Effect of pH on temperature controlled degradation of reactive oxygen species, heat shock protein expression, and mucosal immunity in the sea cucumber Isostichopus badionotus

This study evaluated the effect of pH on the activity of antioxidant and immune enzymes in the sea cucumber Isostichopus badionotus exposed to different temperatures. The organisms (530 ±110 g) were exposed to 16, 20, 24, 28, 30, 34 and 36°C for 6 h to evaluate thermal limits at two water pH values (treatment = 7.70; control = 8.17). For the thermal tolerance experiment, the organisms were exposed to sublethal temperature of 34°C for 3, 6, 12, 24, and 48 h. I. badionotus showed signs of thermal stress by synthesizing heat shock protein 70 (hsp70) at the cold (16°C) and warm thermal limits (34°C). The glutathione peroxidase (GPx) showed a negative correlation with superoxide dismutase (SOD) activity in modulating the effect of oxidative stress at different temperature levels. Specifically, GPx activity was maximal at the extremes of the cold and warm temperatures (16, 20, and 36°C) tested, while contrarily, the SOD activity increased significantly in the narrow range of temperature between 28 and 30°C, as a part of a reaction to offset oxidative damage. The effect of pH on the expression of hsp70 was not significant, whereas the antioxidant enzymes activity was stimulated at pH 7.70. Mucosal immunity, evidenced by the activation of the phenoloxidase (PO) system, increased above the basal level at pH 7.70 and at 28, 30, and 34°C. Independent of pH, the temperature of 34°C was identified as the 12 h-sublethal upper limit for I. badionotus.