Hypoxic Fish Research Articles

Implications of chronic hypoxia during development in red drum.

Respiratory plasticity is a beneficial response to chronic hypoxia in fish. Red drum, a teleost that commonly experiences hypoxia in the Gulf of Mexico, have shown respiratory plasticity following sublethal hypoxia exposure as juveniles, but implications of hypoxia exposure during development are unknown. We exposed red drum embryos to hypoxia (40% air saturation) or normoxia (100% air saturation) for 3 days post fertilization (dpf). This time frame encompasses hatch and exogenous feeding. At 3 dpf, there was no difference in survival or changes in size. After the 3-day hypoxia exposure, all larvae were moved and reared in common normoxic conditions. Fish were reared for ∼3 months and effects of the developmental hypoxia exposure on swim performance and whole-animal aerobic metabolism were measured. We used a cross design wherein fish from normoxia (N=24) were exercised in swim tunnels in both hypoxia (40%, n=12) and normoxia (100%, n=12) conditions, and likewise for hypoxia-exposed fish (n=10 in each group). Oxygen consumption, critical swim speed (Ucrit), critical oxygen threshold (Pcrit) and mitochondrial respiration were measured. Hypoxia-exposed fish had higher aerobic scope, maximum metabolic rate, and higher liver mitochondrial efficiency relative to control fish in normoxia. Interestingly, hypoxia-exposed fish showed increased hypoxia sensitivity (higher Pcrit) and recruited burst swimming at lower swim speeds relative to control fish. These data provide evidence that early hypoxia exposure leads to a complex response in later life.

Hypoxia induces pyroptosis and inflammation in the liver of fat greenling (Hexagrammos otakii)

In fisheries, hypoxia stress is one of the most common environmental stresses that often cause significant harm and losses to the aquaculture industry. The liver is the most important metabolic organ of the body, which is involved in energy metabolism, synthesizes various physiologically active substances, and is an important target organ for hypoxia in fish. This study investigated how acute hypoxia stress induces liver injury and mortality of Hexagrammos otakii. During the hypoxia treatment, reactive oxygen species (ROS), a metabolic intermediary made by the mitochondrial respiratory chain, are highly accumulated, the level of lipid peroxidation increased significantly in the liver tissue. The vacuolation of liver histiocyte is enhanced due to hypoxia. The downstream gene hypoxia-inducible factor (HIF)-1 can react to the elevated ROS in liver tissue. HIF-1α expression levels greatly increase when hypoxia is applied, which prompts the activation of the nlrp3/caspase-1/il-1β and caspase-3/gsdme pathway and encourages the inflammation and pyroptosis of the liver. After reoxygenation, nlrp3/caspase-1/il-1β pathway are reactivated and cause liver damage in H. otakii. In conclusion, it has been shown that hypoxia-stress modulates the expression of HIF-1 through an increase in ROS production, which then encourages the activation of the nlrp3/caspase-1/il-1β and caspase-3/gsdme pathway to cause liver injury. This research sheds fresh light on the mechanism of hypoxia-induced pyroptosis and demonstrates the usefulness of the ROS-HIF-1 signaling pathway as a possible therapeutic target to halt the progression of hypoxia-induced illnesses.

Individual variation in sublethal tolerance of warming and hypoxia in the pacu Piaractus mesopotamicus: an investigation of correlations and dependence on intrinsic metabolic phenotype

We evaluated interindividual variation in traits of warming and hypoxia tolerance in a cohort (n = 24) of juvenile pacu Piaractus mesopotamicus, acclimated to 26°C, to investigate whether individuals tolerant to warming were also tolerant to hypoxia and whether individual tolerance depended on body mass or intrinsic traits of aerobic metabolism. Two traits of warming tolerance were measured, the critical thermal maximum (CTmax) in a static tank, with loss of equilibrium as the endpoint, and the critical thermal maximum for aerobic swimming (CTSmax) in a swim tunnel, with fatigue as the endpoint. Two traits of hypoxia tolerance were derived by static respirometry during progressive hypoxia, the critical saturation for regulation of standard metabolic rate (Scrit) and the regulation index (RI). At 39.7 ± 0.4°C (mean ± SD), CTmax was significantly higher than CTSmax, at 38.0 ± 1.0°C. Both traits had very low coefficient of variation within the cohort (CV, 1.1 and 2.6%, respectively), and CTmax was not correlated with any other trait. Individual Scrit (18.5 ± 7.2% saturation, CV 38.9%) and RI (76.0 ± 16.1% regulation, CV 45.6%) were correlated (R = 0.686), and both were significantly correlated with CTSmax (R = 0.472 and 0.475, respectively). This indicates that individuals tolerant to warming were also tolerant to hypoxia, although two individuals with low CTSmax were drivers of the correlations against Scrit and RI. The CTmax, CTSmax, and Scrit showed no dependence on body mass, but RI increased with mass (R2 = 0.286), indicating that larger individuals were more tolerant to hypoxia. When corrected for body mass, RI was no longer correlated with CTSmax, further revealing that the correlations were rather tenuous. Finally, we found no evidence that individual tolerance was dependent on intrinsic traits of aerobic metabolism. The results indicate that sublethal indicators, such as CTSmax, Scrit, and RI, can be useful in exploring correlations among traits of tolerance to warming and hypoxia in fishes, but more studies on more species with larger sample sizes are required to confirm these results and reveal if there are general patterns.

