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1. Modelling the relationships between haemoglobin oxygen affinity and the oxygen cascade in humans

   https://doi.org/10.1113/JP277591  

   Shepherd, John R. A. ; Dominelli, Paolo B. ; Roy, Tuhin K. ; Secomb, Timothy W. ; Hoyer, James D. ; Oliveira, Jennifer L. ; Joyner, Michael J. ( July 2019 , The Journal of Physiology)

   Haemoglobin affinity is an integral concept in exercise physiology that impacts oxygen uptake, delivery and consumption.

   How chronic alterations in haemoglobin affinity impact physiology is unknown.

   Using human haemoglobin variants, we demonstrate that the affinity of haemoglobin for oxygen is highly correlated with haemoglobin concentration.

   Using the Fick equation, we model how altered haemoglobin affinity and the associated haemoglobin concentration influences oxygen consumption at rest and during exercise via alterations in cardiac output and mixed‐venous.

   The combination of low oxygen affinity haemoglobin and reduced haemoglobin concentration seenin vivomay be unable to support oxygen uptake during moderate or heavy exercise.

   The physiological implications, with regard to exercise, of altered haemoglobin affinity for oxygen are not fully understood. Data from the Mayo Clinic Laboratories database of rare human haemoglobin variants reveal a strong inverse correlation (r = −0.82) between blood haemoglobin concentration andP₅₀, an index of oxygen affinity [Hb = −0.3135(P₅₀) + 23.636]. In the present study, observedP₅₀values for high, normal and low oxygen‐affinity haemoglobin variants (13, 26 and 39 mmHg) and corresponding haemoglobin concentrations (19.5, 15.5 and 11.4 g dL⁻¹respectively) are used to model oxygen consumption as a fraction of delivery at rest ( = 0.25 L min⁻¹, cardiac output = 5.70 L min⁻¹) and during exercise ( = 2.75 L min⁻¹, cardiac output = 18.9 l min⁻¹). With high‐affinity haemoglobin, the model shows that normal levels of oxygen consumption can be achieved at rest and during exercise at the assumed cardiac output levels, with reduced oxygen extraction both at rest (16.8% high affinityvs. 21.7% normal) and during exercise (55.8% high affinityvs. 72.2% normal). With low‐affinity haemoglobin, which predicts low haemoglobin concentration, oxygen consumption at rest can be sustained with the assumed cardiac output, with increased oxygen extraction (31.1% low affinityvs. 21.7% normal). However, exercise at 2.75 l min⁻¹cannot be achieved with the assumed cardiac output, even with 100% oxygen extraction. In conclusion, the model indicates chronic alterations inP₅₀associate directly with Hb concentration, highlighting that human Hb variants can serve as ‘experiments of nature’ to address fundamental hypotheses on oxygen transport and exercise.

    

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2. Worst‐Case Severe Environments for Surface Charging Observed at LANL Satellites as Dependent on Solar Wind and Geomagnetic Conditions

   https://doi.org/10.1029/2021SW002732  

   Ganushkina, N. Yu ; Swiger, B. ; Dubyagin, S. ; Matéo‐Vélez, J. ‐C. ; Liemohn, M. W. ; Sicard, A. ; Payan, D. ( September 2021 , Space Weather)

   The 400 worst‐case severe environments for surface charging detected at Los Alamos National Laboratory satellites during the years of 1990–2005 as binned by the definitions of four criteria developed by Matéo‐Vélez et al. (2018,https://doi.org/10.1002/2017sw001689) and the solar wind and Interplanetary Magnetic Field (IMF) parameters and geomagnetic activity indices are analyzed. The conducted analysis shows that only Auroral Electrojet/Auroral Lower index determines the highest risk for severe environments for surface charging to happen. The presence of a substorm with the southward turning pattern in IMFindicates that the environment can be severe for surface charging to occur but this environment will not depend on whether a substorm was moderate or intense. No clear dependence on IMFis found for risk to a severe environment to occur. Appearances of severe environments for surface charging do not necessarily require high values ofKp(Planetarische Kennziffer) and no storm is needed for such an event to be detected. Among solar wind parameters, solar wind velocityis directly related to the highest risk of severe environments, dependent on thevalue; and number densityis of no importance. Two criteria for severe environment events based on the enhancements of low energy particle fluxes exhibit clearer dependencies on the solar wind and IMF parameters and geomagnetic activity indices with more distinct patterns in their time history.

