Effects of bicarbonate alkalinity on growth, related enzyme activities and relative gene expression of Penaeus monodon under low salinity environment

【Objective】 This study aimed to investigate the effects of bicarbonate alkalinity on growth, related enzyme activities and relative gene expression of Penaeus monodon under low salinity environment, which could provide theoretical reference and data support for the culture of Penaeus monodon in high alkalinity saline waters. 【Method】 With pond water in an experimental base as control（group A）, The bicarbonate alkalinity in the test water was adjusted to 5.36, 6.66, 8.46 and 10.42 mmol/L using analytical grade sodium bicarbonate, and they were set as groups B, C, D and E respectively. After bicarbonate alkalinity stress experiment, the survival rate, wet weight, and specific growth rate（SGR） of Penaeus monodon in each group were calculated. Gill and hepatopancreas tissues from 2 randomly selected Penaeus monodon per group were fixed in paraformaldehyde solution for histopathological observation. Additionally, gill and hepatopancreas tissues from 3 Penaeus monodon in groups A, B, and D were collected to determine the relative expression levels of immune-and antioxidant-related genes. The gill and hepatopancreas tissues from the remaining Penaeus monodon were used to assay the activities of Na⁺-K⁺-ATPase（NKA）, acid phosphatase（ACP）, alkaline phosphatase（AKP）, α-amylase（AMS）, trypsin（TRY）, aspartate aminotransferase（AST）, alanine aminotransferase（ALT）, and the malondialdehyde（MDA） content. 【Result】 Growth indicator analysis revealed that group A exhibited the highest wet weight and SGR, while group E showed the lowest values for both indicators, significantly lower than groups A, B, C and D（P＜0.05, the same below）. The results of tissue structure section observation showed that the gill tissue structure of Penaeus monodon in group A was intact. With increasing bicarbonate alkalinity, gill tissue in groups B-D displayed cuticle thickening, disorganized epithelial cell arrangement, and roughened gill filament surfaces. Group E exhibited the most pronounced structural damage. Hepatopancreas analysis showed intact hepatic tubule basement membranes of Penaeus monodon in group A. As bicarbonate alkalinity increased, groups B-D exhibited a gradual rise in the number of B-cells within hepatopancreatic tissue. Group E displayed abnormal hepatic tubule morphology in hepatopancreas, B-cells volume increased and some ruptured, meanwhile, the number of absorptive R-cells vesicles decreased. Enzyme activity assays indicated lower AKP, ACP, and NKA activities in Penaeus monodon gill tissue of groups B-E compared to group A. In Penaeus monodon hepatopancreatic tissue, groups D and E showed significantly higher ALT activity than groups AC; group A had significantly lower AST activity than groups B-E; groups B and E exhibited significantly lower TRY activity than group A; and group A displayed significantly lower MDA content than groups B, C and E. Real-time fluorescence quantitative PCR detection showed that in Penaeus monodon gill tissue, the relative expression levels of LYZ, HSP70 and GPx genes exhibited declining trends with increasing bicarbonate alkalinity. Conversely, the relative expression levels of CA and CAT genes showed a trend of first increasing and then decreasing with the increase of bicarbonate alkalinity. In Penaeus monodon hepatopancreatic tissue, the relative expression levels of HSP70 and GPx genes showed a downward trend with the increase of bicarbonate alkalinity, while the relative expression levels of CA and CAT genes showed a changing trend of first increasing and then decreasing with the increase of bicarbo-nate alkalinity. 【Conclusion】 Penaeus monodon exhibits strong adaptive capacity to bicarbonate alkalinity stress up to 8.46 mmol/L, maintaining physiological homeostasis through structural adaptations and the regulation of relevant enzyme activities and gene expression. However, when alkalinity stress reaches 10.42 mmol/L, damage occurs to the gill and hepatopancreatic tissues, suppressing digestive and immune functions and restricting growth.