Acclimation of brackish water pearl spot (Etroplus suratensis) to various salinities: relative changes in abundance of branchial Na(+)/K (+)-ATPase and Na (+)/K (+)/2Cl (-) co-transporter in relation to osmoregulatory parameters

Effects of short-term salt exposure on gill damage, serum components and gene expression patterns in juvenile Largemouth bass (Micropterus salmoides)

The Largemouth bass (Micropterus salmoides; LMB) is a freshwater fish that plays a significant role in aquaculture, and its cultural base is expanding into inland saline water areas. To study the effect of short-term salt exposure on LMB, fish with an average body weight of 11.69 (±1.82) g were cultured for 14 days at three different salt concentrations (0 ‰, 6 ‰, and 12 ‰). After 14 days, the second gill arch was collected for tissue sectioning and transcriptome sequencing, while serum samples were collected to analyze serum components. The results showed that the mortality rate in the 0 ‰ and 6 ‰ groups was 0 %, whereas the mortality rate in the 12 ‰ group was 62 %. In the gill tissue sections, no apparent damage was observed in the 0 ‰ and 6 ‰ groups. However, in the 12 ‰ group, the secondary lamellae became shorter, thicker, and exhibited a disordered arrangement. The serum component test results showed that osmolality and K+ significantly increased in the 12 ‰ group, while Na+, K+, and Cl- concentrations showed slight increases, but the differences were not significant. Comparative transcriptome analysis revealed that, along the salinity gradient, gene expression exhibited five profiles. Genes related to ion transport and immunity were highly expressed in the 6 ‰ and 12 ‰ groups, while genes associated with biosynthesis and ATP production showed decreased expression levels as salinity increased. Notably, seven solute carrier genes, two Na+/K+-ATPase genes, and two insulin-like growth factor genes were significantly highly expressed in the 12 ‰ salinity group, playing important roles in the transmembrane transport of ions. Based on the results, the LMB can acclimatize to a salt concentration of at least 6 ‰. However, exposure to 12 ‰ salinity can lead to a series of adverse effects, including organ damage, reduced energy metabolism efficiency, and disruption of ion homeostasis.

Adverse effects of environmentally relevant concentration of microplastics on gill epithelium permeability in the euryhaline Mediterranean killifish Aphanius fasciatus

Estuaries and lagoons are characterized by fluctuating salinity and significant amounts of microplastics (MPs) and are increasingly subjected to various anthropogenic pressures. We investigated whether the accumulation of MPs in the gills of fish inhabiting these fragile ecosystems alters osmoregulation and, consequently, their ability to tolerate fluctuating salinity. The effects of a 15-day exposure to an environmentally relevant concentration (20 μg/L) of spherical polystyrene microplastics (PS-MPs) with a diameter of 5 μm were assessed in the Mediterranean killifish Aphanius fasciatus, focusing on tissue and gene expression changes related to factors of paracellular and transcellular permeability of the gill epithelium during the transition from seawater to freshwater. Our results revealed that PS-MPs indirectly impaired osmoregulation, particularly in fresh water, through their toxic effects on the gill tissue. Toxicity was evidenced by epithelial lifting, a decrease in the proportion of secondary lamellae available for gas exchange, and upregulation of superoxide dismutase and heat shock protein genes. Furthermore, exposure to PS-MPs directly affected gill epithelial permeability by maintaining relatively high paracellular permeability through the downregulation of claudin 3 and by modifying the expression of the transcellular transporter Na+/K+-ATPase and cystic fibrosis transmembrane conductance regulator in the gill epithelium. Overall, these findings confirm the toxic effects of PS-MPs on gill tissue and demonstrate, for the first time, that environmentally relevant concentrations of MPs adversely affect gill epithelium permeability during decreased salinity acclimation in the euryhaline fish A. fasciatus.

