Living in a hypoxic world: A review of the impacts of hypoxia on aquaculture

Effects of hypoxia on survival, apoptosis, and the transcriptome of the Chinese yellow pond turtle (Mauremys mutica)

Mauremys mutica is a widely cultured pond turtle in China that can hold its breath underwater for up to 7 h. However, the adaptive mechanism of this hypoxia-resistant phenotype remains unknown. In the present study, no M. mutica died until 6 h of hypoxic stress. Inflammation was observed in the lungs of dead individuals along with a lung cell apoptosis rate of 12.3% in the death group, which was about six times that of the survival group. Transcriptome sequencing produced 379,659,748 clean reads, and 38,981 genes were obtained. A total of 1566, 1555 and 756 differentially expressed genes (DEGs) were detected between the survival group and the death group, the survival group and the control group, and the death group and the control group, respectively. And the DEGs were enriched in the inflammation, apoptosis, sugar transport, antioxidant, fructose, and mannose metabolism, arginine and proline metabolism, glycosaminoglycan biosynthesis, P53, and MAPK signaling pathways. This study offers new insight into the molecular mechanisms occurring in the lungs of M. mutica during acute hypoxia, which may facilitate genetic selection for hypoxia-resistant lines inM. mutica.

Effects of hypoxia on the reproductive endocrine axis of the pejerrey (Odontesthes bonariensis)

Recently, hypoxic areas have been identified in water bodies of the Pampas region due to human activity. The objective of this work was to study the effect of low concentrations of dissolved oxygen (hypoxia) on the reproductive endocrine axis of a pampas fish (Odontesthes bonariensis). Groups of 8 males and 8 females were subjected to severe hypoxia (2-3 mg l-1) and normoxia (7-9 mg l-1) in 3000 l tanks by duplicate during the reproductive season (spring). After 21 days, 4 males and 4 females from each tank were sacrificed, and blood was drawn to measure estradiol (E2) and testosterone (T). The brain, pituitary gland and a portion of the gonads were extracted and processed to measure the expression of: gnrh1, cyp19a1b, fshβ, lhβ, fshr, lhcgr and cyp19a1a. From the second experimental week, no spawning was found in the hypoxic females, while at the end of the treatment period no male released sperm. Fish under hypoxic conditions showed signs of gonadal regression, reduction of GSI and plasma levels of sex steroids. Furthermore, the expression of gnrh1 in both sexes, cyp19a1b and fshr in males and only fshβ and cyp19a1a in females decreased in comparison with normoxic fish. After 40 days under normal conditions, signs of reproductive recovery were observed in the treated fish. The results obtained demonstrated that hypoxia generated an inhibition of some components of the pejerrey's reproductive endocrine axis, but the effect was reversible.

Differential susceptibility to hypoxia in hypoxia-inducible factor 1-alpha (HIF-1α)-targeted freshwater water flea Daphnia magna mutants

The water flea, Daphnia magna, serves as a key model organism for investigating the response of aquatic organisms to environmental stressors, including hypoxia. Hypoxia-inducible factor 1-alpha (HIF-1α) is a central regulatory protein involved in the cellular response to hypoxic conditions. In this study, we used CRISPR/Cas9 gene editing to create D. magna mutant lines with targeted alterations in the HIF-1α gene. Mutants demonstrated decreased survival and reproductive output and down-regulated genes for the HIF-1α-mediated pathway in low-oxygen conditions. These findings suggest that the HIF-1α pathway is a critical component of resistance to hypoxia in D. magna. This study provides novel insights into the molecular basis of hypoxia tolerance of HIF-1α in D. magna and expands our understanding of how aquatic organisms can adapt to or be challenged by changing oxygen levels in the face of global environmental changes.