Integrated analysis of transcriptome, translatome and proteome reveals insights into yellow catfish (Pelteobagrus fulvidraco) brain in response to hypoxia

Brain plays a central role in adapting to environmental changes and is highly sensitive to the oxygen level. Although previous studies investigated the molecular response of brain exposure to acute hypoxia in fish, the lack of studies at the translational level hinders further understanding of the regulatory mechanism response to hypoxia from multi-omics levels. Yellow catfish (Pelteobagrus fulvidraco) is an important freshwater aquaculture species; however, hypoxia severely restricts the sustainable development of its breeding industry. In the present study, the transcriptome, translatome, and proteome were integrated to study the global landscapes of yellow catfish brain response to hypoxia. The evidently increased amount of cerebral cortical cells with oedema and pyknotic nuclei has been observed in hypoxia group of yellow catfish. A total of 2750 genes were significantly changed at the translational level. Comparative transcriptional and translational analysis suggested the HIF-1 signaling pathway, autophagy and glycolysis/gluconeogenesis were up-regulated after hypoxia exposure. KEGG enrichment of translational efficiency (TE) differential genes suggested that the lysosome and autophagy were highly enriched. Our result showed that yellow catfish tends to inhibit the TE of genes by increasing the translation of uORFs to adapt to hypoxia. Correlation analysis showed that transcriptome and translatome exhibit higher correlation. In summary, this study demonstrated that hypoxia dysregulated the cerebral function of yellow catfish at the transcriptome, translatome, and proteome, which provides a better understanding of hypoxia adaptation in teleost.

Hypoxia-induced HIF-1α promotes Listeria monocytogenes invasion into tilapia.

HIF-1α is a nuclear transcription factor, and its activity is tightly regulated by the level of available oxygen in cells. Here, we investigated the roles of HIF-1α in the invasion of Listeria monocytogenes into tilapia under hypoxic environments. We found that the expression levels of HIF-1α in examined tissues of hypoxic tilapia were significantly upregulated, indicating that the tissue cells have been in hypoxic conditions. After 24-h infection with L. monocytogenes, we found that bacterial burden counts increased significantly in all examined tissues of hypoxic fish. To explore why the bacterial count increased significantly in the tissues of hypoxic fish, we modulated HIF-1α expression through RNAi technology. The results indicated that c-Met expression levels were positively related to HIF-1α expression. Since c-Met is the receptor of InlB that plays critical roles in the adhesion and invasion of L. monocytogenes, the ∆InlB strain was used to further explore the reason for the significant increase in bacterial counts in hypoxic fish. As expected, the decrease in the adhesion ability of ∆InlB suggested that InlB mediates L. monocytogenes infection in tilapia. After being infected with ∆InlB strain, we found that the bacterial counts in hypoxic fish were not affected by hypoxic conditions or HIF-1α expression levels. These findings indicate that HIF-1α may promote the internalization of InlB by upregulating c-Met expression and therefore contributes to the invasion of L. monocytogenes into hypoxic tilapia. IMPORTANCE Listeria monocytogenes is a zoonotic food-borne bacterial pathogen with a solid pathogenicity for humans. After ingestion of highly contaminated food, L. monocytogenes is able to cross the intestine invading phagocytic and nonphagocytic cells and causes listeriosis. China is the world's largest supplier of tilapia. The contamination rate of L. monocytogenes to tilapia products was as high as 2.81%, causing a severe threat to public health. This study revealed the underlying regulatory mechanisms of HIF-1α in the invasion of L. monocytogenes into tilapia under hypoxic environments. This study will be helpful for better understanding the molecular mechanisms of hypoxic environments in L. monocytogenes infection to tilapia. More importantly, our data will provide novel insights into the prevention and control of this pathogen in aquaculture.

