Immunological and histological responses to sulfide in the crab Charybdis japonica

H2O2-Induced Oxidative Stress Responses in Eriocheir sinensis: Antioxidant Defense and Immune Gene Expression Dynamics

Eriocheir sinensis, a key species in China's freshwater aquaculture, is threatened by various diseases, which were verified to be closely associated with oxidative stress. This study aimed to investigate the response of E. sinensis to hydrogen peroxide (H2O2)-induced oxidative stress to understand the biological processes behind these diseases. Crabs were exposed to different concentrations of H2O2 and their antioxidant enzyme activities and gene expressions for defense and immunity were measured. Results showed that activities of antioxidant enzymes-specificallysuperoxide dismutase (SOD), catalase (CAT), total antioxidant capacity(T-AOC), glutathione (GSH), and glutathione peroxidase (GSH-Px)-varied with exposure concentration and duration, initially increasing then decreasing. Notably, SOD, GSH-Px, and T-AOC activities dropped below control levels at 96 h. Concurrently, oxidative damage markers, including malondialdehyde (MDA), H2O2, and 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels, increased with exposure duration. The mRNA expression of SOD, CAT, and GSH-Px also showed an initial increase followed by a decrease, peaking at 72 h. The upregulation of phenoloxidaseloxidase (proPO) and peroxinectin (PX) was also detected, but proPO was suppressed under high levels of H2O2. Heat shock protein 70 (HSP70) expression gradually increased with higher H2O2 concentrations, whereas induced nitrogen monoxide synthase (iNOS) was upregulated but decreased at 96 h. These findings emphasize H2O2's significant impact on the crab's oxidative and immune responses, highlighting the importance of understanding cellular stress responses for disease prevention and therapy development.

Sulfide causes histological damage, oxidative stress, metabolic disorders and gut microbiota dysbiosis in juvenile sea cucumber Apostichopus japonicus Selenka

Sulfide is a common harmful substance in sediments, with an especially high risk for deposit feeder organisms. The sea cucumber Apostichopus japonicus is a typical benthic feeder, and its intestine is the first line of defense and serves as a crucial barrier function. In this study, histological, physiological, gut microbiota, and metabolomic analyses were performed to explore the toxic response in the intestine of juvenile A. japonicus exposed to 0, 0.8, and 1.6 mg/L sulfide stress for 96 h. The results revealed sulfide-induced intestinal inflammatory symptoms and oxidative stress. Moreover, gut bacterial composition was observed after sulfide exposure, with an increase in Proteobacteria and a decrease in Cyanobacteria and Planctomycetes. Specifically, sulfide increased a set of sulfide-removing bacteria and opportunistic pathogens while decreasing several putative beneficial substance-producing bacteria. The metabolomic analysis indicated that sulfide also disturbed metabolic homeostasis, especially lipid and energy metabolism, in intestine. Interestingly, several intestinal bacteria were further identified to be significantly correlated with metabolic changes; for example, the decreased abundance levels of Bacillus, Corynebacterium, and Psychromonas were positively correlated with important energy metabolites, including maleic acid, farnesyl pyrophosphate, thiamine, butynoic acid, and deoxycholic acid. Thus, our research provides new insights into the mechanisms associated with the intestinal metabolic and microbiota response involved in sulfide stress adaptation strategies of juvenile A. japonicus.

Defense System of the Manila Clam Ruditapes philippinarum under High-Temperature and Hydrogen Sulfide Conditions

Hydrogen sulfide (H₂S) acts as an environmental toxin. Despite its toxicity, little is known about the defense strategies of marine bivalves against it. Thus, the tolerance, behavioral characteristics, and physiological response strategies against H₂S treatment in the sentinel organism Manila clam Ruditapes philippinarum were examined. We monitored the survival and behavioral status of Manila clams exposed to different combinations of temperature and H₂S. The physiological response strategies were examined by measuring the enzymatic activity of cytochrome C oxidase (CCO), fumarate reductase (FRD), superoxide dismutase (SOD), and catalase enzymes (CAT). Moreover, adverse effects of H₂S on the tissue and cell structure of Manila clams were also examined under a transmission electron microscope. Manila clams responded to H₂S stress through behavioral and chemical defenses. With exposure to H₂S alone, Manila clams primarily enhanced aerobic respiratory metabolic pathways in the beginning stages by opening the shell and increasing the CCO activity to obtain more oxygen; with increasing exposure time, when aerobic respiration was inhibited, the shell was closed, and FRD, CAT, and SOD were activated. At this point, Manila clams responded to H₂S stress through the anaerobic metabolism and antioxidant defense systems. However, high temperatures (≥28 °C) altered the defense strategy of Manila clams. With co-exposure to high temperatures and high H₂S concentrations (≥20 μmol/L), the Manila clams immediately closed their shells and changed from aerobic respiration to anaerobic metabolism while immediately activating antioxidant defense systems. Nevertheless, this defense strategy was short lived. In addition to this, apparent damage to tissue and cell structures, including mitochondrial ridge dissolution and many vacuoles, was observed in Manila clams exposed to high temperatures and high H₂S concentrations. Thus, prolonged exposure to high temperature and H₂S damages the tissue structure of Manila clams, affecting their behavioral capacity and future survival. In summary, profiling Manila clams' physiological response strategies to H₂S exposure provided ecological behavioral support for our current understanding of H₂S detrimental toxicity on marine bivalves.

