Change in dissolved oxygen of water bodies is the major driver of alteration in intestinal bacterial community of the bottom-feeding fishCirrhinusmrigala

Oxygen depletion, or hypoxia, a common consequence of eutrophication, has adverse effect on aquatic flora and fauna. Bacteria living within or around aquatic macroorganisms struggle to thrive in low dissolved oxygen (DO 2 ) conditions. The goal of our study was to see how the bacterial population in the intestine of a bottom-feeding fish, Cirrhinus mrigala , changed when the DO 2 level in the water decreased from 7 ± 0.5 ppm to 0.35 ± 0.05 ppm. The V3 regions of 16S rRNA genes derived out of metagenome extracted from the intestines of control and experimental fishes were sequenced and analysed. The findings revealed that during DO 2 stress (0.35 ± 0.05ppm), the aerobic bacterial population in the gut (existing when fish lived in 7 ± 0.5ppm DO 2 ) changed significantly, transforming the microbial community into a facultative and/or obligate anaerobic community with dominance of microaerophilic bacterial populations; density of Cetobacterium increased in intestinal samples, which is linked to the prevention of paradoxical anaerobism in fish tissues. The current research advances our understanding of the microbial ecology of intestinal bacterial flora under DO 2 stress, as well as their interactions with changing environmental parameters within the host body.