Long-term hypoxia-induced physiological response in turbot Scophthalmus maximus L

Hypoxia affects fish's survival, growth, and physiological metabolism processes. In this study, turbot plasma glucose and cortisol contents, hepatic glycolysis (hexokinase [HK], phosphofructokinase [PFK], pyruvate kinase [PK]) and lipolysis (fatty acid synthetase [FAS], lipoprotein lipase [LPL]) enzyme activities, anti-oxidant enzyme (superoxide dismutase [SOD], catalase [CAT], glutathione peroxidase [GSH-Px]) activities, malondialdehyde (MDA), lactate and glycogen contents, gill histological parameters (lamellar length [SLL], width [SLW], interlamellar distance [ID]), respiratory frequency (RF), the proportion of the secondary lamellae available for gas exchange (PAGE), and hifs (hif-1α, hif-2α, hif-3α) expression were determined during long-term hypoxia and reoxygenation. Results showed that long-term hypoxia (3.34 ± 0.17 mg L-1) significantly elevated plasma cortisol and glucose contents; increased hepatic HK, PK, PFK, FAS, and LPL activity; decreased hepatic glycogen, lactate contents, and lipid drop numbers; and caused changes of hepatocyte (vacuolation, pyknotic, and lytic nucleus) after treatment for 4 weeks. Hepatic SOD, CAT, GSH-Px activity, and MDA contents; lamellar perimeter, SLL, ID, RF, and PAGE; and hepatic hif-1α, hif-2α, and hif-3α manifested similar results. Meanwhile, hif-1α is significantly higher than hif-2α, and hif-3α. Interestingly, females and males demonstrated no sex dimorphism significantly different from the above parameters (except hepatic FAS, LPL activity, and lipid drop number) under hypoxia. The above parameters recovered to normal levels after reoxygenation treatment for 4 weeks. Thus, long-term hypoxia promotes turbot hepatic glycogenolysis and lipolysis, induces oxidative damage and stimulates hepatic antioxidant capacity, and alters gill morphology to satisfy insufficient energy demand and alleviate potential damage, while hif-1α plays critical roles in the above physiological process.

Acute hypoxia stress mediates HIF-1α-Yki-Cactus axis to facilitate the infection of Vibrio parahaemolyticus in Litopenaeus vannamei

Introduction

Hypoxia stress renders aquatic animals more susceptible to bacterial disease, yet the underlying mechanism remains elusive.

Methods

We conducted an acute hypoxia stress experiment to investigate the impact of stress on the immune response of Litopenaeus vannamei via transcriptome analysis, RT-qPCR and Western blot.

Results

Our results showed that acute hypoxia stress disrupted the tissue architecture, and significantly changed the gene expression profiles in the hepatopancreas of shrimp. More importantly, acute hypoxia stress significantly changed the expression levels of immune-related genes. Ladderlectin, GBP 1, Caspase-1, CLEC4F, MR1 and GBP 2 were significantly down-regulated, but HIF-1α, Cactus, TIPE, Akirin-2, Ivns1abp and TLR3 were significantly up-regulated. We further demonstrated that acute hypoxia activated Yki via HIF-1α to enhance expression level of Cactus, and then Cactus inhibited the phosphorylation of Dorsal and its nuclear translocation, thereby suppressing antibacterial immunity. Subsequently, the challenge experiment following stress revealed that exposure to acute hypoxia stress amplified the infectivity and lethality of Vibrio parahaemolyticus to shrimp. The mechanism of HIF-1α-Yki-Cautus axis provided an explanation for this phenomenon.

Discussion

This study offered new insights into interactions among environmental hypoxia stress, host immunity and pathogens, thereby providing practical guidelines for optimizing shrimp culture practices.

