Physiological and molecular responses to hypoxia stress in Manila clam Ruditapes philippinarum

Ovarian toxicity of 2,6-di-tert-butyl-hydroxytoluene on female Ruditapes philippinarum: Reproductive endocrine disruption and oxidative stress

Synthetic phenolic antioxidants (SPAs) are the most widely used antioxidants in the world. There is a growing concern due to their potential toxic effects and high pollution levels in aquatic environments. Existing studies have confirmed the neurotoxic, immunotoxic, and developmental toxicity of SPAs on aquatic organisms. However, there is limited research on the reproductive toxicity of SPAs, particularly in aquatic invertebrates. In this study, female Ruditapes philippinarum were selected as research objects to investigate the reproductive toxicity effects of typical SPAs 2,6-di-tert-butyl hydroxytoluene (BHT) on clams at different reproductive stages. The results showed that BHT downregulated the level of ovarian steroid hormones by disrupting steroid production, and showed anti-estrogenic effects. This interference impedes meiosis, follicular development, and ovulation, resulting in a decrease in the number of mature oocytes and gonadal index. Furthermore, exposure to BHT increased ROS levels and suppressed antioxidant defenses, resulting in biomacromolecular damage. BHT also induced apoptosis, ferroptosis, and pyroptosis in ovarian cells, impairing ovarian development. Collectively, this study elucidates the potential molecular mechanisms of reproductive toxicity caused by BHT in bivalve shellfish, focusing on endocrine disruption, oxidative damage, and cell death pathways. The findings provide data supporting the conservation of marine shellfish germplasm.

EPO Enhances Adaptation to Hypoxic Environment in the Freshwater Teleost (Micropterus salmoides) through the PI3K/AKT Pathway

Hypoxia has become one of the most common environmental stress events in the life history of aquatic organisms due to accelerated global warming. Exploring the adaptation mechanisms of aquatic organisms in hypoxic environments is important to deepen our understanding of environmental toxicology and to design breeding programs. In this study, the largemouth bass Micropterus salmoides exhibited greater hypoxic adaptability after 4 weeks of intermittent hypoxic exposure (IHE), with the O2 tension for loss of equilibrium decreased from 1.17 ± 0.20 to 0.66 ± 0.10 mg/L. Combined transcriptomics, biochemical detection, and immunostaining results revealed that the hypoxia-tolerant phenotype driven by IHE was strongly correlated with the activation of erythropoietin (EPO). EPO promoted phosphoinositide-3-kinase (PI3K)/protein kinase B (AKT) signaling to alleviate hepatic damage under acute hypoxic exposure (AHE) by selectively regulating the expression of genes related to antioxidant defense, antiapoptosis, and cell proliferation, which plays an important role in regulating hypoxic adaptation. The inhibition of EPO impaired cell survival in hypoxic environments, but intervention with the PI3K agonist 740 Y-P reversed this process. This novel finding provides insights into exploring how aquatic organisms cope with the challenges of hypoxia under increasing environmental risks.

Marine mussel metabolism under stress: Dual effects of nanoplastics and coastal hypoxia

Emerging challenges in marine environments include nanoplastics (NPs) pollution and coastal hypoxia. Although NPs toxicity in marine organisms is being increasingly documented, the complex interactions between coastal hypoxia and NPs remain largely unexplored. This study investigated the dual effects of polystyrene nanoplastics and different oxygen levels on redox homeostasis and bioenergetics in the marine model organism Mytilus galloprovincialis. Both NPs and hypoxia significantly disrupted redox homeostasis in mussels. Exposure to NPs alone increased electron transport chain activity, whereas exposure to hypoxia alone and co-exposure significantly reduced this activity. Metabolomic analysis showed that NPs primarily affected the pentose phosphate pathway (PPP), tricarboxylic acid (TCA) cycle, and amino acid metabolism; hypoxia exposure alone disrupted the TCA cycle, pyruvate metabolism, and glycolysis/gluconeogenesis, whereas combined exposure notably altered the TCA cycle, PPP, and sugar interconversion. This suggests that regulating these pathways would help mussels cope with the combined environmental stress. Furthermore, co-exposure severely disrupted redox homeostasis and energy metabolism in mussels, suggesting that hypoxia exacerbates NPs toxicity. We believe that these new findings would enhance our understanding of the compounded ecological risks posed by NPs in the context of climate change.