Hypoxia activates autophagy by Akt/FoxO1 pathway in fish cells

Hypoxia, a common environmental condition, can affect cell survival and physiological function by triggering oxidative stress. Akt/FoxO pathway has been proven to play a non-negligible role in the regulation of autophagy. However, the role of Akt/FoxO pathway in hypoxia-induced autophagy is unclear in fish. Therefore, in this study, grass carp hepatocyte cells (L8824) were treated by CoCl2 to simulate hypoxia, and the results showed that CoCl2 can increase the expression of Hif-1α protein at different concentrations or different treatment time. Further study found that hypoxia increased intracellular reactive oxygen species (ROS) level, and the expression of autophagy-related genes (LC3-II, pink1, beclin-1 and p62) and foxO1a/1b. The mitochondrial membrane potential (Δψm) was also depolarized, and autophagosomes were intriguingly detected by transmission electron microscope (TEM) after the treatment of hypoxia. Moreover, hypoxia inhibited Akt phosphorylation, while PI3K/Akt pathway inhibitor, LY294002 significantly up-regulated the expression of foxO1a/1b and autophagy-related genes. Additionally, silencing foxo1b also resulted in down-regulation of autophagy-related genes. It was demonstrated that hypoxia induced autophagy via Akt/FoxO1 pathway. These results will provide a new light on further understanding the role of Akt/FoxO pathway in the response to hypoxia in fish.

Melatonin integrates multidimensional regulation of Na+/K+-ATPase in ionocytes and promotes stress and ease response in hypoxia-induced air-breathing fish: lessons from integrative approach.

As circadian regulator, melatonin is involved in many physiological processes including ionosmotic regulation in fishes. Na+/K+-ATPase (NKA), an ubiquitous Na+/K+ transporter in ionocyte epithelia that drives electrochemical Na+ gradients and systemic osmotic integration, is a target of stress in fish. However, it is not certain how melatonin regulates NKA functions in ionocyte epithelia and how it modulates the adaptive response such as stress and ease response in fish particularly in hypoxia condition. We, thus, examined the short-term in vivo action of melatonin on the dynamics of NKA regulation in branchial, renal and intestinal ionocytes of hypoxia-induced air-breathing fish (Anabas testudineus Bloch). Interestingly, we found a rise in plasma melatonin in fish when kept for 30min of forced submergence in water and that indicates a role for melatonin in hypoxia tolerance. A fall in blood [Na+ , K+] occurred in these hypoxic fish which later showed a recovery after melatonin treatment. Similarly, melatonin favored the fall in NKA activity in branchial and renal epithelia of hypoxic fish, though it remarkably stimulated its activities in non-stressed fish. Likewise, melatonin that produced differential pattern of mRNA expression in nkaα1-subunit isoforms (nkaα1a, nkaα1b and nkaα1c) and melatonin receptor isoforms (mtnr1a, mtnr1bb, mtnr1bb x1x2 ) in the tested ionocyte epithelia, showed reversed expression in hypoxic fish. In addition, the rise in NKAα-protein abundance in branchial and renal epithelia of melatonin-treated hypoxic fish indicated a recovery action of melatonin. A higher NKAα-immunoreactivity was found in the immunohistochemical and immunofluorescent images of branchial ionocytes and renal proximal and distal ionocytes of hypoxic fish treated with melatonin. Furthermore, an activation of PKA and PKG-dependent phosphorylation was found in branchial epithelia of hypoxic fish. The generated integrative parabola model showed that melatonin has a maximum targeted action on NKA function in the renal epithelia, suggesting its lead role in the integration of ionosmotic balance during the recovery or ease response. Over all, the data indicate a multidimensional and preferential action of melatonin on NKA regulation in fish ionocytes that integrate the recovery action against hypoxia, thus pointing to a major role for melatonin in stress and ease response in this fish.

The air-breathing Alaska blackfish (Dallia pectoralis) suppresses brain mitochondrial reactive oxygen species to survive cold hypoxic winters

The Alaska blackfish (Dallia pectoralis) is the only air-breathing fish in the Arctic. In the summer, a modified esophagus allows the fish to extract oxygen from the air, but this behavior is not possible in the winter because of ice and snow cover. The lack of oxygen (hypoxia) and near freezing temperatures in winter is expected to severely compromise metabolism, and yet remarkably, overwintering Alaska blackfish remain active. To maintain energy balance in the brain and limit the accumulation of reactive oxygen species (ROS), we hypothesized that cold hypoxic conditions would trigger brain mitochondrial remodeling in the Alaska blackfish. To address this hypothesis, fish were acclimated to warm (15 °C) normoxia, cold (5 °C) normoxia or cold hypoxia (5 °C, 2.1–4.2 kPa; no air access) for 5–8 weeks. Mitochondrial respiration, ADP affinity and H202 production were measured at 10 °C in isolated brain homogenates with an Oroboros respirometer. Cold acclimation and chronic hypoxia had no effects on mitochondrial aerobic capacity or ADP affinity. However, cold acclimation in normoxia led to a suppression of brain mitochondrial H202 production, which persisted and became more pronounced in the cold hypoxic fish. Overall, our study suggests cold acclimation supresses ROS production in Alaska blackfish, which may protect the fish from oxidative stress when oxygen becomes limited during winter.