Decoding the role of HIF-1α in immunoregulation in Litopenaeus vannamei under hypoxic stress

Hypoxia poses a significant challenge to aquatic organisms, especially Litopenaeus vannamei (L. vannamei), which play a vital role in the global aquaculture industry. Hypoxia-inducible factor 1α (HIF-1α) is a pivotal regulator of the organism's adaptation to hypoxic conditions. To understand of how HIF-1α affects the immunity of L. vannamei under hypoxic conditions, we conducted a thorough study involving various approaches. These included observing tissue morphology, analyzing the expression of immune-related genes, assessing the activities of immune-related enzymes, and exploring immune-related pathways. Our study revealed that RNA interference (RNAi)-mediated knockdown of HIF-1α markedly reduced HIF-1α expression in the gill (75-95 %), whereas the reduction ranged from 2 to 43 % in the hepatopancreas. Knockdown of HIF-1α resulted in increased damage to both gill and hepatopancreatic tissues in hypoxic conditions. Additionally, immune-related genes, including Astakine (AST), Hemocyanin (HC), and Ferritin (FT), as well as immune-related enzymes such as Acid Phosphatase (ACP), Alkaline Phosphatase (AKP), and Phenoloxidase (PO), exhibited intricate regulatory patterns in response to hypoxia stress following the knockdown of HIF-1α. Transcriptome analysis revealed that HIF-1α knockdown significantly impacts multiple signaling pathways, including the JAK-STAT signaling pathway, Th17 cell differentiation pathways, PI3K-Akt signaling pathway, ErbB signaling pathway, MAPK signaling pathway, chemokine signaling pathway, ribosomal pathways, apoptosis, lysosomes and arachidonic acid metabolism. These alterations disrupt the organism's immune balance and interfere with normal metabolic processes, potentially leading to various immune-related diseases. We speculate that the weakened immune response resulting from HIF-1 inhibition is due to the reduced metabolic capacity, and the existence of a direct regulatory relationship between them requires further exploration. This study greatly advances our understanding of the vital role that HIF-1α plays in regulating immune responses in shrimp under hypoxic conditions, thereby deepening our comprehension of this critical biological mechanism.

Hypoxia induces reversible gill remodeling in largemouth bass (Micropterus salmoides) through integrins-mediated cell adhesion

Gill remodeling is an important strategy for fish to cope with hypoxia, and many of the teleost possess this ability, but the underlying mechanism is not well understood. To investigate the mechanism of hypoxia-induced gill remodeling, largemouth bass (Micropterus salmoides) exposed to hypoxia (dissolved oxygen level: 2.0 ± 0.2 mg L-1) for 7 days, followed by 7 days of reoxygenation. Hypoxia tests were also performed on primary gill cells from largemouth bass. We found that hypoxia-induced gill remodeling increased the respiratory surface area of the gills. This change in gill morphology was reversible and recovered after reoxygenation. A reduction in the number of mucous cells and rearrangement of mitochondria-rich cells (MRCs) were observed during gill remodeling. After 7 days of reoxygenation, the number of mucous cells and the position of the MRCs were restored. Hypoxia resulted in a 2.92-fold increase in the number of primary gill cells that underwent migration over a 12-h period. The mRNA levels of nine integrin subunits (α1, α2, α5, α7, α8, α10, αL, β1 and β2) were significantly up-regulated after 12 h of hypoxia in vivo, and the changes in the expression of these subunits were consistent with the HIF-1α trend. Immunohistochemistry showed that integrin β1 protein levels were significantly increased and were abundantly expressed in the interlamellar cell mass after exposure to hypoxia. Taken together, the results of the present study demonstrated that changes in mucosal cells and MRCs play an important role in hypoxia-induced gill remodeling in largemouth bass and that these changes are regulated by integrins.

Enhancing water quality monitoring through the integration of deep learning neural networks and fuzzy method

The escalating growth of the global population has led to degraded water quality, particularly in seawater environments. Water quality monitoring is crucial to understanding the dynamic changes and implementing effective management strategies. In this study, water samples from the southwestern regions of Iran were spatially analyzed in a GIS environment using geostatistical methods. Subsequently, a water quality map was generated employing large and small fuzzy membership functions. Additionally, advanced prediction models using neural networks were employed to forecast future water pollution trends. Fuzzy method results indicated higher pollution levels in the northern regions of the study area compared to the southern parts. Furthermore, the water quality prediction models demonstrated that the LSTM model exhibited superior predictive performance (R2 = 0.93, RMSE = 0.007). The findings also underscore the impact of urbanization, power plant construction (2010 to 2020), and inadequate urban wastewater management on water pollution in the studied region.

