Effect of acclimation temperature on oxygen transport in the blood of the carp, Cyprinus carpio

Metabolic resilience of the Australasian snapper (Chrysophrys auratus) to marine heatwaves and hypoxia

Marine organisms are under threat from a simultaneous combination of climate change stressors, including warming sea surface temperatures (SST), marine heatwave (MHW) episodes, and hypoxic events. This study sought to investigate the impacts of these stressors on the Australasian snapper (C. auratus) - a finfish species of high commercial and recreational importance, from the largest snapper fishery in Aotearoa New Zealand (SNA1). A MHW scenario was simulated from 21°C (current February SST average for north-eastern New Zealand) to a future predicted level of 25°C, with the whole-animal and mitochondrial metabolic performance of snapper in response to hypoxia and elevated temperature tested after 1-, 10-, and 30-days of thermal challenge. It was hypothesised that key indicators of snapper metabolic performance would decline after 1-day of MHW stress, but that partial recovery might arise as result of thermal plasticity after chronic (e.g., 30-day) exposures. In contrast to this hypothesis, snapper performance remained high throughout the MHW: 1) Aerobic metabolic scope increased after 1-day of 25°C exposure and remained high. 2) Hypoxia tolerance, measured as the critical O₂ pressure and O₂ pressure where loss of equilibrium occurred, declined after 1-day of warm-acclimation, but recovered quickly with no observable difference from the 21°C control following 30-days at 25°C. 3) The performance of snapper mitochondria was also maintained, with oxidative phosphorylation respiration and proton leak flux across the inner mitochondrial membrane of the heart remaining mostly unaffected. Collectively, the results suggest that heart mitochondria displayed resilience, or plasticity, in snapper chronically exposed to 25°C. Therefore, contrary to the notion of climate change having adverse metabolic effects, future temperatures approaching 25°C may be tolerated by C. auratus in Northern New Zealand. Even in conjunction with supplementary hypoxia, 25°C appears to represent a metabolically optimal temperature for this species.

Thermal plasticity of the cardiorespiratory system provides cross-tolerance protection to fish exposed to elevated nitrate

Exposure to nitrate is toxic to aquatic animals due to the formation of methaemoglobin and a subsequent loss of blood-oxygen carrying capacity. Yet, nitrate toxicity can be modulated by other stressors in the environment, such as elevated temperatures. Acclimation to elevated temperatures has been shown to offset the negative effects of nitrate on whole animal performance in fish, but the mechanisms underlying this cross-tolerance interaction remain unclear. In this study, juvenile silver perch (Bidyanus bidyanus) were exposed to a factorial combination of temperature (28 °C or 32 °C) and nitrate concentrations (0, 50 or 100 mg NO₃^- L^-1) treatments to test the hypothesis that thermal acclimation offsets the effects of nitrate via compensatory changes to the cardiorespiratory system (gills, ventricle and blood oxygen carrying capacity). Following 21 weeks of thermal acclimation, we found that fish acclimated to 32 °C experienced an expansion of gill surface area and an increase in ventricular thickness regardless of nitrate exposure concentration. Exposure to nitrate (both 50 and 100 mg NO₃^- L^-1) reduced the blood oxygen carrying capacity of silver perch due to increases in methaemoglobin concentration and a right shift in oxygen-haemoglobin binding curves in fish from both thermal acclimation treatments. These results indicate that plasticity of the gills and ventricle of warm acclimated fish are potential mechanisms which may provide cross-tolerance protection to elevated nitrate concentrations despite nitrate induced reductions to oxygen transport.