Gene expression profiling and physiological adaptations of pearl spot (Etroplus suratensis) under varying salinity conditions

Eutroplus suratensis (Pearl spot) is naturally found in estuarine environments and has been noted to have a high salinity tolerance. By examining the impact of various salinity levels on the growth and survival of pearl spot, the present study aims to enhance aquaculture profitability by assessing their adaptability and physiological adjustments to changes in salinity and determining their potential to acclimate to a broad range of salinity regimes. Results revealed no mortality in the control group (0 ppt), and in 15, 25 and 35 ppt treatment groups. However, the remaining groups (45, 60, and 75 ppt) showed differing levels of mortality with 44 % mortality observed in the 45 ppt group and 100 % mortality in both the 60 and 75 ppt groups. The expression analysis showed that liver IGF-1 mRNA expression increased by 2.6-fold at 15 ppt, and HSP70 mRNA expression in the liver also showed a significant increase with rising salinity levels. In addition, OSTF1 expression exhibited an increase at 15 ppt, whereas SOD and CAT expression reached their highest levels at 25 ppt. At 15 ppt, the expression of NKA mRNA increased significantly by 2.8-fold. The study's overall findings suggested that utilizing a salinity level of 15 ppt for pearl spot production could be viable for profitable aquaculture.

Salinity-dependent changes in branchial morphometry and Na+ , K+ -ATPase responses of euryhaline Asian sea bass, Lates calcarifer

Asian sea bass (Lates calcarifer Bloch, 1790) is a euryhaline fish widely cultured in Asia and Australia. Although it is common to culture Asian sea bass at different salinities, osmoregulatory responses of Asian sea bass during acclimation to various salinities have not been fully observed. In this study, we used scanning electron microscopy to observe the morphology of the ionocyte apical membrane of Asian sea bass acclimated to fresh water (FW), 10‰ brackish water (BW10), 20‰ brackish water (BW20), and seawater (SW; 35‰). Three types of ionocytes were identified in FW and BW fish: (I) flat type with microvilli, (II) basin type with microvilli, and (III) small^- hole type. Flat type I ionocytes were also observed in the lamellae of the FW fish. In contrast, two types of ionocytes were identified in SW fish: (III) small-hole type and (IV) big-hole type. Furthermore, we observed Na⁺ , K⁺ -ATPase (NKA) immunoreactive cells in the gills, which represent the localization of ionocytes. The highest protein abundance was observed in the SW and FW groups, whereas the highest activity was observed in the SW group. In contrast, the BW10 group had the lowest protein abundance and activity. This study demonstrates the effects of osmoregulatory responses on the morphology and density of ionocytes, as well as protein abundance and activity of NKA. In this study, we found that Asian sea bass had the lowest osmoregulatory response in BW10, because the lowest amounts of ionocytes and NKA were required to maintain osmolality at this salinity.

Rapid blood acid-base regulation by European sea bass (Dicentrarchus labrax) in response to sudden exposure to high environmental CO2

Fish in coastal ecosystems can be exposed to acute variations in CO2 of between 0.2 and 1 kPa CO2 (2000-10,000 µatm). Coping with this environmental challenge will depend on the ability to rapidly compensate for the internal acid-base disturbance caused by sudden exposure to high environmental CO2 (blood and tissue acidosis); however, studies about the speed of acid-base regulatory responses in marine fish are scarce. We observed that upon sudden exposure to ∼1 kPa CO2, European sea bass (Dicentrarchus labrax) completely regulate erythrocyte intracellular pH within ∼40 min, thus restoring haemoglobin-O2 affinity to pre-exposure levels. Moreover, blood pH returned to normal levels within ∼2 h, which is one of the fastest acid-base recoveries documented in any fish. This was achieved via a large upregulation of net acid excretion and accumulation of HCO3- in blood, which increased from ∼4 to ∼22 mmol l-1. While the abundance and intracellular localisation of gill Na+/K+-ATPase (NKA) and Na+/H+ exchanger 3 (NHE3) remained unchanged, the apical surface area of acid-excreting gill ionocytes doubled. This constitutes a novel mechanism for rapidly increasing acid excretion during sudden blood acidosis. Rapid acid-base regulation was completely prevented when the same high CO2 exposure occurred in seawater with experimentally reduced HCO3- and pH, probably because reduced environmental pH inhibited gill H+ excretion via NHE3. The rapid and robust acid-base regulatory responses identified will enable European sea bass to maintain physiological performance during large and sudden CO2 fluctuations that naturally occur in coastal environments.

