Transcriptome Analysis Reveals the Mechanism by which Probiotic Alleviate the Response of Juvenile American Shad (Alosa sapidissima) to High Temperatures.

Probiotics show great potential in mitigating the impacts of temperature stress on fish. American shad (Alosa sapidissima), a species highly sensitive to high temperatures, served as an ideal model for studying fish adaptation to climate change. This study integrated biochemical and hepatic transcriptomic analyses to demonstrate that the probiotic Lactococcus lactis L103 alleviated thermal stress via a regulatory network. The findings revealed that probiotic-mediated upregulation of irf1b and nfil3-2 suppressed pro-inflammatory responses, as evidenced by significantly reduced AST and ALT activities (P < 0.05), while enhanced SOD and GSH-PX activities mitigate oxidative damage. Moreover, activation of PPAR signaling via cpt1ab and dnajc14 promoted fatty acid β-oxidation, and enhanced PPS and TPS activities for energy substrate utilization. Notably, as a core circadian regulator, nr1d1 downregulation was speculated to dampen hypothalamic-pituitary-interrenal (HPI) axis activity potentially reducing stress-induced cortisol secretion. Concomitantly, serpinh1b downregulation alleviated endoplasmic reticulum stress to complement anti-oxidative defense. These changes collectively drove metabolic reprogramming to redirect energy from stress responses to growth and homeostasis, as shown by reduced MDA levels and enhanced metabolic enzyme activities. This research established a mechanistic link between probiotic-mediated circadian resetting, lipid catabolism, and stress resilience, providing molecular insights into fish adaptation to climate change. These results also offered translational strategies for sustainable aquaculture under warming conditions, highlighting the value of probiotics in optimizing energy homeostasis during thermal challenge.