Distinct Gut Bacterial Composition of Mesopelagic Fishes Inside and Outside the Cold Seep Environment

Cold seeps, unique deep-sea ecosystems driven by hydrocarbon- and sulfide-rich fluid fluxes, support mesopelagic fish communities. Fish gut microbiota are vital to host metabolism and health, and interact closely with surrounding water environment. However, information on gut bacterial composition and potential functional profiles of mesopelagic fishes in the cold seep has rarely been explored. In this study, high-throughput sequencing of the 16S rRNA gene was used to compare gut microbiota of mesopelagic fishes collected in the cold seep environment and outside the cold seep environment . Significant differences were found in gut bacterial composition and predicted functional profiles between the two distinct fish habitats. The genera Blastopirellula, Ruegeria, and Rubripirellula, which are involved in sulfide and hydrocarbon transformations, were significantly more abundant in cold seep fish. In contrast, Acinetobacter was significantly enriched in fish outside the cold seep. The difference of gut bacterial composition between the two habitats were primarily driven by the abundan subcommunities. Co-occurrence network analysis revealed more complex gut microbiome networks in cold seep fishes, characterized by higher node degree and connectivity but lower modularity. Together, these findings demonstrate that the cold seep environment strongly shapes both the structure and metabolic potential of mesopelagic fish gut microbiota, and highlight gut microbial contributions to host adaptation in chemosynthesis-based habitats.