Biochemical indices, gene expression, and SNPs associated with salinity adaptation in juvenile chum salmon (Oncorhynchus keta) as determined by comparative transcriptome analysis

Chum salmon (Oncorhynchus keta) migrate from freshwater to saltwater, and incur developmental, physiological and molecular adaptations as the salinity changes. The molecular regulation for salinity adaptation in chum salmon is currently not well defined. In this study, 1-g salmon were cultured under 0 (control group, D0), 8‰ (D8), 16‰ (D16), and 24‰ (D24) salinity conditions for 42 days. Na⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase activities in the gill first increased and then decreased in response to higher salinity environments where D8 exhibited the highest Na⁺/K⁺ATPase and Ca²⁺/Mg²⁺-ATPase activity and D24 exhibited the lowest. Alkaline phosphatase (AKP) activity was elevated in all salinity treatment groups relative to controls, while no significant difference in acid phosphatase (ACP) activity was observed across treatment groups. De novo transcriptome sequencing in the D0 and D24 groups using RNA-Seq analysis identified 187,836 unigenes, of which 2,143 were differentially expressed in response to environmental salinity (71 up-regulated and 2,072 down-regulated). A total of 56,020 putative single nucleotide polymorphisms (SNPs) were also identified. The growth, development, osmoregulation and maturation factors of N-methyl-D-aspartate receptors (nmdas) expressed in memory formation, as well as insulin-like growth factor 1 (igf-1) and igf-binding proteins (igfbps) were further investigated using targeted qRT-PCR. The lowest expression of all these genes occurred in the low salinity environments (D8 or D16), while their highest expression occurred in the high salinity environments (D24). These results provide preliminary insight into salinity adaptation in chum salmon and a foundation for the development of marker-assisted breeding for this species.

Acute sulfide exposure induces hemocyte toxicity and microbiota dysbiosis in blood clam Tegillarca granosa

Sulfide are widely accumulated in aquatic environments under anaerobic conditions, which cause health problems of aquatic animals, yet their toxic effects to benthic bivalves are not well understood. We investigated the effects of sulfide on innate immunity of the blood clam Tegillarca granosa. Immunity-related indicators and hemolymph microbiota were investigated in the clams exposed to sulfide (via 10, 100 and 1000 μmol/L of Na₂S) over a 7-day period. The results showed that cellular immune responses in T. granosa were affected by exposure to high sulfide concentration (1000 μmol/L Na₂S), as indicated by total counts of hemocytes (THC), cell viability, ROS levels and phagocytic activities, suggesting that sulfide stress induces T. granosa more vulnerable to pathogen challenges. In addition, the Na₂S-induced stress also reshaped the hemolymph microbial community structure of T. granosa that some original genera decreased, such as Lactobacillus, Desulfovibrio and Akkermansia; some genera increased, such as Vibrio and Pseudoalteromonas in sulfide stress group. Sulfide exposure promoted the proliferation of opportunistic pathogen and reduced the diversity of microbial community in the hemolymph of T. granosa. In summary, sulfide stress had marked hemocytotoxicity, reduced immune-cell activity and increased bacterial infections in the blood clam.

