Salinity modulates gut microbiota and host transcriptome dynamics in juvenile euryhaline fish yellowfin seabream (Acanthopagrus latus)

Salinity critically modulates physiological adaptations in aquatic animals, particularly in euryhaline species frequently encountering salinity fluctuations. Here, we investigated how environmental salinity (10 ppt vs. 30 ppt) shapes gut microbiota and host transcriptome interactions in juvenile yellowfin seabream ( Acanthopagrus latus ; body weight: 7.31 ± 2.54 g). Following a 30-day comparative experiment, fish in brackish water (10 ppt) significantly reduced gut microbiota alpha diversity (Chao1 and Shannon indices, p < 0.05) and enriched Cetobacterium (72.01 % vs. 11.17 % in seawater), whereas seawater enriched Vibrio (31.46 % vs. 0.29 % in brackish water) and Candidatus Arthromitus (29.25 % vs. 1.88 % in brackish water). Gut transcriptomic profiling identified 398 differentially expressed genes, with brackish water fish upregulating metabolism ( cpa1 , adh1 ), biosynthesis ( cyp46a1 ), and ion transport ( slc26a3 ), while seawater fish activated immune pathways ( ils , ccl20 ) and redox homeostasis ( coq5 ). Critically, Cetobacterium abundance positively correlated with metabolic genes ( dmgdh , adh1 , b4galnt1 ), whereas Vibrio abundance coincided with immune gene expression ( vdac2 , ils ). These re s ults suggest that salinity-driven microbiota-host crosstalk may contribute to environmental adaptation, suggesting potential benefits of brackish water aquaculture. • Fish in brackish water enriched Cetobacterium , whereas seawater promoted Vibrio . • Low salinity upregulates metabolism, and high salinity activates immune pathways. • Cetobacterium links metabolism genes, Vibrio correlates with pro-inflammatory markers.