Effect of high salinity acclimation on glucose homeostasis in Mozambique tilapia (Oreochromis mossambicus)

During salinity stress, osmoregulatory processes in euryhaline fish need to modify for their survival, and glucose is the preferred mode of extra energy during such conditions. These organisms must have a proper mechanism to maintain glucose homeostasis during such modified osmoregulatory process across different body fluids. Hence, we studied high salinity effect on regulation of glucose homeostasis in Mozambique tilapia. The fish were induced to 15‰ salinity for 21 days. Glucose, glycogen, ion concentrations, Na⁺-K⁺-ATPase, pyruvate kinase, γ-amylase activities and GLUT mRNA expressions were investigated in liver, intestine, gill and white muscle tissues. At the end of experiment, Na⁺ ion concentrations, glucose content and activity of Na⁺-K⁺-ATPase especially in the gill and intestine were increased, while decrease in liver and gill glycogen content was seen. Lower concentration of glycogen decrease was observed in the intestine and white muscle of the treated group. High pyruvate kinase activity was noticed in liver and gill tissues that correlates with high Na⁺-K⁺-ATPase activity. Elevated γ-amylase activity was observed in the liver and intestine suggesting breakdown of glycogen; however, gill and white muscle did not show any increased activity. Increase in GLUT1 and GLUT4 mRNA expressions was observed especially in the gill and intestine, while increase in GLUT2 mRNA expressions was observed in the liver. Upregulations of GLUTs suggest higher influx of glucose into the cell for catabolism to provide energy and further to drive the enhanced osmoregulatory process. These findings suggest glucose homeostasis being regulated in Mozambique tilapia during salinity acclimation.

Hepatic Toxicity in Etroplus suratensis (Bloch 1790): An Economically Important Edible Fish in Vembanad Fresh Water Lake, Kerala, India

Fish consumption from contaminated water-bodies is a serious health issue. This study conducted to reveal the presence of heavy metals and bisphenols in Vembanad lake, an exploiting tourist spot in Kerala, receiving untreated agricultural, domestic, municipal, and industrial effluents. We evaluated aquatic contaminant impact on hepatic stress markers in Etroplus suratensis from fragile Vembanad lake. The significant difference in water physiochemical parameters, the concentration of heavy metals, and bisphenols (BPA and BPS) were studied. Hepatic tissue of E. suratensis inhabited in lake featured with high iron (11.29 ± 0.39 ppm) and BPA (0.02412 ± 0.0031 µg/mL) content along with an increased hepatic stress marker and distorted hepatic structure. The study highlights the presence of high iron and BPA in edible fish. The study recommends monitoring of physiochemical characters of freshwater lakes is essential for better survival of freshwater flora and fauna.

Pseudobranch mimics gill in expressing Na+K+-ATPase 1 α-subunit and carbonic anhydrase in concert with H+-ATPase in adult hilsa (Tenualosa ilisha) during river migration

In hilsa (Tenualosa ilisha), pseudobranch comprises a row of parallel filaments bear numerous leaf-like lamellae arranged on both sides throughout its length. The purpose of this study was to elucidate involvement of pseudobranchial Na⁺, K⁺-ATPase (NKA) 1 α-subunit, and carbonic anhydrase (CA) in concert with H⁺-ATPase (HAT) compared to their branchial counterparts in freshwater acclimation of hilsa during spawning migration from off-shore of the Bay of Bengal to the Bhagirathi-Hooghly zones of the Ganga river system in India. Adult hilsa fish were collected from seawater (SW), freshwater 1 (FW1), and freshwater 2 (FW2) locations, where the salinity level was 26-28‰, 1-5‰, and 0-0.04‰, respectively. Hilsa migrating through freshwater showed a consistent decrease in the plasma osmolality, sodium (Na⁺) and chloride (Cl^-) ion levels indicates unstable ionic homeostasis. The mRNA expression and activity of NKA 1 α-subunit in pseudobranch as well as in true gills declined with the migration to upstream locations. The pseudobranchial CA activity almost mirrors its branchial counterpart most notably while hilsa entered the freshwater zone, in the upstream river suggesting its diverse role in hypo-osmotic regulatory acclimation. Nevertheless, the H⁺-ATPase activity of both the tissues increased with the freshwater entry and remained similar during up-river movement into the freshwater environment. The results confirm that the pseudobranchial NKA 1 α-subunit mRNA expression and activity mimic its branchial counterpart in the process of ionoregulatory acclimation during migration through salt barriers. Also, the increase in the activities of pseudobranchial and branchial CA in concert with H⁺-ATPase (HAT) during freshwater acclimation of hilsa suggests their critical involvement in ion uptake.

