are especially damaging to population viability of long-lived species with slow life histories. Our goal was to elucidate the cause(s) of a MME of 53 Blanding’s Turtles (Emydoidea blandingii (Holbrook, 1838)), a globally endangered species, in a protected area. We investigated disease, winter-kill, and depredation as potential causes of the mortality. The turtle carcasses lacked soft tissue to test for disease, so we examined tissue from co-occurring live Leopard Frogs (Lithobates pipiens (Schreber, 1782)) and found no evidence of ranavirus, indicating that the disease was not present at our study site. Water temperature and dissolved aquatic oxygen at known overwintering sites and sites which yielded carcasses did not differ, suggesting that winter-kill did not cause the MME. Carcass condition, comparisons with descriptions of turtle depredation events in the literature, and trail cameras paired with turtle decoys identified potential predators within the study site and suggested that mass depredation, enabled by low water levels and a concomitant reduction in aquatic habitat, was the most likely cause of mortality. Our study can inform conservation of the study population and the management of MMEs of long-lived species elsewhere.

Ontogenetic Shifts in Oxygen Uptake in Common Mudskipper (Periophthalmus kalolo) and its Role in Microhabitat Selection

Aims: The common mudskipper, Periophthalmus kalolo is a tropical, amphibious fish that utilizes both air and water as a respiratory medium. Adult oxygen uptake in water and air is well studied, but requirements of post-metamorphosed fish are virtually unknown. Our study quantifies how ontogenetic shifts across mudskipper life stage affects microhabitat choice.
 Study Design: metabolic rates in air and water were estimated for common mudskippers with mass values from 0.03 to 28.9 grams. Fish in each media were divided into 5 standard length classes (≤ 2.00, 2.01-4.00, 4.01-6.00, 6.01-8.00, ≥ 8.01 cm). Oxygen consumption for each class was calculated as the mean individual oxygen consumption for the group.
 Place and Duration of Study: Hoga Research Laboratory, Wakatobi National Park, Sulawesi, Indonesia, between June 14 and August 26, 2005.
 Methodology: Gilson manometry, and flow-through respirometery was used to determine respective aerial and aquatic oxygen uptake values for common mudskippers.
 Results: Mudskipper lengths varied by an order of magnitude (1.0 to 10.3 cm) and mass varied by three orders of magnitude (0.03 to 17.3 g). Mass-adjusted aerial metabolic rates of fish 2.00 cm or less in length, consumed up to ten times more oxygen than larger fish. whereas, aerial consumption values for size classes 2.01 cm and above did not differ significantly from each other (ANOVA: F4,112 = 40.29, P < 0.0001; SNK, α = 0.05).
 Metabolic rates of submerged fish fell into two statistically distinct subsets. mudskippers with lengths of 4.00 cm or less had significantly higher metabolic rates than fish in remaining size classes; however, mean oxygen uptake values of fish within subsets did not differ significantly (ANOVA: F4,87 = 6.89, P < 0.0001; SNK, α = 0.05). When oxygen consumption values at each size class were compared, only the smallest mudskipper size class (£ 2.00 cm) differed significantly between air and water (t-test: df = 34, t = 3.44, P < 0.0001). Mudskippers 2 cm in standard length or smaller consumed over seven times more oxygen in air than water. All other size classes had similar rates of oxygen uptake in air and water and air:water oxygen uptake ratios fell to approximately 1:1.
 Conclusions: (1) Common mudskippers select different mangal habitats based on developmental life stage. (2) Small, post-metamorphosed fish <2 mm in standard length, prefer cool shaded mangal areas, whereas, larger mudskippers utilize sun-exposed zones.
 (3) Post-metamorphosed fish, consume 10X more oxygen than all other size groups, confirming that juveniles are well suited to aerial respiration oxygen shortly after hatching. (4) Emerged post-metamorphosed fish exploit cooler, wetter pool habitats, thereby ameliorating problems of desiccation, excretion, and/or predation. (5) Ontogenetic shifts are a key life-history event in P. kalolo that promotes wider mangrove habitat use, and plays an important role in establishing common mudskipper as a key mangal species.

