Dietary niches drive microbial community assembly, network reorganization, and symbiont evolution in freshwater fish gut microbiomes

Host diet is a fundamental ecological factor shaping the assembly and evolution of host-associated microbiomes, yet how dietary niches influence the structure of microbial associations and functional adaptation in freshwater fish remains poorly understood. This study selected five dominant farmed freshwater fish species in China with distinct feeding habits (herbivory, omnivory, filter-feeding, and carnivory) and systematically investigated the adaptive mechanisms of their gut microbiomes by integrating metagenomics, targeted cultivation, comparative genomics, and in vitro assays. We show that dietary niches exert a strong deterministic effect on microbial community assembly, leading to pronounced differences in ecological network topology, including connectivity, modularity, and keystone taxa. Cetobacterium was detected in all five fish species but exhibited a higher relative abundance in omnivorous (16.0%) compared to carnivorous fish (5.4%), suggesting that it may be a core genus within the gut microbiota of freshwater fish. Comparative genomics further revealed that Cetobacterium symbionts exhibit streamlined genome architectures and conserved core metabolic functions, indicative of adaptive evolution toward stable host-associated lifestyles. Guided by metagenomic insights, we isolated multiple Cetobacterium strains displaying host-adapted functional traits, linking community-level ecological patterns to cultivable symbiont resources. In summary, our findings demonstrate that freshwater fish guts function as ecological niches that deterministically structure microbial community assembly and drive symbiont evolution, providing a conceptual framework for understanding host-microbiome co-adaptation in aquatic ecosystems.