Ammonia exposure induces endoplasmic reticulum stress and apoptosis in Chinese striped-necked turtle (Mauremys sinensis)

Ammonia is a major pollutant in the water environment, which could cause severe harm to aquatic organisms. To explore the pathological and physiological effects of ammonia in Chinese striped-necked turtles (Mauremys sinensis), the individuals (body mass: 218.26 ± 12.65 g) were divided into two groups: control group and ammonia exposed group (6.25 mM total ammonia), then the expression levels of signaling factors involved in the endoplasmic reticulum stress and apoptotic pathways were determined. The results showed that ammonia exposure up-regulated the transcriptional and protein levels of endoplasmic reticulum stress marker gene Bip. Meanwhile, the relative mRNA levels of key genes (PERK, ATF6, eIF2α, ATF4, IRE1α and XBP1) involved in unfolded protein response up-regulated, and the phosphorylation levels of PERK, eIF2α and IRE1α increased correspondingly. In addition, the protein and transcriptional levels of CHOP and JNK related to apoptotic pathway induced by unfolded protein reaction increased under ammonia exposure. Moreover, Bcl-2 mRNA expression levels and protein levels decreased, whereas BAX and caspase-3 showed an opposite trend, and the cleaved protein of caspase-3 appeared when the turtles in the elevated ammonia. Furthermore, the apoptotic cells in liver increased after ammonia exposure. These results suggested ammonia exposure induced endoplasmic reticulum stress, then activated unfolded protein response, followed by apoptosis in M. sinensis. The results will contribute to a better understanding of the toxicity mechanism of ammonia to aquatic turtles.

Two-sided effects of prolonged hypoxia and sulfide exposure on juvenile ark shells (Anadara broughtonii)

Oxygen deficit and sulfide have been restrictive factors in mariculture zones. However, the adaptive mechanism in aquatic lives is still unclear. The commercial ark shells Anadara broughtonii were selected to test the tolerance and adaptive responses to prolonged and intermittent hypoxia with or without exogenous sulfide (mild, moderate, high) by evaluating their behavior, mortality, oxidative level, antioxidant responses, and the MAPK-mediated apoptosis in gills. The results indicated that the clams were tolerant to hypoxia and sulfide exposure but vulnerable during reoxygenation from the challenges. Even so, sulfide had remarkable effect on attenuating the accumulation of reactive oxygen species (ROS) and lipid peroxides caused by reoxygenation from prolonged hypoxia. The increase of glutathione level was probably as an early and primary protective response to prevent the expected reperfusion injury from reoxygenation. The challenges suppressed the oxidative level with a dose-dependent effect of sulfide, with an exception when exposed to mild sulfide. Synchronously, biphasic effects of exogenous sulfide on apoptotic cascade, which was induced by mild sulfide while it was inhibited by higher sulfide, were also detected in gills. The induced or inhibited apoptosis by hypoxia and sulfide kept to a typical ROS-MAPK-CASPASE cascade, desiderating further investigation.

Individual and combined effects of ammonia-N and sulfide on the immune function and intestinal microbiota of Pacific white shrimp Litopenaeus vannamei

In this study, we explored the individual and combined effects of ammonia-N and sulfide stress (1 mg/L sulfide and 15 mg/L ammonia-N) on the oxidation resistance, immune response and intestinal health of Litopenaeus vannamei during 72 h exposure. The total antioxidant capacity (T-AOC), malonaldehyde (MDA) and nitric oxide (NO) content, superoxide dismutase (SOD) and catalase activity (CAT), the immune-relative gene (caspase-3, hsp70 and IMD) expression in hepatopancreas and intestine of L.vannamei and the intestinal microbiota were measured. The result showed that MDA and NO contents in hepatopancreas of L. vannamei in all treatment groups increased and remain were at high levels at the end of the stress exposure. The L. vannamei employ antioxidant defense system by increasing the activities of T-AOC, SOD and CAT enzymes in hepatopancereas and intestine to reduce oxidant damage. More severe damages with combined ammonia-N and sulfide stress to antioxidant systems were observed. The gene expression results also demonstrated that antioxidant capacity of L. vannamei was severely impaired and the apoptosis cell was initiated under the ammonia-N and sulfide stress. In addition, the environmental stress also reshaped the intestinal microbial community structure of L. vannamei that a number of original genera decreased, such as Cellvibrio, Vibrio and Rheinheimera; some new genera increased or appeared, such as Photobacterium in all treatment groups, Arcobacter and Fusibacter in sulfide stress group. Therefore, the health of L. vannamei was severely impacted when exposed to the stress of ammonia nitrogen and sulfide and these two factors can have weak synergic effects.