Alteration in branchial NKA and NKCC ion-transporter expression and ionocyte distribution in adult hilsa during up-river migration

Hilsa (Tenualosa ilisha) is a clupeid that migrates from the off-shore area through the freshwater river for spawning. The purpose of this study was to investigate the involvement of branchial Na⁺/K⁺-ATPase (NKA) and Na⁺/K⁺/2Cl^- cotransporter (NKCC) in maintaining ionic homeostasis in hilsa while moving across the salt barriers. Hilsa, migrating through marine and brackish waters, did not show any significant decline in NKA activity, plasma osmolality, and plasma ionic concentration. In contrast, all the parameters declined significantly, after the fish reached in freshwater zone of the river. Immunoblotting with NKA α antibody recognized two bands in gill homogenates. The intensity of the higher molecular NKA band decreased, while the other band subsequently increased accompanying the movement of hilsa from marine water (MW) to freshwater. Nevertheless, total NKA expression in marine water did not change prior to freshwater entry. NKCC expression was down-regulated in gill, parallel with NKA activity, as the fish approached to the freshwater stretch of river. The NKA α-1 and NKCC1 protein abundance decreased in freshwater individuals by 40% and 31%, respectively, compared to MW. NKA and NKCC1 were explicitly localized to branchial ionocytes and immunoreactive signal appeared throughout the cytoplasm except for the nucleus and the most apical region indicates a basolateral/tubular distribution. Immunoreactive ionocytes were distributed on the filaments and lamellae; lamellar ionocytes were more in number irrespective of habitat salinity. The decrease in salinity caused a slight reduction in ionocyte number, but not in size and the underlying distribution pattern did not alter. The overall results support previously proposed models that both the ion transporters are involved in maintaining ionic homeostasis and lamellar ionocytes may have the function in hypo-osmoregulation in migrating hilsa, unlike other anadromous teleosts.

Isolation and characterization of Aquaporin 1 (AQP1), sodium/potassium-transporting ATPase subunit alpha-1 (Na/K-ATPase α1), Heat Shock Protein 90 (HSP90), Heat Shock Cognate 71 (HSC71), Osmotic Stress Transcription Factor 1 (OSTF1) and Transcription Factor II B (TFIIB) genes from a euryhaline fish, Etroplus suratensis

The present study reports the complete sequences of Aquaporin 1 (AQP1) gene and partial sequences of genes, Sodium/potassium-transporting ATPase subunit alpha-1 (Na/K-ATPase α1 subunit), Osmotic Stress Transcription Factor 1 (OSTF1), Transcription Factor II B (TFIIB), Heat Shock Cognate 71 (HSC71) and Heat Shock Protein 90 (HSP90) obtained from mRNA and genomic DNA of Etroplus suratensis. They are candidate genes involved in stress responses of fishes. AQP1 gene was 2163 bp long. Its mRNA sequence has 55 bp 5' UTR, 783 bp open reading frame (ORF), 119 bp 3' UTR, three intronic regions and 90% identity with AQP1 of Oreochromis niloticus. The partial Na/K-ATPase α1subunit gene obtained 5998 bp length with an ORF of 2213 bp and 12 intronic regions. The partial OSTF1, TF IIB, HSC71 and HSP90 mRNA sequences obtained were 1473 bp, 587 bp, 1708 bp and 151 bp in length respectively. All the genes showed a high sequence similarity with respective genes reported from fishes. Comparison of AQP1 and Na/K-ATPase α1 genomic DNA sequence of E. suratensis collected from different water system showed two type of AQP1 with one synonymous mutation in exon-1 and higher sequence difference in intronic regions (including addition, deletion, transition and transversion mutations) with few synonymous and non-synonymous mutations in the exons of Na/K-ATPase α1. The sequence information of these major candidate genes involved in stress responses will help in further studies on population genetics, adaptive variations and genetic improvement programs of this cichlid species having aquaculture, ornamental and evolutionary importance.