Establishment of the first cell line from the small yellow croaker (Larimichthys polyactis) and its application in unraveling the mechanism of ROS-induced apoptosis under hypoxia

Cell lines play an important role in studying biological processes in vitro. In this study, we aimed to establish and characterize the first cell line isolated from the small yellow croaker (Larimichthys polyactis), an economically important marine fish in China. The small yellow croaker fry (SYCF) cells proliferated well in Leibovitz's 15 medium containing 2% antibiotics, 0.1 M NaCl, and 10% fetal bovine serum at 27 °C. Sequence analyses of mitochondrial genes confirmed the origin of SYCF cells. Furthermore, we unraveled the mechanism of hypoxia-induced apoptosis, a major threat to aquatic animals using the SYCF cell line. The HIF-PHD pathway of the SYCF cells can be activated under hypoxic stress, suggesting that this cell line is impressible for hypoxia and can serve as a superb model in vitro to study the cell physiology in L. polyactis in a hypoxic setting. We found that hypoxia results in morphological abnormalities, excess reactive oxygen species (ROS) production, and dysfunctional redox homeostasis, indicating the occurrence of oxidative stress in the SYCF cells. In addition, decreased cell viability and severe apoptosis were detected under hypoxic conditions. Through monitoring the indicators of the intrinsic and extrinsic apoptosis pathways, including the mitochondrial membrane potential, the expression of bax, bcl2, bnip3, and fas, Bcl2, and Caspase-3 protein, and activity of Caspases-3/8/9, we found that both pathways are involved. Considering that ROS-induced oxidative stress may be the main reason of apoptosis, we investigated whether the addition of an antioxidant, N-Acetyl-L-cysteine (NAC), will ameliorate the apoptosis. Interestingly, we found that both intrinsic and extrinsic apoptosis indicators were markedly recovered by adding NAC in culture media, significant remissions were also observed in the cell viability and apoptosis rate. These results suggested that hypoxia-induced apoptosis of SYCF cells may be mediated by the overproduction of ROS, and the antioxidant may be a useful material to relieve cell damage under hypoxia in fish. In general, this study established a marine fish cell line of SYCF cells, which can be used as an in vitro research model of L. polyactis and for revealing the mechanisms of ROS-induced apoptosis under hypoxic conditions in this species. This provides important insights into the cellular physiology of marine fish under hypoxia.

Characterizing the hypoxic performance of a fish using a new metric: PAAS-50.

The hypoxic constraint on peak oxygen uptake (ṀO2,peak) was characterized in rainbow trout over a range of ambient oxygen tensions with different testing protocols and statistical models. The best-fit model was selected using both statistical criteria (R2 and AIC) and the model's prediction of three anchor points for hypoxic performance: critical PO2(Pcrit), maximum ṀO2and a new metric, the minimum PO2that supports 50% of absolute aerobic scope (PAAS-50). The best-fitting model was curvilinear using five strategically selected PO2values. This model predicted PAAS-50 as 70 mmHg (coefficient of variation, CV=9%) for rainbow trout. Thus, while a five-point hypoxic performance curve can characterize the limiting effects of hypoxia in fish, as envisaged by Fry over 75 years ago, PAAS-50 is a promising metric to compare hypoxic constraints on performance in a standardized manner both within and across fish species.

Metabolomic Profiling Analysis of Physiological Responses to Acute Hypoxia and Reoxygenation in Juvenile Qingtian Paddy Field Carp Cyprinus Carpio Var Qingtianensis.

