Metagenomic profiling of fish-associated microbiota: ecological perspectives from freshwater to marine environment—a review

Microorganisms play pivotal roles in maintaining host physiology and ecosystem balance, with fish-associated microbiomes offering unique insights due to the diverse habitats and feeding behaviours of their hosts. This review comprehensively explores the diversity, composition, and functional roles of gut and skin-associated microbial communities in fish across freshwater, brackish, and marine environments, with emphasis on recent advancements in metagenomic methodologies. Culture-independent techniques, particularly high-throughput and third-generation sequencing technologies, have revolutionized our ability to uncover microbial diversity, gene functions, and interspecies interactions. The fish gut microbiome, heavily influenced by factors such as diet, habitat, and host species, contributes significantly to nutrient metabolism, immune modulation, and physiological adaptation. Similarly, the skin microbiota provides a critical first line of defence, offering protection through competitive exclusion and antimicrobial activity. Functional metagenomics reveals microbial contributions to host metabolism, energy homeostasis, xenobiotic degradation, and environmental adaptation via the gut-brain axis and metabolic pathways. Emerging evidence highlights the bidirectional relationships between microbiota and host phenotypic plasticity. This review underscores the importance of integrative metagenomic approaches to decode complex microbial functions and their ecological relevance in aquaculture, with implications for sustainable fish health management, disease prevention, and improved productivity.