Abstract
Water alkalinization is a critical global stressor for freshwater fish, yet the systemic patterns of multi-organ responses and injury remain insufficiently understood. This study integrates histopathology, biochemistry, and multi-organ transcriptomics to provide an integrated, time-resolved assessment of stress responses in European perch (Perca fluviatilis) exposed to acute alkaline stress (20 mmol/L). The analysis indicated that alkaline stress initially causes structural disturbance of gill tissue (lamellar fusion, necrosis) within 96 h, associated with impaired osmoregulatory functions. This primary dysfunction was followed by progressive hepatic impairment, characterized by uncontrolled oxidative stress (elevated levels in Malondialdehyde, MDA) and widespread hepatocyte necrosis. Transcriptomic analysis identified extensive transcriptional shifts associated with these alterations: large-scale differential expression in the liver (3629 Differentially Expressed Genes, DEGs) and kidney (478 DEGs). Notably, the liver exhibited a stress-responsive transcriptional profile involving activation of the HIF-1 signaling pathway and mobilizing protein quality control systems (e.g., 'Proteasome,' 'Lysosome') consistent with mitigation of proteotoxic stress. This compensatory response appeared insufficient to prevent severe metabolic disruption and cellular injury. This study presents a time-associated sequence of organ-specific stress responses under acute alkalinity, identifying candidate stress-associated genes (slc7a11, egln3, klhl38b) as potential targets for future functional studies and breeding alkali-tolerant strains.