The Qingtian paddy field carp (Cyprinus carpio var qingtianensis) is a local carp cultivated in the rice field of Qingtian county, Zhejiang province, China. The paddy field environment is distinct from the pond environment. Due to the inability to artificially increase oxygen, the dissolved oxygen greatly changes during the day. Therefore, investigating the physiological regulation to the changes of acute dissolved oxygen in Qingtian paddy field carp (PF-carp) will dramatically clarify how it adapts to the paddy breeding environment. The high tolerance of Qingtian paddy field carp to hypoxia makes it an ideal organism for studying molecular regulatory mechanisms during hypoxia process and reoxygenation following hypoxia in fish. In this study, we compared the changes of metabolites in the hepatopancreas during hypoxia stress and the following reoxygenation through comparative metabolomics. The results showed 131 differentially expressed metabolites between the hypoxic groups and control groups. Among them, 95 were up-regulated, and 36 were down-regulated. KEGG Pathway enrichment analysis showed that these differential metabolites were mainly involved in regulating lipid, protein, and purine metabolism PF-carps could require energy during hypoxia by enhancing the gluconeogenesis pathway with core glutamic acid and glutamine metabolism. A total of 63 differentially expressed metabolites were screened by a comparison between the reoxygenated groups and the hypoxic groups. Specifically, 15 were up-regulated, and 48 were down-regulated. The KEGG Pathway enrichment analysis supported that PF-carp could continue to gain energy by consuming glutamic acid and the glutamine accumulated during hypoxia and simultaneously weaken the ammonia-transferring effect of amino acids and the toxicity of ammonia. By consuming glycerophospholipids and maintaining the Prostaglandin E content, cell damage was improved, sphingosinol synthesis was reduced, and apoptosis was inhibited. Additionally, it could enhance the salvage synthesis and de novo synthesis of purine, reduce purine accumulation, promote the synthesis of nucleotide and energy carriers, and assist in recovering physiological metabolism. Overall, results explained the physiological regulation mechanism of PF-carp adapting to the acute changes of dissolved oxygen at the metabolic level and also provided novel evidence for physiological regulation of other fish in an environment with acute changes in dissolved oxygen levels.

Different patterns of chronic hypoxia lead to hierarchical adaptive mechanisms in goldfish metabolism.

Some hypoxia-tolerant species, such as goldfish, experience intermittent and severe hypoxia in their natural habitat, causing them to develop multiple physiological adaptations. However, in fish, the metabolic impact of regular hypoxic exposure on swimming performance in normoxia is less well understood. Therefore, we experimentally tested whether chronic exposure to constant (30 days at 10% air saturation) or intermittent hypoxia (3 h in normoxia and 21 h in hypoxia, 5 days a week) would result in similar metabolic and swimming performance benefits after reoxygenation. Moreover, half of the normoxic and intermittent hypoxic fish were put on a 20-day normoxic training regime. After these treatments, metabolic rate (standard and maximum metabolic rates: SMR and MMR) and swimming performance [critical swimming speed (Ucrit) and cost of transport (COT)] were assessed. In addition, enzyme activities [citrate synthase (CS), cytochrome c oxidase (COX) and lactate dehydrogenase (LDH)] and mitochondrial respiration were examined in red muscle fibres. We found that acclimation to constant hypoxia resulted in (1) metabolic suppression (-45% SMR and -27% MMR), (2) increased anaerobic capacity (+117% LDH), (3) improved swimming performance (+80% Ucrit, -71% COT) and (4) no changes at the mitochondrial level. Conversely, the enhancement of swimming performance was reduced following acclimation to intermittent hypoxia (+45% Ucrit, -41% COT), with a 55% decrease in aerobic scope, despite a significant increase in oxidative metabolism (+201% COX, +49% CS). This study demonstrates that constant hypoxia leads to the greatest benefit in swimming performance and that mitochondrial metabolic adjustments only provide minor help in coping with hypoxia.