Sub-acute exposure of sea urchin (Strongylocentrotus intermedius) to environmentally relevant concentrations of PFOA and GenX influences gonadal development

Perfluorooctanoic acid (PFOA) and its substitute, hexafluoropropylene oxide dimer acid (GenX), are widely used perfluorinated compounds (PFCs) that pose significant risks to marine ecosystems. However, the specific impacts of these contaminants on marine invertebrates, particularly echinoderms, remain poorly understood. Strongylocentrotus intermedius, a globally significant benthic aquacultural species, may be potentially affected by PFCs. This study aimed to assess the reproductive toxicity of PFOA and GenX in S. intermedius. After exposing S. intermedius to either PFOA or GenX for 7 or 14 days, it was observed that even at environmentally relevant concentrations (2 μg/L), both compounds inhibited normal growth and gonadal development in S. intermedius, with effects becoming more pronounced over time. Further analysis revealed that prolonged exposure to PFCs resulted in a significant reduction in energy reserves (glycogen, lipids, and proteins) and caused abnormal changes in metabolic enzyme activities, with PFOA exhibiting more pronounced effects compared to GenX. At the genetic level, the expression of genes related to gonadal development initially increased and then decreased as the concentrations of the compounds rose. Additionally, integrated biomarker response analysis indicated that PFOA had greater reproductive toxicity than GenX, in terms of both concentration and exposure duration. These results provided a preliminary evaluation of the impact of PFCs on marine invertebrates, offering a foundation for further research into their ecological risks and contributing to the development of more comprehensive environmental risk assessments for these contaminants.

Understanding the probiotic potential of Lactobacillus plantarum: Antioxidant capacity, non-specific immunity and intestinal microbiota improvement effects on Manila clam Ruditapes philippinarum

Lactic acid bacteria (LAB) have beneficial effects on aquatic animals, improving their immune system and intestinal microbiota. Nevertheless, the probiotic effects of LAB on the Manila clam Ruditapes philippinarum remain poorly understood. Herein, the effects of administering Lactobacillus plantarum at final doses of 1 × 105 CFU/L (T5 group), 1 × 107 CFU/L (T7 group), and 1 × 109 CFU/L (T9 group) in the rearing water for eight weeks were evaluated for the antioxidant capacity, non-specific immunity, resistance to Vibrio parahaemolyticus infection, and intestinal microbiota of R. philippinarum. The rearing water without the addition of L. plantarum served as a control. The results showed that the T7 and T9 groups demonstrated a significant elevation in the disease resistance of clams against V. parahaemolyticus, in the activities of alkaline phosphatase and lysozyme in the hepatopancreas, and in the expression of antioxidant- and immune-related genes, including SOD, GPx, and GST. Meanwhile, the T7 group showed a significant enhancement in superoxide dismutase and catalase activities and CAT expression, while the T9 group experienced a remarkable elevation in reduced glutathione content. Only catalase activity was markedly elevated in the T5 group. The expression of SOD, CAT, GPx, and GST was significantly elevated in three treatment groups following the V. parahaemolyticus challenge. The T7 group exhibited a significant increase in intestinal microbiota richness. Significant increases were noted in Firmicutes abundance across all three treatment groups and in Actinobacteriota in the T5 and T7 groups. Additionally, the opportunistic pathogen Escherichia-Shigella abundance significantly decreased in three treatment groups. Furthermore, administration of 1 × 107 CFU/L L. plantarum enhanced the stability of the intestinal microecosystem, whereas a dose of 1 × 109 CFU/L might have a negative effect. The application of three doses of L. plantarum significantly enhanced intestinal microbiota functions related to the immune response and oxidative stress regulation, while a higher dose (1 × 109 CFU/L) might inhibit several functions. In conclusion, the application of L. plantarum in the rearing water exerted beneficial effects on the antioxidant capacity, non-specific immunity, resistance to V. parahaemolyticus, and the intestinal microbiota stability and functions of R. philippinarum. The beneficial effects of L. plantarum on R. philippinarum were dose-dependent, and the final dose of 1 × 107 CFU/L exhibited the optimal effects.