The gut microbiota community and antioxidant enzymes activity of barramundi reared at seawater and freshwater

To understand the physiological responses of euryhaline fish to reared salinity, the gut microbiota composition and antioxidant enzymes activity of same batch barramundi Lates calcarifer reared in two extreme salinity condition (seawater and freshwater) were studied, under laboratory condition. The gut bacterial composition was analyzed using high-throughput Illumina sequencing. Acid phosphatase (ACP), catalase (CAT), glutathione peroxidase (GSH-Px), lipid peroxide (LPO), lysozyme (LZM), malonaldehyde (MDA), peroxidase (POD), and superoxide dismutase (SOD) were used as biochemical indicators. The reared salinity did not change the major composition of barramundi gut bacteria under circulating aquaculture system. Proteobacteria, Firmicutes were the most phylum in barramundi gut microbiota community, and Exiguobacterium, Citrobacter, Acinetobacter, Pseudomonas were the dominate genus. CAT and ACP activity in barramundi liver were found significantly different between freshwater and seawater group. GSH-Px, LZM, POD, SOD activity and MDA, LPO levels were not significantly affected by reared salinity. This study is the first high-throughput analyses of the gut microbiota diversity in barramundi from same batch of siblings reared under two extreme salinity condition. And the findings in the present study can be instructive to the management of animal health in barramundi circulating farming activities, and further euryhaline fish gut microecology research.

Dietary supplementation of selenium yeast enhances the antioxidant capacity and immune response of juvenile Eriocheir Sinensis under nitrite stress

This study elucidates the response to nitrite stress and the effect of dietary selenium supplements on the growth, antioxidant activity, immunity and transcriptome of juvenile Chinese mitten crab Eriocheir sinensis. In the control group, the crabs were fed the diet without selenium supplementation and there was no nitrite addition to the water. In the test group, the crabs were fed diets with three levels of selenium 0 (N1), 0.5 (N2) and 1.0 (N3) mg/kg in the water containing 2 mg/L NO₂N as a stress factor for eight weeks. Feed conversion ratio (FCR) was improved by adding dietary selenium. There was no significant difference in specific growth rate and weight gain between N1 and the control groups, or among different selenium levels in the test group. The superoxide dismutase (SOD) activity was significantly lower, but malondialdehyde (MDA) was higher in the N1 group than those in the serum and hepatopancreas of the control group. The activities of SOD, glutathione peroxidase (GPx) and acid phosphatase increased at the medium level of selenium but decreased as the level of dietary selenium increased to 1.0 mg/kg. The serum lysozyme (LZM) activity increased but the MDA content in both serum and hepatopancreas decreased with the increase of selenium levels. The total clean reads of the crabs in the control group, N1 and N3 groups reached 390.7M and were assembled into 106 471 transcripts. Compared with the control group, 1196 gene were significantly expressed (588-up and 608-down) in the N1 group under nitrite stress. Between the N1 and N3 groups, the expression of 1537 genes (751-up and 786-down) were significantly different. KEGG pathway analysis reveals that 11 and 19 pathways were significantly different between N1 and control and between N3 and N1 groups, respectively. Transcriptome results demonstrate that nutrient metabolism is much more active in crabs fed additional selenium under nitrite stress. This study indicates that dietary selenium can improve both antioxidant capacity and immune response and alter the protein and carbohydrate metabolism of E. sinensis under nitrite stress.

Response of gut health and microbiota to sulfide exposure in Pacific white shrimp Litopenaeus vannamei

