Impact of ocean acidification on the intestinal microflora of the Pacific oyster Crassostrea gigas

The intestinal microflora is critical for the health of hosts by affecting their nutrient absorption and immune response. Increasing evidences demonstrate that environmental stress can lead to the dysbiosis of intestinal microflora, which increases the susceptibility of host to pathogens. Ocean acidification (OA) is one of the greatest environmental threats for marine mollusks with the negative effects on growth and calcification, but its impact on intestinal microflora is poorly understood. In the present study, the intestinal microflora of the Pacific oyster Crassostrea gigas reared at seawater with pH values of 8.1 (control group), 7.8 (AC78 group) and 7.4 (AC74 group) were characterized and compared using 16S rRNA gene sequencing. The composition of oyster intestinal microflora changed significantly after acidification, while no significant difference of alpha-diversity was observed between the control and acidification groups. At the phylum level, the relative abundance of Firmicutes decreased in acidification groups, and the Firmicutes/Bacteroides ratio in AC78 and AC74 groups were 0.53 and 0.31-fold of that in the control group, respectively. At the genus level, the intestinal microflora was dominated by Mycoplasma, which was significantly enriched in the two acidification groups. LEfSe analysis showed that Mycoplasma was one of the most discriminative biomarkers in the AC78 group, while Alteromonadales, Amphritea, Salinirepens and Alteromonas were the biomarker taxa in the AC74 group. The functional prediction results indicated that the pathways related to protein and nucleic acid synthesis were enriched in the two acidification groups, while those related to carbohydrate catabolism were blocked in the AC78 group but enhanced in the AC74 group. These results suggested that the relative abundance of probiotic bacteria decreased upon ocean acidification, which favored the proliferation of pathogenic species in the intestine of oysters. The increased consumption of nutrients caused by microflora proliferation would aggravate the susceptibility of oysters to pathogens. Under greater OA stress, the intestinal microflora would enhance the competition for energy source with their hosts, consequently posing a great challenge to the host health. The information contributed to the better understanding of the oyster-microflora interactions under environmental stress.

Automated summary

Environmental stress can lead to dysbiosis of the intestinal microflora, increasing host susceptibility to pathogens. Ocean acidification has a significant impact on the intestinal microflora of Pacific oysters, reducing the abundance of probiotic bacteria and increasing pathogenic species. This can exacerbate the susceptibility of oysters to pathogens and pose a challenge to host health.