Green hydrogen: The only oxygen and water balanced fuel

The use of any fuel depletes the oxygen content of the atmosphere, with one exception: hydrogen produced from water. Water electrolysis liberates oxygen from water in the precise stoichiometric ratio required to oxidise (and hence release energy from) the co-produced hydrogen. As a commercial fuel production process, electrolysis is unique in providing the oxidant as well as the fuel; electrolytic oxygen can thereby replenish the consumption of atmospheric oxygen due to hydrogen use. Furthermore, the amount of water consumed during electrolysis is reproduced when the hydrogen is oxidised. So the use of electrolysers and electrolytic hydrogen does not affect global oxygen and water resources: ‘green’ hydrogen may thus be described as the only oxygen and water balanced fuel. Conversely, the use of hydrogen derived from fossil fuels (with or without carbon capture and storage, CCS) depletes the oxygen resource and increases water vapour emissions to the atmosphere, which enhances the rate of global warming. Therefore, a worldwide multi-TW deployment of electrolysers could provide very substantial amounts of hydrogen for the energy system, and oxygen for the global ecosystem. This should be done in combination with other measures for combatting oxygen depletion (such as reducing combustion, increasing forestation, and reducing nutrient inputs to the ocean from sewage and agriculture). In this way the long-term objective should be to stabilise, or even increase slightly, the concentrations of atmospheric and aquatic oxygen, and possibly speed up the decay of atmospheric methane. Clearly the production-and-use of hydrogen derived from fossil fuels contravenes this objective, and should cease without delay.

Aquatic Biogeochemical Eddy Covariance Fluxes in the Presence of Waves

AbstractThe eddy covariance (EC) technique is a powerful tool for measuring atmospheric exchange rates that was recently adapted by biogeochemists to measure aquatic oxygen fluxes. A review of aquatic biogeochemical EC literature revealed that the majority of studies were conducted in shallow waters where waves were likely present, and that waves biased sensor and turbulence measurements. This review identified that larger measurement heights shifted turbulence to lower frequencies, producing a spectral gap between turbulence and wave frequencies. However, some studies sampled too close to the boundary to allow for a spectral turbulence‐wave gap, and a change in how EC measurements are conducted and analyzed is needed to remove wave‐bias. EC fluxes have only been derived from the time‐averaged product of vertical velocity and oxygen, often resulting in wave‐bias. Presented is a new analysis framework for removing wave‐bias by accumulation of cross‐power spectral densities below wave frequencies. This analysis framework also includes new measurement guidelines based on wave period, currents, and measurement heights. This framework is applied to sand, seagrass, and reef environments where traditional EC analysis resulted in wave‐bias of 7.0% ± 9.2% error in biogeochemical (oxygen and H+) fluxes, while more variable and higher error was evident in momentum fluxes (10.5% ± 21.0% error). It is anticipated that this framework will lead to significant changes in how EC measurements are conducted and evaluated, and help overcome the major limitations caused by wave‐sensitive and slow‐response sensors, potentially expanding new chemical tracer applications and more widespread use of the EC technique.

Rising CO2 saves lives

Anthropogenic climate change is a multi-faceted problem. Rising CO2 levels in the atmosphere cause a warming of global temperatures, acidify the oceans and may ultimately deplete aquatic oxygen, leading to hypoxia. By itself, any one of these conditions can pose a real challenge to an animal, but how the stressors interact is still largely unexplored. Aquatic hypoxia may be especially intricate and, unlike the hypoxic conditions that one might experience when climbing a mountain, is almost always caused by organismal respiration in the water and, thus, is associated with an additional, simultaneous increase in CO2. The linkage between oxygen and CO2 levels in water has long been known, but how fish are affected by the combined challenge is poorly studied.To address the issue, Daniel Montgomery and his colleagues at the University of Exeter, UK, set out to test the effect of rising CO2 on the hypoxia tolerance of European seabass. They measured the oxygen consumption of the fish while lowering the available oxygen in the water; the point at which the animal can no longer extract oxygen from the depleted water and their oxygen consumption starts to decline is a popular indicator of hypoxia tolerance in fish. Knowing that higher levels of CO2 tend to acidify the blood of fish – compromising the ability of haemoglobin to bind oxygen and hindering the fish's ability to extract oxygen from the water – Montgomery then supplemented one of the treatments with a simultaneous increase in CO2, expecting that the combined challenge would render the fish more susceptible to hypoxia.However, contrary to these predictions, CO2-treated fish excelled at extracting oxygen from the water and outperformed the low-CO2 group in terms of hypoxia tolerance. When Montgomery took a closer look at the fish's blood, he found no difference in blood pH between the treatments, indicating a remarkable capacity of the seabass to counteract the acidifying effect of elevated CO2; surely this is good news for fish that must cope with global rises in CO2. In addition, fish in the rising CO2 treatment had a higher haemoglobin–oxygen affinity compared with the low-CO2 group – a surprising finding, given the common blood pH in the two groups – and this may explain their improved ability to tolerate hypoxia. Haemoglobin is housed within red blood cells, which may actively alter their internal environment to modulate oxygen transport. Whether this is part of the mechanism by which CO2-exposed seabass increased their haemoglobin–oxygen affinity remains to be seen.Studying the effects of climate change is complicated as numerous factors may interact to challenge an organism. Understandably, researchers often choose to study single factors in isolation, as this simplifies the interpretation of the data. However, Montgomery and colleagues show that overlooking interaction effects may limit our ability to predict the response of animals to climate change in the wild. In European seabass it appears that the combined condition of a decreasing oxygen level and an increasing CO2 level in the water, as is found in nature, improves hypoxia tolerance. To account for such effects in future work, current methods to assess hypoxia tolerance in fish should be revised to control for, or at least measure and report, CO2 levels in the water. Rising CO2 levels receive a lot of bad press and, in the context of climate change, rightfully so. However, at least for seabass that find themselves gasping for oxygen, the natural rise in CO2 that comes with aquatic hypoxia may actually save lives.

