The intestinal tract serves as a critical immune regulator in aquatic species, maintaining homeostasis and environmental stress resistance. This study evaluates the protective effects of melatonin (MT) on Litopenaeus vannamei (L. vannamei) under acute alkaline stress through a comprehensive analysis of intestinal morphology, antioxidant responses, apoptosis regulation, and microbial community dynamics. A total of six groups of melatonin treatment groups were designed. After another 2 months of breeding, a 96 h acute alkalinity stress experiment was conducted. Experimental supplementation revealed dose-dependent outcomes: 82.7 mg/kg MT significantly improved survival rates without affecting growth parameters, while higher concentrations (329.2 mg/kg) induced elevated apoptosis (p < 0.05). Histological examination demonstrated mitigated intestinal structural damage in MT-treated groups compared to non-supplemented controls under alkaline stress. Antioxidant capacity initially increased and then stabilized at optimal MT doses (82.7–165.1 mg/kg), accompanied by enhanced immune marker expression (p < 0.05). Microbial profiling indicated MT-mediated enrichment of commensal bacteria associated with polysaccharide metabolism, energy utilization, and intestinal immunity. This study establishes that melatonin exerts dose-dependent protection in L. vannamei under alkaline stress, balancing antioxidant enhancement, apoptosis modulation, and microbiome regulation to fortify intestinal health, with 82.7–165.1 mg/kg identified as the optimal therapeutic range for mitigating environmental stress without compromising physiological homeostasis. The results of this study establish an empirical framework for optimizing MT application in crustacean aquaculture, particularly highlighting its role in maintaining intestinal barrier integrity and microbial homeostasis under alkaline environmental challenges.

Salinity is one of the most critical environmental factors for fish, influencing their reproduction, growth, and physiological and metabolic activities. Lateolabrax japonicus is a major commercially important marine fish that has been widely cultured in China. However, there are few reports on the growth tolerance of fish juveniles of different species under various salinity conditions. In this study, we investigated the effects of acute low-salt stress on the survival, plasma osmolality, blood ion concentration, and Na⁺/K⁺-ATPase (NKA) activity in L. japonicus juveniles of two size groups. Our findings revealed no significant difference in survival rates between 5 cm and 10 cm juveniles at salinities of 0.2, 1, 3, 5, 10, 15, and 25. Plasma osmolality and blood ions exhibited a “decrease-increase-stabilization” pattern; 5 cm juveniles stabilized at 6 h at all salinities, whereas 10 cm juveniles required 48 h to stabilize at a salinity of 1, with isosmotic points of 10.91 and 11.00, respectively. Gill and kidney NKA activity followed an “increase-decrease-stabilization” pattern, with 5 cm juveniles achieving stability 12–24 h earlier than 10 cm individuals under low salinities (1, 3, 5). In conclusion, 5 cm L. japonicus juveniles exhibited superior low-salinity tolerance, accelerated osmoregulatory responses, and enhanced adaptation compared to 10 cm juveniles. These findings strongly support the prioritizing of smaller-sized L. japonicus for low-salinity aquaculture practices.

In northwestern China, there is an abundance of saline-alkali water resources, but their high alkalinity severely restricts the development of inland saline-alkali water aquaculture. As an important aquaculture species, the whiteleg shrimp, Litopenaeus vannamei, shows an unclear physiological adaptation mechanism under high-alkaline stress. In this study, multi-omics and physiological methods were used to systematically reveal the effects of high-alkaline stress on the molt, antioxidation response, and immune defense in L. vannamei. The results showed that high-alkaline stress caused damage to the intestinal tissues of the shrimp and weakened the mucous barrier function, which was accompanied by a significant decrease in the activities of antioxidant enzymes (SOD, CAT, and GPx) and non-specific immune indicators (PO and LZM) (p < 0.05). The transcriptome results showed that the expression of genes related to chitin metabolism and calcium ion binding was upregulated, whereas that of genes related to muscle contraction and cell skeleton construction was downregulated. The structure of the intestinal microbiota changed significantly, with a decrease in microbiota diversity, whereas the abundance of potential pathogenic species (e.g., Photobacterium) increased. These results provide a theoretical basis for clarifying the molting response and antioxidant defense mechanism of L. vannamei in high-alkaline environments, with significance for saline-alkali water aquaculture practices.