Effect of hypoxia on the post-hatching growth of the body of the fry and the caudal fin of the Atlantic Salmon (Salmo salar)

Introduction

Hypoxia is a recurring problem in the fish farming industry. Currently, it is known that the exposure of fish and fry to a hypoxic environment induces important changes in their metabolism, compromising not only their development but also their reproduction and mortality rates. Our hypothesis is that hypoxia constitutes one of the etiological factors causing deformation of the body and caudal fin in this species, as well as affecting its growth.

Methods

We analyzed two hundred forty Salmo salar salmon fry, differentially cultured at 100% saturation (normoxia condition) and 60% (hypoxia condition) for 2, 4, 6, and 8 days, including a group under continuous hypoxia. We performed diaphanization and Alcian blue staining, along with standard histological techniques. The polyclonal anti-HIF-1a antibody was used as a marker of hypoxia in Salmo salar, and hypoxia in these fish was associated with the immunopositivity of this antibody.

Results and discussion

The results indicate that there is an association between exposure to hypoxia and the deformation of the body and fin, as well as an agreement between hypoxia and the total length of the fry and fin. Several months after the event occurred, we were able to find and describe angiogenesis, blood vessel disorganization, and vasodilation histologically. Finally, hypoxic cells in the fry (HIF-1a) could be recognized and confirmed as hypoxia sensors. All of this indicates that hypoxia not only affects the fry during the development phase of the event, but that its results can be evident much later and affect the fry throughout their entire ontogeny.

Impact of anthropogenic global hypoxia on the physiological response of bivalves

Dissolved oxygen (DO) is an important parameter that affects the biology, physiology, and immunology of aquatic animals. In recent decades, DO levels in the global oceans have sharply decreased, partly due to an increase in atmospheric carbon dioxide, temperature, and anthropogenic nutrient loads. Although there have been many reports on the effects of hypoxia on the survival, growth, behavior, and immunity of bivalves, this information has not been well organized. Therefore, this article provides a comprehensive review of the effects of hypoxia on bivalves. In general, hypoxia negatively impacts the food consumption rate and assimilation efficiency, as well as increasing respiration rates in many bivalves. As a result, it reduces the energy allocation for bivalve growth, shell formation, and reproduction. In severe cases, prolonged exposure to hypoxia can result in mass mortality in bivalves. Moreover, hypoxia also has adverse effects on the immunity and response of bivalves to predators, including decreased burial depths, sensitivity to predators, impairment of byssus production, and negatively impacts on the integrity, strength, and composition of bivalve shells. The tolerance of bivalves to hypoxia largely depends on size and species, with larger bivalves being more susceptible to hypoxia and intertidal species being relatively more tolerant to hypoxia. The information in this article is very useful for elucidating the current research status of hypoxia on bivalves and determining future research directions.

Wetland Destruction in a Headwater River Leads to Disturbing Decline of In-stream Nitrogen Removal

Wetlands have long been recognized as efficient nitrogen (N) processing systems. While widespread interest is in constructing wetlands to mitigate N pollution, there is a dearth of information about the environmental consequences following wetland dismantlement. This study elucidated the changing trajectories of water quality and N removal capacity in a headwater river that initially contained a series of constructed wetlands but later underwent wetland destruction. An estimated 17% surge in total N concentration has been reported since the wetlands' destruction. This adverse trend is primarily attributed to a weakened in-stream N removal capacity, which was reduced to a mere 25% of the levels observed when the wetlands were operational. Further analysis confirms that the presence of wetlands actively shapes desirable environmental settings for N processing. In stark contrast, wetland destruction leads to unfavorable environmental conditions, which not only restrain in-stream anaerobic metabolisms but also trigger algal proliferation and biological N fixation. Collectively, this research provides compelling evidence of the detrimental consequences associated with wetland destruction, emphasizing the need for remedial strategies to mitigate these negative effects.