Hypoxic hagfish gills binge calcium

Slimy, tentacle-faced hagfish look like aliens and, accordingly, possess several PhD theses’ worth of unusual traits. Their body fluids have a similar number of dissolved salts to seawater, although because hagfish control the levels of some ions like calcium in their blood, the mix of salts is slightly different. Hagfish are also famously tolerant of low oxygen conditions (hypoxia), which comes in handy when they burrow inside rotting whales to dine. In some fishes, hypoxia disrupts how they manage the delicate balance between their internal salts and the salts in their surroundings as they cut back on the energy-intensive process of ion movement to save energy. But what about hagfish with their unusual balance of salts? And what happens when oxygen is scarce? Chris Glover of Athabasca University, Canada, and Greg Goss of the University of Alberta, Canada, decided to find out.Glover and Goss hypothesized that their slimy beasts would do the same as other fish when oxygen was in short supply: decreasing calcium uptake and accumulation. They tested this idea by placing hagfish in air-tight jars and tracking the movement of mildly radioactive calcium into their tissues as they used up the available oxygen.Unlike the hagfish in open jars, the hagfish in sealed jars moved calcium quickly into most of their tissues, including skin, liver, muscle, heart, plasma and brain. Low oxygen levels also bumped up the rates of calcium build-up in their gut and especially their gills; the guts of the fish in the low oxygen conditions had nearly 4 times more calcium than the guts of the fish in oxygen-rich conditions after adjusting for calcium contained in the blood flowing through the tissue. Since hagfish have no bones to leech calcium from, these results meant that, contrary to expectations, hagfish responded to hypoxia by importing more calcium into their bodies.When an animal does something you don't expect, the next questions are usually: how and why? Tackling the first question, Glover and Goss focused on how calcium moved across hagfish skin and gut tissue when it was isolated from the rest of the body with the goal of understanding the role each tissue played in the hypoxic fish's calcium binge.They discovered that low oxygen in the environment reduced calcium uptake by the skin, but only when the experiment was performed in unnaturally low calcium conditions, so this was unlikely to be a major mechanism of calcium accumulation in living animals. However, the results in the gut tissue were more puzzling. Despite surging calcium levels in the gut under hypoxia, there were no differences in the calcium uptake rates between the guts of the hagfish that had been kept in normal levels of oxygen and those that were oxygen starved. The authors mused that perhaps the sealed-in hagfish had gulped down seawater as a reaction to the stress, accidently flushing the tissue with calcium in the process. With the skin and gut ruled out as major importers of calcium into the body, the researchers determined that the gills were the source of the calcium detected in the closed-jar hagfish, which squares with the impressive rate of calcium accumulation observed in this tissue.Hagfish are peculiar creatures, so it's perhaps unsurprising that they respond so differently from most other earthbound fishes. As for why hypoxic hagfish pull in so much calcium through their gills, that remains a mystery. Calcium could possibly aid in coping with low oxygen conditions by constricting blood vessels or protecting brains cells. Alternatively, the calcium accumulation might not be a strategy at all and could simply be a result of hagfish moving more water – and consequently calcium – through their gills; but that's a study for another time.

Field trials to determine the efficacy of aerators to mitigate hypoxia in inland waterways

Following a series of fish deaths in the lower Darling River in the austral summer of 2018–19, several field trials were undertaken over two summers to determine the efficacy of various aerators to mitigate the effects of hypoxia to prevent fish deaths. The aerators evaluated included: low-powered, solar bubble-plume diffusers; high-flow, single-port Venturi systems; pontoon-mounted multi-port Venturi systems; and an ultra-fine bubble generator. The effectiveness of bubble diffusers appears related to their power, the number of units used and their running time. Pontoon-mounted multi-port Venturi aerators and high-flow, single-port Venturi aerators were much more successful in maintaining oxygen levels and may have prevented fish deaths in the Darling River in early 2019. However, the Venturi aerators were expensive to run, consuming hundreds to thousands of litres of diesel fuel per week. Trial of an ultra-fine oxygen bubble pump showed promise because it supersaturated the water with dissolved oxygen while not elevating total gas pressure to levels that would cause gas-bubble trauma in fish. Although aerators may play a role in emergency responses to hypoxic fish deaths, they have limitations regarding their area of influence (at best thousands of square metres) and their cost to deploy and operate.

Molecular Characterization and Expression of Cytochrome P450 Aromatase in Atlantic Croaker Brain: Regulation by Antioxidant Status and Nitric Oxide Synthase During Hypoxia Stress.

We have previously shown that nitric oxide synthase (NOS, an enzyme) is significantly increased during hypoxic stress in Atlantic croaker brains and modulated by an antioxidant (AOX). However, the influence of NOS and AOX on cytochrome P450 aromatase (AROM, CYP19a1, an enzyme) activity on vertebrate brains during hypoxic stress is largely unknown. In this study, we characterized brain AROM (bAROM, CYP19a1b) cDNA in croaker and examined the interactive effects of hypoxia and a NOS-inhibitor or AOX on AROM activity. The amino acid sequence of croaker bAROM cDNA is highly homologous (76–80%) to other marine teleost bAROM cDNAs. Both real-time PCR and Northern blot analyses showed that bAROM transcript (size: ∼2.8 kb) is highly expressed in the preoptic-anterior hypothalamus (POAH). Hypoxia exposure (dissolved oxygen, DO: 1.7 mg/L for 4 weeks) caused significant decreases in hypothalamic AROM activity, bAROM mRNA and protein expressions. Hypothalamic AROM activity and mRNA levels were also decreased by pharmacological treatment with N-ethylmaleimide (NEM, an alkylating drug that modifies sulfhydryl groups) of fish exposed to normoxic (DO: ∼6.5 mg/L) conditions. On the other hand, treatments with Nω-nitro-L-arginine methyl ester (NAME, a competitive NOS-inhibitor) or vitamin-E (Vit-E, a powerful AOX) prevented the downregulation of hypothalamic AROM activity and mRNA levels in hypoxic fish. Moreover, NAME and Vit-E treatments also restored gonadal growth in hypoxic fish. Double-labeled immunohistochemistry results showed that AROM and NOS proteins are co-expressed with NADPH oxidase (generates superoxide anion) in the POAH. Collectively, these results suggest that the hypoxia-induced downregulation of AROM activity in teleost brains is influenced by neuronal NOS activity and AOX status. The present study provides, to the best of our knowledge, the first evidence of restoration of AROM levels in vertebrate brains by a competitive NOS-inhibitor and potent AOX during hypoxic stress.