Air breathing in the Arctic: influence of temperature, hypoxia, activity and restricted air access on respiratory physiology of the Alaska blackfish Dallia pectoralis

The Alaska blackfish (Dallia pectoralis) is an air-breathing fish native to Alaska and the Bering Sea islands, where it inhabits lakes that are ice-covered in the winter, but enters warm and hypoxic waters in the summer to forage and reproduce. To understand the respiratory physiology of this species under these conditions and the selective pressures that maintain the ability to breathe air, we acclimated fish to 5°C and 15°C and used respirometry to measure: standard oxygen uptake (Ṁ(O₂)) in normoxia (19.8 kPa P(O₂)) and hypoxia (2.5 kPa), with and without access to air; partitioning of standard Ṁ(O₂) in normoxia and hypoxia; maximum Ṁ(O₂) and partitioning after exercise; and critical oxygen tension (P(crit)). Additionally, the effects of temperature acclimation on haematocrit, haemoglobin oxygen affinity and gill morphology were assessed. Standard Ṁ(O₂) was higher, but air breathing was not increased, at 15°C or after exercise at both temperatures. Fish acclimated to 5°C or 15°C increased air breathing to compensate and fully maintain standard Ṁ(O₂) in hypoxia. Fish were able to maintain Ṁ(O₂) through aquatic respiration when air was denied in normoxia, but when air was denied in hypoxia, standard Ṁ(O₂) was reduced by ∼30-50%. P(crit) was relatively high (5 kPa) and there were no differences in P(crit), gill morphology, haematocrit or haemoglobin oxygen affinity at the two temperatures. Therefore, Alaska blackfish depends on air breathing in hypoxia and additional mechanisms must thus be utilised to survive hypoxic submergence during the winter, such as hypoxia-induced enhancement in the capacities for carrying and binding blood oxygen, behavioural avoidance of hypoxia and suppression of metabolic rate.

Respiratory responses to progressive hypoxia in the Amazonian oscar, Astronotus ocellatus

This study determined the respiratory responses to progressive hypoxia in oscar, an extremely hypoxia-tolerant Amazonian cichlid. Oscar depressed oxygen consumption rates (MO2), beginning at a critical O2 tension (Pcrit) of 46Torr, to only 14% of normoxic rates at 10Torr. Total ventilation (Vw) increased up to 4-fold, entirely due to a rise in ventilatory stroke volume (no change in ventilatory frequency), and water convection requirement (Vw/MO2) increased substantially (up to 15-fold). Gill O2 extraction fell steadily, from 60% down to 40%. Although O2 transfer factor (an index of gill O2 diffusion capacity) increased transiently in moderate hypoxia, it decreased at 10Torr, which may have caused the increased expired-arterial PO2 difference. Venous PO2 was always very low (< or =7Torr). Anaerobic metabolism made a significant contribution to ATP supply, indicated by a 3-fold increase in plasma lactate that resulted in an uncompensated metabolic acidosis. Respiration of isolated gill cells was not inhibited until below 5Torr; because gill water PO2 always exceeded this value, hypoxic ion flux arrest in oscars [Wood et al., Am. J. Physiol. Reg. Integr. Comp. Physiol. 292, R2048-R2058, 2007] is probably not caused by O2 limitation in ionocytes. We conclude that metabolic depression and tolerance of anaerobic bi-products, rather than a superior capacity for O2 supply, allow oscar to thrive in extreme hypoxia in the Amazon.

Whole Blood–Oxygen Binding Properties of Four Cold‐Temperate Marine Fishes: Blood Affinity Is Independent of pH‐Dependent Binding, Routine Swimming Performance, and Environmental Hypoxia

The relationship between whole blood-oxygen affinity (P(50)) and pH-dependent binding (i.e., cooperativity and the Bohr [ Phi ] and Root effects) was examined statistically under standardized conditions (10.0 degrees Celsius) in four unrelated cold-temperate marine fishes that differ widely in their swimming performance and their expected responses to hypoxia: cod (Gadus morhua), herring (Clupea harengus), mackerel (Scomber scombrus), and plaice (Pleuronectes platessa). An unexpected difference in blood-oxygen affinity was found (herring>plaice>mackerel>cod), and this was independent of both swimming performance and the predicted low O(2) response of each species. The ecotype of the four marine species was also unrelated to pH-dependent binding because no difference in the Bohr effect was apparent ( Phi varied insignificantly from -0.90 to -1.06), and differences in the magnitude of the cooperative binding reaction were associated only with the presence of the Root effect. Although several reviews propose a generalized link between blood-oxygen affinity and pH-dependent binding, our results advise against overestimating the adaptive functional properties of hemoglobin across unrelated species.