Antioxidant Capacity, Enzyme Activities Related to Energy Metabolism, and Transcriptome Analysis of Crassostrea hongkongensis Exposed to Hypoxia

Crassostrea hongkongensis (C. hongkongensis) is one of the three most commonly cultivated oyster species in China. Seasonal hypoxia is one of the most serious threats to its metabolism, reproductive behavior, and survival. To investigate the effects of hypoxia stress on the antioxidant capacity and energy metabolism of C. hongkongensis, the total antioxidant capacity (T-AOC), glycogen content, and enzyme activities (phosphofructokinase, PFK; pyruvate kinase, PK; phosphoenolpyruvate carboxykinase, PEPCK) of oysters were determined under normoxic (DO 6 ± 0.2 mg/L) and hypoxic (DO 1.5 mg/L) conditions at 0 h, 6 h, 48 h, and 72 h. We also determined the T-AOC, glycogen content, and enzyme activities of oysters under reoxygenation (recovered to normoxia for 24 h). To further examine the potential molecular regulatory mechanism of hypoxic adaptation, a transcriptome analysis was conducted on the gill of C. hongkongensis under normoxia (N, 72 h), hypoxia (H, 72 h), and reoxygenation (R). After being exposed to hypoxia for 6 h, the T-AOC, glycogen content, and enzyme activities of PK, PFK, and PEPCK in C. hongkongensis were significantly decreased. However, after prolonging the duration of hypoxia exposure for 72 h, the T-AOC, glycogen content, and enzyme activities increased compared to that of 48 h. After 24 h reoxygenation, the T-AOC, glycogen content, and enzyme activity of PK and PFK returned to close to initial levels. In addition, a transcriptome analysis discovered 6097 novel genes by mapping the C. hongkongensis genome with the clean reads. In total, 352 differentially expressed genes (DEGs) were identified in the H vs. N comparison group (235 upregulated and 117 downregulated genes). After recovery to normoxia, 292 DEGs (134 upregulated and 158 downregulated genes) were identified in the R vs. N comparison group, and 632 DEGs were identified (253 upregulated and 379 downregulated genes) in the R vs. H comparison group. The DEGs included some hypoxia-tolerant genes, such as phosphoenolpyruvate carboxykinase (PEPCK), mitochondrial (AOX), tyramine beta-hydroxylase (TBH), superoxide dismutase (SOD), glutathione S-transferase (GST), and egl nine homolog 1 isoform X2 (EGLN1). Additionally, DEGs were significantly enriched in the KEGG pathways that are involved in hypoxia tolerance, including the metabolism of xenobiotics by cytochrome P450 pathways and the HIF-1 signaling pathway. Then, we selected the five hypoxic-tolerant candidate DEGs for real-time quantitative polymerase chain reaction (RT-qPCR) validation, and the results were consistent with the transcriptome sequencing data. These discoveries have increased our understanding of hypoxia tolerance, recovery ability after reoxygenation, and molecular mechanisms governing the responses to hypoxia in C. hongkongensis.

In Vitro Toxicity and Modeling Reveal Nanoplastic Effects on Marine Bivalves

Nanoplastics (NPs) represent a growing concern for global environmental health, particularly in marine ecosystems where they predominantly accumulate. The impact of NPs on marine benthic organisms, such as bivalves, raises critical questions regarding ecological integrity and food safety. Traditional methods for assessing NP toxicity are often limited by their time-intensive nature and ethical considerations. Herein, we explore the toxicological effects of NPs on the marine bivalve Ruditapes philippinarum, employing a combination of in vitro cellular assays and advanced modeling techniques. Results indicate a range of adverse effects at the organismal level, including growth inhibition (69.5-108%), oxidative stress, lipid peroxidation, and DNA damage in bivalves, following exposure to NPs at concentrations in the range of 1.6 × 109-1.6 × 1011 particles/mL (p/mL). Interestingly, the growth inhibition predicted by models (54.7-104%), based on in vitro cellular proliferation assays, shows strong agreement with the in vivo outcomes of NP exposure. Furthermore, we establish a clear correlation between cytotoxicity observed in vitro and the toxicological responses at the organismal level. Taken together, this work suggests that the integration of computational modeling with in vitro toxicity assays can predict the detrimental effects of NPs on bivalves, offering insightful references for assessing the environmental risk assessment of NPs in marine benthic ecosystems.