Metabolic rate and hypoxia tolerance in Girardinichthys multiradiatus (Pisces: Goodeidae), an endemic fish at high altitude in tropical Mexico

The darkedged splitfin (Amarillo fish), Girardinichthys multiradiatus is a vulnerable endemic fish species inhabiting central Mexico's high altitude Upper Lerma Basin, where aquatic hypoxia is exacerbated by low barometric pressures (lower PO2s), large aquatic oxygen changes, poor aquatic systems management and urban, agricultural and industrial pollution. The respiratory physiology of G. multiradiatus under such challenging conditions is unknown - therefore the main goal of the present study was to determine metabolic rates and hypoxia tolerance to elucidate possible physiological adaptations allowing this fish to survive high altitude and increasingly eutrophic conditions. Fish came from two artificial reservoirs – San Elías and Ex Hacienda - considered refuges for this species. Both reservoirs showed high dial PO2 variation, with hypoxic conditions before midday and after 20:00 h, ~4 h of normoxia (15 kPa) from 16:00–20:00, and ~4 h of hyperoxia (16–33 kPa) from 12:00–16:00. Standard metabolic rate at 20 ± 0.5 °C of larvae from Ex Hacienda was significantly higher than those from San Elías, but these differences disappeared in juveniles and adults. Metabolic rate at 20 ± 0.5 °C for adults was 9.8 ± 0.1 SEM μmol O2/g/h. The metabolic scaling exponent for adults was 0.58 for San Elías fish and 0.83 for Ex Hacienda fish, indicating possible ecological effects on this variable. Post-larval fish in Ex Hacienda and all stages in San Elias site showed considerable hypoxia tolerance, with PCrit mean values ranging from 1.9–3.1 kPa, lower than those of many tropical fish at comparable temperatures. Collectively, these data indicate that G. multiradiatus is well adapted for the hypoxia associated with their high-altitude habitat.

Controlling reduced iron and manganese in a drinking water reservoir by hypolimnetic aeration and artificial destratification

The concentrations of iron (Fe) and manganese (Mn) in the water column have extremely important effects on the water quality of drinking water reservoirs; however, reservoirs often experience problematic Fe and Mn levels during seasonal stratification and rainfall events. Water-lifting aerators (WLAs) were deployed in the Jinpen Reservoir to control these issues with Fe and Mn at the source via bottom aeration and artificial destratification. In this study, variations of Fe and Mn concentrations in the water column, porewater, and sediments, were used to characterize behaviors of reduced Fe and Mn under the conditions of hypolimnetic aeration and artificial destratification during periods of hypolimnion hypoxia and rainfall events. The results showed that replenishing aquatic oxygen levels by aeration can effectively decrease the dissolved Fe and Mn in the water column thereby increasing the sedimentation rate and the diffusive flux of Fe and Mn at the sediment-water interface (SWI). The dissolved Fe was significantly chemically oxidized and the concentration remained relatively low in the water column during WLA operations, while dissolved Mn persistently accumulated in the near-sediment regions because of its complex kinetics. Our in situ profiles of labile Fe and Mn in the sediments demonstrated that the diffusive flux of Mn (JMn) was largely increased by the increased concentration gradient at the SWI, while the diffusive flux of Fe (JFe) decreased. The sediments were observed to rapidly become anoxic and release Fe and Mn after WLA deactivation; this emphasized the importance of appropriate operations linking the artificial and natural mixing periods to prevent SWI hypoxia and the release of reduced substances.