Pulmonary Oxygen Diffusing Capacity of the South American LungfishLepidosiren paradoxa: Physiological Values by the Bohr Method

Lungfish (Dipnoi) may represent the sister group to all land vertebrates and are therefore important for reconstructing the conquest of land by tetrapods. We determined venous and arterial blood gases, pulmonary O(2) uptake, and the form of the hemoglobin-O(2) dissociation curves in the South American lungfish Lepidosiren paradoxa. Measurements were performed at 25 degrees and 35 degrees C. Based on this information, we calculated its pulmonary O(2) diffusing capacity (D(L)O(2)), using the Bohr integration procedure. D(L)O(2) increased with temperature to reach about 0.04 mL stpd kg(-1) min(-1) mmHg(-1) at 35 degrees C. This value represents about 40% of the morphometric diffusing capacity and is similar to physiological values in some amphibians and reptiles.

Cardiorespiratory responses of the common carp (Cyprinus carpio) to severe hypoxia at three acclimation temperatures

In vivo measurements of the cardiovascular responses of anoxia-tolerant teleosts to severe prolonged hypoxia are limited. Here, we report the first direct measurements of cardiac output (Q), heart rate (fH) and stroke volume during prolonged severe hypoxia (&lt;0.3 mg O2 l–1) in common carp (Cyprinus carpio L.) that had been acclimated to 6, 10 and 15°C. While routine Q and fH values varied with temperature under normoxic conditions (Q10 values of 1.7 and 2.6, respectively), severe hypoxic exposure significantly depressed fH and Q to similar minimum values that were largely independent of acclimation temperature (Q10 values of 1.2). In contrast, the duration of cardiac depression and the subsequent time period during which carp could tolerate severe hypoxia were inversely related to acclimation temperature (24 h at 6°C, 6 h at 10°C, and 2.5 h at 6°C). Likewise, respiration rate during hypoxia showed a temperature dependence. An unusual finding was that cardiorespiratory status partially recovered during the latter stages of severe hypoxic exposure. We conclude that the cardiorespiratory responses to severe prolonged hypoxia in common carp involved a mixture of temperature-independent, temperature-dependent and time domain phases.

Catecholamine-induced changes in oxygen affinity of carp and trout blood

Carp and trout blood maintained at low constant oxygen and carbon dioxide tensions was beta-stimulated. This activated the Na+/H(+)-exchanger of the red cell membrane, leading to increases in red cell pH (pHi) and cell water content, the latter resulting in dilution of hemoglobin and organic phosphates. The increase in pHi was rapid and maintained throughout the experimental period, the trout red cells showing the largest increase. Likewise swelling of the red cells was larger in trout than in carp blood. As a consequence of beta-stimulation the oxygen affinity of the blood increased. In trout the intracellular Bohr factor of unstimulated blood combined with the pHi increase upon stimulation could account for 85% of the increase in oxygen affinity, whereas it only covered 65% of the increase in carp blood. We therefore conclude that blood oxygen affinity is dependent on the red cell hemoglobin concentration in both species, the effect being more marked in carp.

Influence of blood sample collection on the haematocrit value of two teleosts: Rainbow trout (Oncorhynchus mykiss) and european sea bass (Dicentrarchus labrax L.)

1. The effects of the volume of blood extracted, number of extractions, and weight upon the haematocrit value of a freshwater teleost (Oncorhynchus mykiss) and a marine one (Dicentrarchus labrax) have been studied. 2. For trout with the same weight, the increase in the volume of blood extracted results in a significant increase in the haematocrit value. 3. For the same volume of blood extracted--in trout of the same weight--the second extraction results in a significant increase in the haematocrit value. 4. In sea bass, haematocrit variations in relation to the aforementioned parameters are not produced. 5. For both species there is a positive and significant correlation between weight (W) and haematocrit value (Ht), which coincides with the Ht = aWb model, in the range of weights used in this study.

