Synergistic Regulation by FoxO Signaling Pathway and Muscle Remodeling Defines the Adaptive Strategy of Largemouth Bass (Micropterus salmoides) Under Saline–Alkaline Stress

This study investigates the effects of saline, alkaline, and combined saline-alkaline water environments on the growth, muscle quality, gene expression, and metabolic profiles of largemouth bass (<i>Micropterus salmoides</i>). Juvenile fish were exposed to five water conditions for 60 days: freshwater (FW), saline water (SW, 10 ppt), alkaline water (AW, 15 mmol/L), and two saline-alkaline combinations (SAW-1: 4 ppt + 10 mmol/L; SAW-2: 6 ppt + 15 mmol/L). While growth rate was similar across groups, SAW-2 caused a significant decrease in survival rate and induced notable alterations in muscle texture and fiber structure. Transcriptomic analyses revealed group-specific enrichment of stress-responsive pathways. The FoxO signaling pathway acts as a central regulator of muscle maintenance and energy reallocation. The solute carrier gene <i>slc38a4</i> and <i>glula</i> (glutamine synthetase), both closely associated with ammonia detoxification via glutamine synthesis and transport, were upregulated under saline-alkaline stress, indicating enhanced capacity for nitrogen metabolism. In addition, two key regulators of muscle remodeling, <i>loc119898415</i> and <i>tbx18</i>, were significantly upregulated, suggesting a potential chromatin-transcription program underlying compensatory myogenesis and muscle fiber adaptation in response to environmental challenges. Metabolomic profiling showed an accumulation of osmoprotectants (betaine, taurine) in SW and SAW-2 groups, suggesting enhanced stress resistance. Multiomics integration further indicated coordinated regulation between lipid metabolism and insulin signaling, potentially mediated by the FoxO pathway. These results offer practical guidance for improving largemouth bass aquaculture under inland saline-alkaline conditions.