The oxidative stress of the Pacific oyster Crassostrea gigas under high-temperature stress

Oxidative stress always occurs in the Pacific oyster Crassostrea gigas when they are exposed to high temperature, which will eventually lead to serious summer mortalities. In the present study, the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), the total antioxidant capacity (T-AOC), as well as the content of malondialdehyde (MDA) and glutathione (GSH) in gill were employed as the oxidative stress indexes (OSIs) in oxidative stress process, and they were measured under indoor high temperature and during outdoor aquaculture period to investigate the induction of oxidative stress by high temperature. Two evaluation formulas were established to determine the oxidative stress level (represented by oxidative stress score, OSS) in oysters based on the data matrix of OSIs. In the indoor experiment at 12 h under different temperatures (18 °C, 20 °C, 22 °C, 24 °C, 26 °C, 28 °C, 30 °C, 32 °C, and 34 °C) with 16 °C as control group, the GSH content, activities of SOD, CAT and POD reached the bottom at 28 °C in the high-temperature treated groups, which were 1.15-fold, 0.61-fold, 0.68-fold and 0.26-fold of those in the control group, respectively. MDA content increased to 19.95 nmol/mgprot at 30 °C, which was significantly higher than that in the control group (p < 0.05). The mRNA expression levels of CgNrf2 and CgKeap1 reached the bottom at 30 °C, which were 0.78-fold and 0.63-fold of those in the control group, respectively. The indoor OSS (OSSin) sharply decreased to 22.97 at 28 °C and then rebounded to 48.73 at 30 °C. In the outdoor aquaculture period from April to August, the temperature increased from 8.3 °C to 23.9 °C, which was significantly negatively correlated with MDA content (r = −0.743, p < 0.001). MDA content decreased significantly (p < 0.05) from 7.94 nmol/mgprot in April to the lowest value of 3.41 nmol/mgprot in August. The outdoor OSS (OSSout) increased and peaked in June (32.05) and decreased to the lowest value in September (21.64). OSSout of the replicated samples in July and August exhibited a significant larger range of variation compared with the other months, ranging from 22.70 to 41.00 and 20.73 to 37.00, respectively. An average high mortality rate of 33.93% was observed in the period from July to August, but not in the other months. Collectively, it is suggested that oxidative stress occurred around 28 °C after the treatment with indoor high temperature of 12 h, and in July and August 2021 during outdoor aquaculture period. These findings will be helpful to further understand and evaluate the oxidative stress in Pacific oyster exposed to high temperature.