    

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3. Probabilistic Evaluation of Drought in CMIP6 Simulations

   https://doi.org/10.1029/2021EF002150  

   Papalexiou, Simon Michael ; Rajulapati, Chandra Rupa ; Andreadis, Konstantinos M. ; Foufoula‐Georgiou, Efi ; Clark, Martyn P. ; Trenberth, Kevin E. ( October 2021 , Earth's Future)

   As droughts have widespread social and ecological impacts, it is critical to develop long‐term adaptation and mitigation strategies to reduce drought vulnerability. Climate models are important in quantifying drought changes. Here, we assess the ability of 285 CMIP6 historical simulations, from 17 models, to reproduce drought duration and severity in three observational data sets using the Standardized Precipitation Index (SPI). We used summary statistics beyond the mean and standard deviation, and devised a novel probabilistic framework, based on the Hellinger distance, to quantify the difference between observed and simulated drought characteristics. Results show that many simulations have less thanerror in reproducing the observed drought summary statistics. The hypothesis that simulations and observations are described by the same distribution cannot be rejected for more thanof the grids based on ourdistance framework. No single model stood out as demonstrating consistently better performance over large regions of the globe. The variance in drought statistics among the simulations is higher in the tropics compared to other latitudinal zones. Though the models capture the characteristics of dry spells well, there is considerable bias in low precipitation values. Good model performance in terms of SPI does not imply good performance in simulating low precipitation. Our study emphasizes the need to probabilistically evaluate climate model simulations in order to both pinpoint model weaknesses and identify a subset of best‐performing models that are useful for impact assessments.

    

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4. Perfect divisibility and 2‐divisibility

   https://doi.org/10.1002/jgt.22367  

   Chudnovsky, Maria ; Sivaraman, Vaidy ( June 2018 , Journal of Graph Theory)

   A graphGis said to be 2‐divisible if for all (nonempty) induced subgraphsHofG,can be partitioned into two setssuch thatand. (Heredenotes the clique number ofG, the number of vertices in a largest clique ofG). A graphGis said to be perfectly divisible if for all induced subgraphsHofG,can be partitioned into two setssuch thatis perfect and. We prove that if a graph is‐free, then it is 2‐divisible. We also prove that if a graph is bull‐free and either odd‐hole‐free orP₅‐free, then it is perfectly divisible.

    

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5. Developmental changes in oxygen consumption and hypoxia tolerance in the heat and hypoxia‐adapted tabasco line of the Nile tilapia Oreochromis niloticus , with a survey of the metabolic literature for the genus Oreochromis

   https://doi.org/10.1111/jfb.13945  

   Burggren, Warren W. ; Mendez‐Sanchez, Jose F. ; Martínez Bautista, Gil ; Peña, Emyr ; Martínez García, Rafael ; Alvarez González, Carlos A. ( April 2019 , Journal of Fish Biology)

   The genusOreochromisis among the most popular of the tilapiine cichlid tribe for aquaculture. However, their temperature and hypoxia tolerance, if tested at all, is usually tested at temperatures of 20–25°C, rather than at the considerably higher temperatures of 30–35°C typical of tropical aquaculture.

   We hypothesized that both larvae and adults of the heat and hypoxia‐adapted Tabasco‐line of the Nile tilapiaOreochromis niloticuswould be relatively hypoxia‐tolerant. Oxygen consumption rate (), Q₁₀and aquatic surface respiration (ASR) was measured using closed respirometry at 2 (c. 0.2 g), 30 (c. 2–5 g), 105c. (10–15 g) and 240 (c. 250 g) days of development, at 25°C, 30°C and 35°C.at 30°C was inversely related to body mass:c. 90 μM O₂g⁻¹/h in larvae down toc. 1 μM O₂g⁻¹/h in young adults. Q₁₀forwas typical for fish over the range 25–35°C of 1.5–2.0. ASR was exhibited by 50% of the fish at pO₂of 15–50 mmHg in a temperature‐dependent fashion. However, the largest adults showed notable ASR only when pO₂fell to below 10 mmHg. Remarkably, p_critforwas 12–17 mmHg at 25–30°C and still only 20–25 mmHg across development at 35°C. These values are among the lowest measured for teleost fishes. Noteworthy is that all fish maintain equilibrium, ventilated their gills and showed routine locomotor action for 10–20 min afterceased at near anoxia and when then returned to oxygenated waters, all fish survived, further indicating a remarkable hypoxic tolerance. Remarkably, data assembled forfrom >30 studies showed a > x2000 difference, which we attribute to calculation or conversion errors. Nonetheless, p_critwas very low for allOreochromissp. and lowest in the heat and hypoxia‐adapted Tabasco line.

    

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