Comprehensive Proteomics Reveals Acute Osmoregulatory Responses in the Intestine of Oreochromis mossambicus Under Salinity, Alkalinity, and Combined Saline–Alkaline Stress

Osmotic stress induces pleiotropic physiological disruptions in aquatic organisms; however, the intestinal proteomic responses of euryhaline teleosts to salinity, alkalinity, and saline–alkalinity exposure remain insufficiently characterized. In the present study, intestinal histopathology, antioxidant enzymatic activities, and proteomic profiles were investigated in Oreochromis mossambicus (Peters, 1852) subjected to 24-h acute individual and concurrent salinity-alkalinity challenges. Pronounced histopathological lesions and significant alterations in enzymatic antioxidant defenses were observed, as reflected by elevated glutathione peroxidase (GSH-Px) and malondialdehyde (MDA) levels alongside diminished catalase (CAT) activity. Proteomic analysis identified multiple differentially abundant proteins implicated in osmoregulation across all treatment groups, and subsequent pathway enrichment analysis indicated that stress exposure perturbed oxidative phosphorylation and immune-related signaling cascades, particularly under alkalinity and combined conditions. Collectively, this work provides the first comparative proteomic characterization of intestinal acute stress responses to osmotic stress in O. mossambicus, thereby enhancing our understanding of the molecular regulatory networks governing osmoregulation in euryhaline fish confronted with multifactorial environmental fluctuations.