The ethylene receptor regulates Typha angustifolia leaf aerenchyma morphogenesis and cell fate.

Ethylene receptor is crucial for PCD and aerenchyma formation in Typha angustifolia leaves. Not only does it receive and deliver the ethylene signal, but it probably can determine the cell fate during aerenchyma morphogenesis, which is due to the receptor expression quantity. Aquatic plant oxygen delivery relies on aerenchyma, which is formed by a programmed cell death (PCD) procedure. However, cells in the outer edge of the aerenchyma (palisade cells and septum cells) remain intact, and the mechanism is unclear. Here, we offer a hypothesis: cells that have a higher content of ethylene receptors do not undergo PCD. In this study, we investigated the leaf aerenchyma of the aquatic plant Typha angustifolia. Ethephon and pyrazinamide (PZA, an inhibitor of ACC oxidase) were used to confirm that ethylene is an essential hormone for PCD of leaf aerenchyma cells in T. angustifolia. That the ethylene receptor was an indispensable factor in this PCD was confirmed by 1-MCP (an inhibitor of the ethylene receptor) treatment. Although PCD can be avoided by blocking the ethylene receptor, excessive ethylene receptors also protect cells from PCD. TaETR1, TaETR2 and TaEIN4 in the T. angustifolia leaf were detected by immunofluorescence (IF) using polyclonal antibodies. The result showed that the content of ethylene receptors in PCD-unsusceptible cells was 4-14 times higher than that one in PCD-susceptible cells, suggesting that PCD-susceptible cells undergo the PCD programme, while PCD-unsusceptible cells do not due to the content difference in the ethylene receptor in different cells. A higher level of ethylene receptor content makes the cells insensitive to ethylene, thereby avoiding cell death and degradation.

Aggression supersedes individual oxygen demand to drive group air-breathing in a social catfish.

Group‐living is widespread among animals and comes with numerous costs and benefits. To date, research examining group‐living has focused on trade‐offs surrounding foraging, while other forms of resource acquisition have been largely overlooked.Air‐breathing has evolved in many fish lineages, allowing animals to obtain oxygen in hypoxic aquatic environments. Breathing air increases the threat of predation, so some species perform group air‐breathing, to reduce individual risk. Within species, individual air‐breathing can be influenced by metabolic rate as well as personality, but the mechanisms of group air‐breathing remain unexplored. It is conceivable that keystone individuals with high metabolic demand or intrinsic tendency to breathe air may drive social breathing, especially in hypoxia.We examined social air‐breathing in African sharptooth catfish Clarias gariepinus, to determine whether individual physiological traits and spontaneous tendency to breathe air influence the behaviour of entire groups, and whether such influences vary in relation to aquatic oxygen availability.We studied 11 groups of four catfish in a laboratory arena and recorded air‐breathing behaviour, activity and agonistic interactions at varying levels of hypoxia. Bimodal respirometry was used to estimate individual standard metabolic rate (SMR) and the tendency to utilize aerial oxygen when alone.Fish took more air breaths in groups as compared to when they were alone, regardless of water oxygen content, and displayed temporally clustered air‐breathing behaviour, consistent with existing definitions of synchronous air‐breathing. However, groups displayed tremendous variability in surfacing behaviour. Aggression by dominant individuals within groups was the main factor influencing air‐breathing of the entire group. There was no association between individual SMR, or the tendency to obtain oxygen from air when in isolation, and group air‐breathing.For C. gariepinus, synchronous air‐breathing is strongly influenced by agonistic interactions, which may expose subordinate individuals to risk of predation. Influential individuals exerted an overriding effect on risk‐taking by the entire group, for reasons independent of their physiological oxygen requirements. Overall, this illustrates that social context can obscure interactions between an individual's physiological and behavioural traits and their tendency to take risks to obtain resources.