Na+/K+-ATPase response to salinity change and its correlation with FXYD11 expression in Anguilla marmorata

The Na⁺/K⁺-ATPase (NKA) is a primary electrogenic protein that promotes ion transport in teleosts. FXYD11 is a putative regulatory subunit of the NKA pump. The regulation of Na ⁺/K ⁺ -ATPase and FXYD11 is of critical importance for osmotic homeostasis. To investigate the changes of the two genes under different salinity environments, we first identified NKA (AmNKAα1) and FXYD11 (AmFXYD11) in Anguilla marmorata, and then evaluated the mRNA levels of NKA and FXYD11 as well as the activity of NKA in the gill and kidney at different timepoints (0, 1, 3, 6, 12, 24, 48, 72, 96, and 360 h) under three salinity conditions-0‰ (fresh water: FW), 10‰ (brackish water: BW), and 25‰ (seawater: SW). In the gill, the mRNA levels of AmNKAα1 and AmFXYD11 and the enzyme activity of AmNKAα1 were higher in BW and SW than in FW; the protein abundance was positively correlated with the specific activity of NKA in BW/SW. However, in the kidney, the mRNA level of AmNKAα1 in the BW group was higher than that in the FW group. In addition, AmFXYD mRNA levels in both BW and SW groups were significantly lower than that in the FW control group. These results suggested that AmFXYD11 was tissue specific in response to different salinity environment. Our results clearly demonstrated the important roles of AmNKAα1 and AmFXYD11 in osmotic homeostasis of juvenile A. marmorata under saline environment.

A Stenohaline Medaka, Oryzias woworae, Increases Expression of Gill Na(+), K(+)-ATPase and Na(+), K(+), 2Cl(-) Cotransporter 1 to Tolerate Osmotic Stress

The present study aimed to evaluate the osmoregulatory mechanism of Daisy's medaka, O. woworae,as well as demonstrate the major factors affecting the hypo-osmoregulatory characteristics of euryhaline and stenohaline medaka. The medaka phylogenetic tree indicates that Daisy's medaka belongs to the celebensis species group. The salinity tolerance of Daisy's medaka was assessed. Our findings revealed that 20‰ (hypertonic) saltwater (SW) was lethal to Daisy's medaka. However, 62.5% of individuals survived 10‰ (isotonic) SW with pre-acclimation to 5‰ SW for one week. This transfer regime, "Experimental (Exp.) 10‰ SW", was used in the following experiments. After 10‰ SW-transfer, the plasma osmolality of Daisy's medaka significantly increased. The protein abundance and distribution of branchial Na(+), K(+)-ATPase (NKA) and Na(+), K(+), 2Cl(-) cotransporter 1 (NKCC1) were also examined after transfer to 10‰ SW for one week. Gill NKA activity increased significantly after transfer to 10‰ SW. Meanwhile, elevation of gill NKA αα-subunit protein-abundance was found in the 10‰ SW-acclimated fish. In gill cross-sections, more and larger NKA-immunoreactive (NKA-IR) cells were observed in the Exp. 10‰ SW medaka. The relative abundance of branchial NKCC1 protein increased significantly after transfer to 10‰ SW. NKCC1 was distributed in the basolateral membrane of NKA-IR cells of the Exp. 10‰ SW group. Furthermore, a higher abundance of NKCC1 protein was found in the gill homogenates of the euryhaline medaka, O. dancena, than in that of the stenohaline medaka, O. woworae.

Effects of salinity acclimation and eyestalk ablation on Na(+), K(+), 2Cl(-) cotransporter gene expression in the gill of Portunus trituberculatus:a molecular correlate for salt-tolerant trait

The Na(+), K(+), 2Cl(-) cotransporter (NKCC) is an important gene in ion transport. In order to elucidate its function, and regulatory mechanisms, in salinity acclimation, the complete cDNA sequence of NKCC (4218 bp) from Portunus trituberculatus (PtNKCC) was first cloned and characterized. It was found to encode 1055 amino acids containing conserved AA-permease and SLC12 motifs. Results show that PtNKCC is expressed to the greatest extent in gills. High salinity stress exposure led to significant increases (9.6-fold) of PtNKCC mRNA expression in the gills 12 h after treatment, declining to less than the levels seen in the control group between 48 and 72 h. During low salinity stress, expression levels of PtNKCC in gills were found to be upregulated at each sampling time, reaching their peak after 6 h (a 12.4-fold increase). Eyestalk ablation also triggered an 11.3-fold increase in PtNKCC mRNA, while re-injection with eyestalk homogenates significantly reduced the expression of PtNKCC mRNA. Four single nucleotide polymorphisms (SNPs) were detected in the PtNKCC open reading frame, and one SNP was associated with salt tolerance. Our results indicate that PtNKCC plays an important role in the salinity acclimation of P. trituberculatus, while there may be a compound present in the XOSG that inhibits the expression of PtNKCC.