Physiological and Transcriptome Analyses Offer Insights into Revealing the Mechanisms of Red Tilapia (Oreochromis spp.) in Response to Carbonate Alkalinity Stress

The utilization of saline-alkali water resources presents a promising approach for freshwater aquaculture. Red tilapia (<i>Oreochromis</i> spp.) exhibits moderate salinity tolerance, but its adaptation mechanism to alkaline conditions remains poorly understood. In the current study, five alkaline carbonate concentrations in a 60-day chronic stress experiment on red tilapia were evaluated. The experimental design included a control group (CA0, 0 mmol/L) and three treatment groups (CA10, 20 mmol/L; CA30, 30 mmol/L; and CA40 40 mmol/L). The results indicated that at alkaline carbonate concentrations exceeding 20 mmol/L, the gill filaments exhibited curling and deformation, the hepatocytes displayed migration, and tissue damage increased significantly. The gill's antioxidant capacity initially decreased and then increased, with severe gill injury in the CA40 group, leading to significantly reduced levels of SOD, CAT, and GSH-PX compared to the CA40 group (<i>p</i> < 0.05). Conversely, the enzymatic activities related to energy metabolism showed an opposite trend under alkaline carbonate stress. The transcriptome analyses of gill tissues across five groups identified significant alterations in key pathways, including the metabolic process (endocytosis, focal adhesion, PI3K-Akt signaling pathway, MAPK signaling pathway, and Citrate cycle (TCA cycle)), and immune responses (mTOR signaling and NOD-like receptor signaling pathways). Additionally, we screened 13 differentially expressed genes (DEGs) as potential regulators of alkaline stress and validated their expression levels using quantitative real-time PCR (qPCR). This study preliminarily elucidated the molecular mechanism of red tilapia in the physiological regulation process under chronic alkaline stress, and offers a theoretical foundation for breeding programs aimed at developing alkali-tolerant strains for aquaculture in alkaline water environments.