Effects of hypoxia stress on the intestinal microflora of juvenile of cobia (Rachycentron canadum)

Hypoxia is one of the most frequently arising stressors in aquaculture. Hypoxia causes huge financial losses and degradation of resources. However, the response of cobia (Rachycentron canadum) to hypoxia stress is not completely understood. In this study, the effects of hypoxia on the intestinal microflora of juvenile cobia were investigated. The diversity and structure of microflora after 4 weeks of hypoxia stress were analyzed using high-throughput sequencing technology. A total of 210 fish were divided into two treatment groups, namely, hypoxic stress and normoxic control groups, with 3 replicates per group and 35 fish per replicate. The results showed that the richness and diversity of intestinal microflora increased significantly first and subsequently decreased significantly after invitation of the hypoxia stress (P < 0.05). Principal coordinate analysis (PCoA) showed that hypoxia changed the structure of the intestinal microflora. At the phylum level, Proteobacteria, Firmicutes, Spirochaetes, Tenericutes, and Bacteroidetes were the dominant bacteria. Compared to the control groups, the relative abundance of Proteobacteria increased significantly (P < 0.05) in H7, H14, and H28 groups, which was opposite to that of Tenericutes. The abundances of Firmicutes and Tenericutes both increased significantly (P < 0.05) in the H14 group. At the genus level, the relative abundance of Photobacterium increased significantly (P < 0.05) in the H7, H14, and H28 groups, while that of Brevinema decreased significantly (P < 0.05) in the H1 and H7 groups. Mycoplasma decreased significantly (P < 0.05) at all the time points of hypoxia, while Methylobacterium decreased significantly (P < 0.05) in the H7 and H28 groups. In addition, linear discriminant analysis (LDA) effect size (LefSe) analysis results indicated that hypoxia caused a significant increase in the relative abundance of Actinomyces, Pseudoalteromonas, Staphylococcus, gamma-Proteobacteria, Vibrionaceae, and Pseudomonadales, which were the biomarkers of the hypoxia group. These results provide an understanding of the effects of hypoxia stress on the intestinal microflora of juvenile cobia.