Sulfide is a natural and widely distributed toxicant. It can be commonly found on the interface between water and sediment in the aquatic environment. The Pacific white shrimp Litopenaeus vannamei starts life in the benthic zone soon after the mysis stage, an early stage of post larvae. Therefore, L. vannamei is inevitably affected by exposure to sulfide released from pond sediment. This study explored the toxicant effect of different concentrations of sulfide on the intestinal health and microbiota of Pacific white shrimp by monitoring the change of expression of inflammatory, immune related cytokines, and the structure of the intestinal microbiota. The gut histology, expressions of inflammatory and immune related cytokines (tumor necrosis factor-alpha, C-type lectin 3, myostatin and heat shock transcription factor 1), and the microbiota were determined in L. vannamei after exposure to 0 (control), 425.5 (1/10 LC 50-96 h), and 851 μg/L (1/5 LC 50-96 h) of sulfide for 21 days. With the increase of sulfide concentration, intestinal injury was aggravated and the inflammatory and immune related cytokines generated a range of reactions. The expression of myostatin (MSTN) was significantly down-regulated by the concentration of sulfide exposure. No difference in the expression of heat shock transcription factor 1 (HSF1) was found between the control and shrimp exposed to 425.5 μg/L, but significantly higher HSF1 expression was found in shrimp exposed to 851 μg/L of sulfide. Significantly higher values of tumor necrosis factor-alpha (TNF-α) and C-type lectin 3 (CTL3) were found in the shrimp exposed to 425.5 μg/L of sulfide compared to the control, but a lower value was found in the shrimp exposed to 851 μg/L (P < 0.05). Sulfide also changed the intestinal microbial communities. The abundance of pathogenic bacteria, such as Cyanobacteria, Vibrio and Photobacterium, increased significantly with exposure to the increasing concentration of sulfide. The abundance of some anti-stress bacteria, such as Chlorobi and Fusobacterium, increased. Nitrospirae which can alleviate nitrite toxicity decreased. Microbacterium, Parachlamydia, and Shewanella were all commonly found and down-regulated in both sulfide groups, which is associated with an adaptation to sulfide stimulation. This study indicates that chronic exposure to sub-lethal levels of sulfide could lead to damage of the gut structure, stimulate the response of the inflammatory and immune systems, and shape the structure of the gut microbiota in L. vannamei.

Dietary nano-selenium relieves hypoxia stress and, improves immunity and disease resistance in the Chinese mitten crab (Eriocheir sinensis)

Hypoxia is a relevant physiological challenge for crab culture, and the hemolymph plays a crucial role in response to the hypoxia. In a 60 d feeding trial, Chinese mitten crabs (Eriocheir sinensis) fed a diet containing 0.2 mg/kg nano-selenium (nanoSe) showed a significantly increased weight gain rate (WGR) and a reduced feed coefficient (FC) compared to those fed diets with 0, 0.1, 0.4, 0.8, and 1.6 mg/kg nanoSe. Another 90 d feeding trial was conducted to determine the influence of dietary nanoSe on the immune response in juvenile Chinese mitten crabs kept under the condition of hypoxia. The results showed that hypoxia stress resulted in significantly increased hemocyte counts (THC, LGC, SGC, and HC), expression levels of the hemocyanin gene and protein, lactic acid level, and antioxidant capacity (T-AOC activities, SOD activities, GSH-Px and GSH content) in hemolymph supernatant. When these crabs were infected with Aeromonas hydrophila bacteria, hypoxia exposure increased mortality, but it was alleviated by a diet supplemented with 0.2 mg/kg nanoSe. The up-regulative effects of nanoSe (0.2 mg/kg) on antioxidant capacity, hemocyte counts, and hemocyanin expression under hypoxia exposure were further strengthened throughout, whereas lactic acid levels induced by hypoxia stress were restored. Thus, the observations in this study indicate that the level of dietary nanoSe is important in regulating immunity and disease resistance in crabs kept under hypoxia stress.

Effects of perfluorooctane sulfonate on the immune responses and expression of immune-related genes in Chinese mitten-handed crab Eriocheir sinensis

Perfluorooctane sulfonate (PFOS) has been widely studied due to its global distribution, slow degradation, high bioaccumulation and toxicological effects on vertebrates. However, the potential toxicity of PFOS to crustaceans is little known. The present study investigated the effects of PFOS on the immune responses and expression of immune-related genes in the Chinese mitten-handed crab Eriocheir sinensis. Crabs were exposed to 0, 0.01, 0.1, 1.0 and 10mg/L of PFOS, and sampled on 1, 4, 7, 14 and 21days respectively. The total hemocyte count and lysozyme activity in PFOS-treated crab were significantly lower than in the control. The exposure to 10mg/L of PFOS led to a marked inhibition in phenoloxidase and superoxide dismutase activities. At other PFOS levels, phenoloxidase and superoxide dismutase showed an initial increase and a subsequent decrease over time. The alkaline and acid phosphatase activities were stimulated in 10mg/L PFOS until 21days. The mRNA expression of immune related genes including hepatopancreas-specific C-type lectin and prophenoloxidase activating factors were up-regulated after the exposure to the concentrations of 1 and 10mg/L of PFOS, while the expression of lysozyme gene was up-regulated only in the crab exposed to 0.1mg/L PFOS. The results demonstrate that the high dose of PFOS leads to immune toxicity and the hepatopancreas is a major target organ for PFOS accumulation and immunotoxicity. Hemocyte counts, phenoloxidase and acid phosphatase are useful biomarkers for the risk assessment of PFOS toxicity to crustaceans.