Disruption and resilience: a systematic review on how heat stress affects gut microbiota and immunity in wild freshwater fish

Climate change is increasingly disrupting freshwater ecosystems, with elevated temperatures altering the gut microbiota and immune homeostasis of wild fish. This review provides an integrative synthesis of current knowledge on how thermal stress impacts intestinal microbial communities and immune responses in freshwater fish species. A systematic literature search across Web of Science, PubMed, Scopus, and Google Scholar identified studies exploring the links between heat-induced dysbiosis, oxidative and osmotic stress, and immune modulation. Findings indicate that thermal stress reduces microbial diversity, particularly depleting beneficial phyla such as Firmicutes and Actinobacteria, while promoting opportunistic pathogens. Moreover, emerging data suggest that epigenetic mechanisms, including DNA methylation, may modulate immune gene expression in response to environmental stressors. By bridging microbial ecology, fish immunology, and climate resilience, this review highlights the overlooked role of gut microbiota and epigenetic plasticity in the adaptive responses of wild freshwater fish. The study also identifies key knowledge gaps and proposes future research directions for better understanding host–microbe–environment interactions under climate change.