Saline‐alkali cultivation is a crucial approach to alleviate the fresh water crisis. However, prolonged saline‐alkali stresses might bring adverse effects on fish growth, health, and even death. This study evaluated the effects of different carbohydrate supplementation on growth performance, carbohydrate metabolism, antioxidant capacity, and tissue morphology of mandarin fish ( Siniperca chuatsi ) under saline‐alkali stress. Fish with an initial weight of 31.03 ± 0.12 g, were fed with three carbohydrate levels (8%, 13%, and 18%) in either a freshwater or a saline‐alkali water environment (salinity = 6, alkalinity = 0.7 g/L). Results indicated that fish cultured in saline‐alkali water performed a higher specific growth rate (SGR) and weight gain (WG). In addition, the activities of the glycolysis pathway, pentose phosphate pathway, and gluconeogenesis pathway were enhanced prominently under this stress. The growth performances were also improved significantly by the increased carbohydrate diets. However, the activities of amylase were inhibited with the increased carbohydrate intake, which impeded the process of glycometabolism. In contrast, the pentose phosphate pathway and gluconeogenesis pathway responded positively to dietary carbohydrate levels, especially under saline‐alkali stresses. Moreover, improving the carbohydrate level appropriately could significantly reduce the accumulation of malondialdehyde, promote the activities of superoxide dismutase, and total antioxidant capacity under saline‐alkali stress. Furthermore, the expression of insulin receptors showed an ascending tendency in freshwater, while opposite in saline‐alkali water with increased carbohydrate intake. In summary, an appropriate saline‐alkali environment was more conducive to the growth of mandarin fish. Moreover, 13% carbohydrate diets could enhance its growth performance in saline‐alkali environments. It was noteworthy that the effects of insulin on the regulation of carbohydrate metabolism might not be the same as normal conditions during the adaptation process in mandarin fish. The pentose phosphate pathway played a critical part in osmoregulation under saline‐alkali stress.

Free amino acids (FAAs) are important low-molecular-weight metabolites involved in osmotic regulation, acid–base balance, and nitrogen metabolism in fish exposed to saline–alkaline environments. To characterize tissue-specific FAA responses in mandarin fish (Siniperca chuatsi), 10 cm juveniles were exposed for 96 h to freshwater control (FW), salinity stress (S, salinity 8), alkalinity stress (A, alkalinity 20 mmol/L), or combined saline–alkaline stress (SA, salinity 8 + alkalinity 20 mmol/L). The contents of 19 FAAs were compared among plasma, muscle, liver, brain, and kidney. FAA profiles showed clear tissue specificity. Total FAA (17) decreased in plasma under all stress treatments, increased in muscle under S and SA but decreased under A, increased in liver and kidney, and decreased under single stress but increased under combined stress in brain. Distinct tissue distribution patterns were observed for functional FAA groups. Under salinity stress, osmoregulation-related FAAs, particularly Ala and Pro, showed higher contents mainly in muscle, liver, and kidney. Under alkalinity stress, kidney showed concurrent increases in multiple FAAs, including Ala, Pro, Glu, Gln, Val, Ile, and Leu, whereas brain was characterized by a high Gln content. Under combined saline–alkaline stress, liver was the main tissue in which multiple functional FAA groups increased simultaneously, kidney maintained elevated levels of several FAAs, and brain showed treatment-specific high levels of Gln and Tau. Redundancy analysis (RDA) indicated weak constrained explanatory power of salinity and alkalinity for the overall FAA profile, whereas tissue-specific differentiation was evident. Glu, Gln, and Pro showed directional consistency with the salinity vector, whereas Val and Leu tended to align with the alkalinity-related ordination direction. Overall, acute saline–alkaline exposure induced a functional and tissue-specific distribution pattern of FAAs rather than a uniform whole-body shift in mandarin fish.

Extreme saline–alkaline environments impose simultaneous osmotic and nitrogen stresses, yet the traits that enable microbial persistence and nitrogen utilization under these conditions remain incompletely understood. Here, we isolated and characterized a novel bacterium, Halomonas shizuishanensis DWK9^T, from a saline–alkaline soil (salinity 5.75 g/kg, pH 8.35) in Shizuishan City, Ningxia, China, using a polyphasic taxonomic and genomic approach. Strain DWK9^T grew at 0–20% (w/v) NaCl and pH 7.0–10.0. Although its 16S ribosomal RNA gene shared 98.4% similarity with H. lutescens Q1UT^T, phylogenomic analysis and genome-based indices, including average nucleotide identity (88.12%) and digital DNA-DNA hybridization (32.5%), supported its classification as a distinct species. Genome analysis identified a complete ectABC cluster and genes annotated for ammonium assimilation and nitrate/nitrite metabolism, including glnA, gdhA, nasA, narGK, and nirBD. Physiological experiments showed that extracellular ammonium nitrogen concentrations decreased in inoculated cultures relative to the corresponding sterile control, concomitantly with bacterial growth, while nitrite nitrogen accumulated only to a limited extent. These findings provide genomic and physiological observations relevant to osmotic adaptation and inorganic-nitrogen dynamics in a halotolerant bacterium from a saline–alkaline ecosystem.