Impact of ocean acidification on physiology and microbiota in hepatopancreas of Pacific oyster Crassostrea gigas

The hepatopancreas is an important tissue involved in various biological metabolism for mollusks, but its responses to ocean acidification (OA) have not been well evaluated. In this study, the oysters were cultured in simulated conditions by continuously bubbling with ambient air (pH=8.10) or air-CO 2 (pH=7.50) for up to two months, and the variations on the antioxidant capacity, digestive ability, and microbiota composition in hepatopancreas of Crassostrea gigas were analyzed. The results show that although superoxide dismutase and glutathione responded quickly to OA stress, the antioxidant capacity of the hepatopancreas was inhibited, as revealed by the decrease of the total antioxidant capacity, which led to an upward trend of the malondialdehyde, demonstrating that the oxidative damages were accumulated under the OA process. The determination of the digestive ability manifested as the decrease of pepsin activity and the recovery of lipase and amylase activity after long-term acidification, which may be helpful to improve the adaptability of oysters. In addition, analysis on 16S rDNA amplicon revealed that the total species abundance and diversity of the hepatopancreas microbiota experienced a dynamic change, but finally it decreased greatly after long-term acidification. The structure of the hepatopancreas microbiota was changed drastically with the change of the dominant species from aerobic to the anaerobic and facultative anaerobic bacteria, and the abnormal proliferation of some species, such as genus of Mycoplasma and order Clostridiales, which may aggravate the adverse effects of OA on the physiological functions of the hepatopancreas. As a result, our findings enrich our understanding of the accumulated oxidative damage and adaptive digestive ability in oyster hepatopancreas caused by OA. For the first time, the changes of the hepatopancreas microbiota under long-term acidification conditions are described, proving a good reference for the study of the response and adaptation mechanisms of bivalve mollusks in a wide range of oceans OA.