Effects of normoxic and hypoxic conditions on the immune response and gut microbiota of Bostrichthys sinensis

Dissolved oxygen (DO) content is crucial for the development and health of aquatic animals. DO content is closely related to the antioxidant response of aquatic animals, while the interplay between DO content and the gut microbiota of aquatic animals is unclear. Thus, the objective of this study was to compare the changes in gut microbiota in Bostrichthys sinensis under normoxic (CT group) and hypoxic (Y group) conditions. Superoxide dismutase and catalase activities were similar in the CT and Y groups. The diversity and richness of the community decreased significantly in the Y group, concurrent with an altered composition in the gut microbiota. A dissimilarity test demonstrated that the structure of the gut microbiota was strictly separate between the CT and Y groups. The relative abundance of Gammaproteobacteria increased significantly while the relative abundance of Deltaproteobacteria decreased significantly in the Y group compared to the CT group. A significant increase in the relative abundance of Proteus was observed in the Y group, whereas the relative abundance of Desulfovibrio declined sharply. A network analysis revealed that average connectivity and sub-module numbers decreased in the Y group, suggesting that the complexity, efficiency, and robustness of the gut microbiota network in B. sinensis declined distinctly under the hypoxic condition. Clostridia was the main component serving as a module hub in the CT and Y group networks. Furthermore, most of the shared operational taxonomic units (OTUs) belonged to Clostridia. These results suggested that Clostridia both occupied an important position in the two networks of the B. sinensis gut microbiota, however the niche of Clostridia was notably altered based on its interactions with other species under the hypoxic condition. The interactions between Clostridia and other OTUs in the Y group network converted, indicating that a hypoxic condition could affect the function of Clostridia in the network. The homeostasis of the B. sinensis gut microbiota was disrupted under the hypoxic condition due to deterioration in the network structure resulting from a decrease in the number of module hubs and average connectivity. Microbial metabolism could be affected by the hypoxic conditions, and the abundant microbial functional pathways had advantage to the survival of B. sinensis. This study revealed the effects of DO on the composition of gut microbiota in B. sinensis and further illustrated the importance of interspecific interactions and species roles for evaluating the development and health of B. sinensis using an ecological network analysis of the gut microbiota.