Ecological adaptations of Amazonian fishes acquired during evolution under environmental variations in dissolved oxygen: A review of responses to hypoxia in fishes, featuring the hypoxia-tolerant Astronotus spp.

The Amazon Basin presents a dynamic regime of dissolved oxygen (DO) oscillations, which varies among habitats within the basin, including spatially, daily, and seasonally. Fish species inhabiting these environments have developed many physiological adaptations to deal with the frequent and periodic events of low (hypoxia), or no (anoxia) DO in the water. Cichlid fishes, especially the genus Astronotus (A. ocellatus and A. crassipinnis), are hypoxic-tolerant species that can survive in very low DO levels for long periods, while adults often inhabit places where DO is close to zero. The present review will focus on some metabolic adjustments that Amazonian fish use in response to hypoxic conditions, which include many strategies from behavioral, morphological, physiological, and biochemical strategies. These strategies include ASR (aerial surface respiration), lip expansion, branchial tissue remodeling, increases in glycolytic metabolism with the increase of blood glucose levels, and increases in anaerobic metabolism with increases of plasma lactate levels. Other groups over evolutionary time developed obligate aerial respiration with changes in pharyngeal and swim bladder vascularization as well as the development of a true lung. However, most species are water-breathing species, such as A. ocellatus and A. crassipinnis, which are detailed in this study because they are used as hypoxia-tolerant model fish. Herein, we draw together the literature data of the physiological mechanisms by which these species decrease aerobic metabolism and increase anaerobic metabolism to survive hypoxia. This is the first attempt to synthesize the physiological mechanisms of the hypoxia-tolerant Astronotus species.

Histopathological, hematological, and biochemical changes in high-latitude fish Phoxinus lagowskii exposed to hypoxia.

Hypoxia is one of the most significant threats to biodiversity in aquatic systems. The ability of high-latitude fish to tolerate hypoxia with histological and physiological responses is mostly unknown. We address this knowledge gap by investigating the effects of exposures to different oxygen levels using Phoxinus lagowskii (a high-latitude, cold-water fish) as a model. Fish were exposed to different oxygen levels (0.5 mg/L and 3 mg/L) for 24 h. The loss of equilibrium (LOE), an indicator of acute hypoxia tolerance, was 0.21 ± 0.01 mg/L, revealing the ability of fish to tolerate low-oxygen conditions. We sought to determine if, in P. lagowskii, the histology of gills and liver, blood indicators, enzyme activities of carbohydrate and lipid metabolism, and antioxidants changed to relieve stress in response to acute hypoxia. Notably, changes in vigorous jumping behavior under low oxygen revealed the exceptional hypoxia acclimation response compared with other low-latitude fish. A decrease in blood parameters, including RBC, WBC, and Hb, as well as an increase in MCV was observed compared to the controls. The increased total area in lamella and decreased ILCM volume in P. lagowskii gills were detected in the present study. Our results also showed the size of vacuoles in the livers of the hypoxic fish shrunk. Interestingly, an increase in the enzyme activity of lipid metabolism but not glucose metabolism was observed in the groups exposed to hypoxia at 6 h and 24 h. After combining histology and physiology results, our findings provide evidence that lipid metabolism plays a crucial role in enhancing hypoxia acclimation in P. lagowskii. Additionally, SOD activity significantly increased during hypoxia, suggesting the presence of an antioxidant response of P. lagowskii during hypoxia. High expression levels of lipogenesis and lipolysis-related genes were detected in the 6 h 3 mg/L and 24 h 3 mg/L hypoxia group. Enhanced expression of lipid-metabolism genes (ALS4, PGC-1, and FASN) was detected during hypoxia exposure. Together, these data suggest that P. lagowskii's ability to tolerate hypoxic events is likely mediated by a comprehensive strategy.