Transcriptional and physiological profiles reveal the respiratory, antioxidant and metabolic adaption to intermittent hypoxia in the clam Tegillarca granosa

Tegillarca granosa can survive intermittent hypoxia for a long-term. We used the clam T. granosa as model to investigate the respiratory, antioxidant and metabolic responses to consecutive hypoxia-reoxygenation (H/R) stress at both physiological and transcriptional levels. The results showed that the clams were able to rapidly regulate oxygen consumption and ammonia excretion during H/R stress, and alleviate oxidative stress during the second-time challenge. The clams also efficiently balanced energy metabolism through the rapid conversion and decomposition of glycogen. According to the transcriptome profile, KEGG pathways of starch and sucrose metabolism, ECM-receptor interaction, and protein processing in endoplasmic reticulum were significantly enriched in H group (the second-time 24 h hypoxia exposure), while pathways associated with lipid metabolism were significantly enriched in h group (the first-time 24 h hypoxia exposure). DEGs including hspa5, birc2/3, and map3k5 might play important roles in alleviating endoplasmic reticulum stress, cpla2 and pla2g16 might mitigate oxidative stress by adjusting the composition of cellular membrane. In conclusions, our findings suggest that rapid adjustment of oxygen consumption, ammonia metabolism, glycogen metabolism, and the ability to adjust the composition of the membrane lipid may be critical for T. granosa in maintaining energy homeostasis and reducing oxidative damage during intermittent H/R exposure. This study preliminarily clarified the response of T. granosa to intermittent hypoxia stress on the physiological and molecular levels, offering insights into the hypoxia-tolerant mechanisms in this species and providing a reference for the following study on the other hypoxic-tolerant species.

Effects of acute hypoxia and reoxygenation on histological structure, antioxidant response, and apoptosis in razor clam Sinonovacula constricta

Hypoxia is one of the major environmental problems limiting the healthy development of intensive aquaculture. Marine benthic shellfish are encountering heightened problems related to hypoxic stress as a result of ongoing human activities and aquaculture operations. Razor clam Sinonovacula constricta, a commercially valuable shellfish, has not yet been reported in studies on physiological changes caused by hypoxia and reoxygenation. To understand the negative effects of hypoxia and reoxygenation on the clams, we set up two low-oxygen concentration groups (DO 2.0 mg/L and DO 0.5 mg/L) and assessed multiple aspects of oxidative damage to their hepatopancreas and gills. After the hypoxic stress, the two tissues of the razor clam suffered varying degrees of damage, including cell degeneration and disruption of mitochondrial cristae. After reoxygenation, the 2.0 mg/L group recovered substantially, but the clams in the 0.5 mg/L group still unrecovered. The activities of antioxidant enzymes (MDA, T-AOC, SOD, GPX, and CAT) in clams were considerably altered by acute hypoxia and reoxygenation. Briefly, there was a growing and then declining trend in MDA, T-AOC, and SOD activities in the hepatopancreas, whereas GPX and CAT activities showed the converse trend. In the hepatopancreas and gills, the level of anti-apoptotic gene Bcl-2 transcripts gradually decreased with the duration of hypoxia and increased following reoxygenation. However, changes in the transcript level of the pro-apoptotic gene Bax were in contrast to that of Bcl-2. The TUNEL assay revealed that hypoxia caused apoptosis. Furthermore, at DO 0.5 mg/L, the degree of apoptosis was more significant than at DO 2.0 mg/L, and hepatopancreatic apoptosis was more severe than gill apoptosis. Collectively, our findings imply that hypoxia induces oxidative stress, histological damage, and apoptosis in razor clams in a concentration-dependent and tissue-specific manner. These consequences serve as a reminder that prolonged recovery periods may be required for razor clams to fully recover from oxidative damage resulting from hypoxia-reoxygenation episodes.

Metabolic adaptation of the clam Ruditapes philippinarum during air exposure and the positive effects of sodium nitroprusside pretreatment

The Manila clam (Ruditapes philippinarum), as one of the shellfish living in the intertidal zone, is known for its strong ability to withstand air exposure. Sodium nitroprusside (SNP), a donor of nitric oxide (NO), has been shown to be useful for antioxidant and immune regulation in aquatic animals. In this study, an untargeted metabolomics (LC-MS/MS) technique was employed for the first time in Manila clam to analyze the metabolic and histological impacts after air exposure and the positive effects of SNP pretreatment. During air exposure, a significant increase in taurine, L-glutamate, and several polyunsaturated fatty acids in clams was detected, which indicates that clams may experience inflammatory reactions, oxidative stress, and an increase in blood ammonia content. When clams were exposed to SNP for 6 h, arginine, spermine, L-glutamic acid, and glutathione content were all upregulated, indicating that the SNP exposure induced NO production and improved antioxidant capacity in clams. When the clams were exposed to air after SNP pretreatment, there were no significant differences in the levels of taurine, L-glutamate, or aliphatic acids between the experimental and control groups. Gill tissue was more severely damaged in clams directly exposed to air than in those that experienced air exposure after SNP pretreatment, especially in clams exposed to air for a long time (72 h). Both metabolomics and tissue section structure indicated that SNP pretreatment decreased the stress responses caused by air exposure in R. philippinarum. These findings provided fresh insights and a theoretical foundation for understanding the tolerance to air exposure and physiological functions of SNP (or NO) in R. philippinarum.

