Effects of hypoxic stress and recovery on oxidative stress, apoptosis, and intestinal microorganisms in Pelteobagrus vachelli

In aquatic environments, various natural and man-made factors reduce dissolved oxygen, threatening the survival of some aquatic organisms. However, there are few reports on the effects of hypoxia on the intestinal microbiota of fish. To study the effects of hypoxia and the restoration of normoxia, oxidation-related enzyme activity, apoptotic gene expression, and microbiota in intestines of Pelteobagrus vachelli, we examined P. vachelli after 72 h of hypoxia exposure and after 24 h of oxygen-level restoration. The results of our morphological observations showed that hypoxia clearly affected intestinal morphology. Intestinal villi eroded, and the number of enlarged goblet cells increased. Under hypoxic stress, intestinal antioxidant levels and antioxidant enzyme (superoxide dismutase, catalase, malondialdehyde, and glutathione) activities exhibited varying degrees of change compared to normoxia. Additionally, the intestinal bacterial community structure in the hypoxia group was significantly different to that of the normoxia group; the level of pathogenic bacteria (e.g., Edwardsiella) significantly increased, while beneficial bacteria (e.g., Clostridium) decreased. Moreover, the apoptotic index increased continuously with increasing hypoxia treatment time, and the expression of apoptotic genes (p53, Caspase-3, Caspase-9, Bcl-2, and Bax) was closely correlated with the degree of tissue necrosis. The study uncovered the mechanisms by which hypoxia exposure changed intestinal physiology and biochemistry of P. vachelli, which may provide new methods to assess the physical condition of animals for aquaculture.