Research on sablefish (Anoplopoma fimbria) suggests that limited capacity to increase heart function leaves hypoxic fish susceptible to heat waves.

Studies of heart function and metabolism have been used to predict the impact of global warming on fish survival and distribution, and their susceptibility to acute and chronic temperature increases. Yet, despite the fact that hypoxia and high temperatures often co-occur, only one study has examined the effects of hypoxia on fish thermal tolerance, and the consequences of hypoxia for fish cardiac responses to acute warming have not been investigated. We report that sablefish (Anoplopoma fimbria) did not increase heart rate or cardiac output when warmed while hypoxic, and that this response was associated with reductions in maximum O2 consumption and thermal tolerance (CTmax) of 66% and approximately 3°C, respectively. Further, acclimation to hypoxia for four to six months did not substantially alter the sablefish's temperature-dependent physiological responses or improve its CTmax. These results provide novel, and compelling, evidence that hypoxia can impair the cardiac and metabolic response to increased temperatures in fish, and suggest that some coastal species may be more vulnerable to climate change-related heat waves than previously thought. Further, they support research showing that cross-tolerance and physiological plasticity in fish following hypoxia acclimation are limited.

Exposure to Nitrate Increases Susceptibility to Hypoxia in Fish.

AbstractAquatic hypoxic events are increasing in frequency and intensity as concentrations of nutrients, such as nitrate, continue to rise from human activities. Many fish species can alter their behavior and physiology to cope with drops in oxygen, but these compensatory strategies may be compromised under high levels of nitrate pollution. Hence, we investigated whether exposure to elevated nitrate concentrations affects key behavioral (avoidance and aquatic surface respiration [ASR]) and physiological (hemoglobin and hematocrit levels, ventilation frequency, and burst and prolonged swimming performance) responses of fish to mitigate the impacts of acute hypoxia. Juvenile silver perch (Bidyanus bidyanus) were exposed to one of three nitrate concentrations (0, 50, or 100 mg NO3- L-1) for 3 wk, after which behavioral and physiological responses of fish to progressive hypoxia were assessed. Fish exposed to nitrate utilized ASR at a higher threshold of partial pressure of oxygen during progressive hypoxia compared with control animals but did not alter behavioral avoidance of low oxygen levels. In these nitrate-exposed fish, the early onset of ASR behaviors is likely a behavioral, compensatory strategy to cope with nitrate-induced physiological disruptions, namely, increases in ventilation frequency and lower levels of hemoglobin and hematocrit. The physiological constraints placed by nitrate and acute hypoxia exposures manifested to lower the swimming performance of silver perch. Collectively, these data suggest that exposure to elevated nitrate is likely to disrupt key behavioral and physiological strategies used by fish to cope with short-term hypoxia.

Yucca schidigeraextract mediated the growth performance, hepato‐renal function, antioxidative status and histopathological alterations in Nile tilapia (Oreochromis niloticus) exposed to hypoxia stress

The present study investigated influence of Yucca schidigera extract (Ys E) on the growth performance, hepato-renal function, antioxidative status and histopathological alterations in Nile tilapia (Oreochromis niloticus) exposed to hypoxia stress. 120 Nile tilapia fish (42.39 ± 1.25 g) were distributed into four equal groups, where group (a) kept as a control group, group (b) supplied with YsE (8 mg/L water every 2 days), group (c) exposed to hypoxia after 2 weeks from the start of the experiment and group (d) provided with YsE for 2 weeks and then exposed to hypoxia. The treatments were conducted for 30 days. The results showed that the level of DO was significantly decreased (2.2 ± 0.61) in groups of fish exposed to hypoxia when compared to other groups (p < 0.05). Notably, the pre-supplementation with YsE to hypoxic stressed fish positively affected the WG, SGR, FCR and PER, relieved stress picture of leukogram, reduced the elevated hepatic marker enzymes: ALT, AST and LDH. Furthermore, hypoxic fish pre-treated with YsE succeeded to decline the increased urea, glucose levels and the elevated serum amlyase and lipase enzyme activities. In addition to improving total proteins, albumin, total cholesterol, triglycerides, VLDL-C and HDL-C declined levels compared to hypoxic fish. Meanwhile hypoxia-stressed fish exhibited significantly enhanced hepatic GPX, SOD and GSH and inhibited hepatic MDA content compared to fish supplied with YsE then exposed to hypoxia. Fish group pre-supplemented with YsE then exposed to hypoxic stress condition was protected against the pathological changes of the liver, spleen, brain and gills tissues induced by hypoxic stress condition. These results suggest that YsE could exert a beneficial effect against hypoxia-induced oxidative stress.