The Multifaceted Effects of Short-Term Acute Hypoxia Stress: Insights into the Tolerance Mechanism of Propsilocerus akamusi (Diptera: Chironomidae)

Plenty of freshwater species, especially macroinvertebrates that are essential to the provision of numerous ecosystem functions, encounter higher mortality due to acute hypoxia. However, within the family Chironomidae, a wide range of tolerance to hypoxia/anoxia is displayed. Propsilocerus akamusi depends on this great tolerance to become a dominant species in eutrophic lakes. To further understand how P. akamusi responds to acute hypoxic stress, we used multi-omics analysis in combination with histomorphological characteristics and physiological indicators. Thus, we set up two groups-a control group (DO 8.4 mg/L) and a hypoxic group (DO 0.39 mg/L)-to evaluate enzyme activity and the transcriptome, metabolome, and histomorphological characteristics. With blue-black chromatin, cell tightness, cell membrane invagination, and the production of apoptotic vesicles, tissue cells displayed typical apoptotic features in the hypoxic group. Although lactate dehydrogenase (LDH), alcohol dehydrogenase (ADH), catalase (CAT), and Na+/K+ -ATPase (NKA) activities were dramatically enhanced under hypoxic stress, glycogen content, and superoxide dismutase (SOD) activities were significantly reduced compared to the control group. The combined analysis of the transcriptome and metabolome, which further demonstrated, in addition to carbohydrates, including glycogen, the involvement of energy metabolism pathways, including fatty acid, protein, trehalose, and glyoxylate cycles, provided additional support for the aforementioned findings. Lactate is the end product of glycogen degradation, and HIF-1 plays an important role in promoting glycogenolysis in acute hypoxic conditions. However, we discovered that the ethanol tested under hypoxic stress likely originates from the symbiodinium of P. akamusi. These results imply that some parameters related to energy metabolism, antioxidant enzyme activities, and histomorphological features may be used as biomarkers of eutrophic lakes in Chironomus riparius larvae. The study also provides a scientific reference for assessing toxicity and favoring policies to reduce their impact on the environment.

Hypoxia in aquatic invertebrates: Occurrence and phenotypic and molecular responses

Global deoxygenation in aquatic systems is an increasing environmental problem, and substantial oxygen loss has been reported. Aquatic animals have been continuously exposed to hypoxic environments, so-called "dead zones," in which severe die-offs among organisms are driven by low-oxygen events. Multiple studies of hypoxia exposure have focused on in vivo endpoints, metabolism, oxidative stress, and immune responses in aquatic invertebrates such as molluscs, crustaceans, echinoderms, and cnidarians. They have shown that acute and chronic exposure to hypoxia induces significant decreases in locomotion, respiration, feeding, growth, and reproduction rates. Also, several studies have examined the molecular responses of aquatic invertebrates, such as anaerobic metabolism, reactive oxygen species induction, increased antioxidant enzymes, immune response mechanisms, regulation of hypoxia-inducible factor 1-alpha (HIF-1α) genes, and differently expressed hemoglobin/hemocyanin. The genetic basis of those molecular responses involves HIF-1α pathway genes, which are highly expressed in hypoxic conditions. However, the identification of HIF-1α-related genes and understanding of their applications in some aquatic invertebrates remain inadequate. Also, some species of crustaceans, rotifers, sponges, and ctenophores that lack HIF-1α are thought to have alternative defense mechanisms to cope with hypoxia, but those factors are still unclear. This review covers the formation of hypoxia in aquatic environments and the various adverse effects of hypoxia on aquatic invertebrates. The limitations of current hypoxia research and genetic information about the HIF-1α pathway are also discussed. Finally, this paper explains the underlying processes of the hypoxia response and presents an integrated program for research about the molecular mechanisms of hypoxic stresses in aquatic invertebrates.