Modest effect of temperature on the porcine oxygen dissociation curve

It has been generally assumed that normally endothermic mammals have hemoglobins with greater temperature sensitivity than ectotherms or hibernating mammals. We found the pig to be an exception to this rule. We measured 101 dissociation curves using biotonometry on fresh heparinized blood from 9 pigs at 4 temperatures. The partial pressures of O2 at 50% saturation (P50 +/- SE) were 27.8 +/- 1.2, 30.0 +/- 1.3, 35.7 +/- 0.6 and 41.6 +/- 1.8 mm Hg at 30, 33, 37, and 41 degrees C, respectively. The temperature coefficient d log P50/dT was 0.016 +/- 0.002, about two-thirds that of human and dog blood. It was also saturation dependent, being significantly greater at lower saturations than at high saturations. This saturation dependence causes an increase in heme-heme cooperativity in binding oxygen at higher temperatures. The fixed acid Bohr coefficient was -0.441 +/- 0.005 at 37 degrees C and was not temperature sensitive. We conclude that the effect of temperature on the porcine dissociation curve is significantly lower than that reported for other endotherms, and is similar to that previously reported for hibernating mammals and some ectotherms.

H+ and Cl- ion equilibrium across the red cell membrane in the carp

H2O and electrolyte distribution were studied in carp erythrocytes at various pH values achieved by CO2/O2 equilibration in vitro. Intracellular pH was measured by means of glass electrode and by DMO-14C. rH+, rCl- and rDMO- varied linearly and in a comparable manner with pHe. At pHe = 7.8, rH+, rCl- and rDMO- were 0.21, 0.29 and 0.30. rCl- and rDMO- were closely correlated and exhibited only minor differences. rH+ was closely correlated with rCl- and rDMO-, but was, however, significantly lower than rCl- or rDMO-. This difference is considered to be due to a systematic error of the glass electrode when used in highly concentrated protein solutions. The coulometric determination of chloride in packed red cells is shown to be highly susceptible to protein. The results are consistent with the assumption that H+ and Cl- ions are passively distributed across the red cell membrane.

Temperature acclimation and oxygen binding properties of blood of the European eel, Anguilla anguilla

Temperature acclimation of the European eel, Anguilla anguilla, resulted in red cell GTP/Hb molar ratios of 1.20, 1.77 and 0.80 at 2, 17 and 29 degrees C, respectively. A small increase in blood oxygen capacity was present in 29 degrees C acclimated eels. The CO2 Bohr effect and the shape of the oxygen binding curve (n-Hill) were invariant with both temperature and GTP/Hb. The significant differences in the GTP/Hb ratio corresponded with a strong enhancement of the temperature effect on blood oxygen affinity between 2 and 17 degrees C and a similarly strong compensation between 17 and 29 degrees C. Predicted in vivo P50 values were 3.0, 13.8 and 17.6 mmHg at 2 degrees C, 17 and 29 degrees C, respectively. The adaptational value of these findings are discussed in relation to standard metabolic rates at the various temperatures. A tentative hypothesis is proposed that the present study confirms and expands earlier work and supports the contention that adjustments in blood oxygen affinity of thermally acclimated teleosts serve to provide them with an unloading O2 tension for diffusion closely matching the standard oxygen requirements at the various temperatures.

Effect of acclimation temperature on intraerythrocytic acid-base balance and nucleoside triphosphates in the carp, Cyprinus carpio

In carp acclimated to 20 degrees C or 10 degrees C intraerythrocytic pH (pHi) and plasma pH (pHe) were determined in vitro after equilibration with CO2 in either O2 or N2. ATP and GTP were determined with an enzymatic assay described in detail. The relationship between pHi, pHe and oxygen saturation was not affected by the acclimation temperature and was (pHi-6.10) = (0.853-0.159 X S) X (pHe-6.21) There was a slight but significant decrease in ATP at 20 degrees C. Apparent buffer values were affected by oxygenation and temperature. It is concluded from the recalculated CO2 Bohr factor and from the temperature effect on the buffer value that carp hemoglobin forms carbamate which decreases at a higher temperature. These changes in ATP and carbamate can partly account for the increase in whole blood oxygen affinity in carp acclimated at a high temperature.