Effects of Hypoxic Preconditioning on the Physiological and Biochemical Characteristics of Scapharca broughtonii under Hypoxia Stress

Abstract

Dissolved oxygen (DO) is one of essential factor for the survival of aquatic organisms, affecting many physiological processes including growth, survival, reproduction and development. Due to the change of global environment and the influence of human activities, hypoxia often occurs in both natural and cultured water bodies. Studies showed that water is in a hypoxic state when DO is lower than 2 mg/L, and aquatic animals in this condition may often suffer from hypoxic stress. The blood clam Scapharca broughtonii is an important economic bivalve because of its delicious taste and rich protein, vitamins and hemoglobin. At present, the S. broughtonii has become an important species in Northern China, and the better economic benefit of breeding drives the rapid development of seedling production and culture. However, hypoxia often threatens the cultivation of S. broughtonii. Hypoxic preconditioning refers to the endogenous protective mechanism generated after the body is stimulated by multiple transient and non-fatal hypoxic stimuli, which can protect the body tissues and improve the tolerance of the body. To explore the effects of hypoxic preconditioning on the physiological and biochemical characteristics of S. broughtonii under hypoxia stress, the clams were divided into three groups containing H2 (hypoxic preconditioning for two times), H4 (hypoxic preconditioning for four times) and C (no hypoxic preconditioning, as control group). And then, we analyzed the feeding rate (IR), energy metabolism and three enzymes activity of three groups during the process of hypoxia stress for 48 hours with DO concentration of ~2.0 mg/L. The first item we explored was changes in feeding rates. During hypoxia stress, the IR of the three groups showed a tendency of decreasing first and then increasing. At the late stage of stress, the feeding rate of the three groups recovered to some extent, and the recovery degree of groups H4 and H2 were better than that of group C. The energy metabolism we studied contains the oxygen consumption rate (OR) and the ammonia excretion rate (NR). The results showed that the OR of the three groups increased gradually under hypoxia stress and reached the maximum value at 48 h after treatment. The OR of group C, H2 and H4 increased by 1.15, 1.08 and 0.73 times, respectively, after 48 hours of treatment. The trends in NR were that the NR in group C gradually increased during treatment, while that in group H reduced first followed by increasing and then decreased. The NR of group C, H2 and H4 showed different trends during the stress period which were 1.67, 1.30 and 0.97 times of 0 h at 48 h, respectively. And the ratio of oxygen to nitrogen in group C was relatively stable, while that in group H was relatively wide. The following are the experimental results of enzyme activity changes, which include cytochrome c oxidase (COX), lactate dehydrogenase (LDH) and glutathione (GSH). The COX of the three groups decreased gradually with the extension of hypoxia stress time. LDH activity and GSH enzyme content all showed an upward tendency. Compared with the control group C, the enzyme activity of the pre-hypoxic group H2 and H4 were relatively stable during the hypoxic stress. In total, the results showed that S. broughtonii pretreated with hypoxia had higher feeding rate, lower oxygen consumption rate and relatively stable enzyme activity compared with the control group, which revealed hypoxia pretreatment could improve its ability of hypoxia tolerance. It showed that hypoxia preconditioning had a more positive effect on the hypoxia tolerance of S. broughtonii. In this study, the effects of hypoxic preconditioning on physiological and biochemical of S. broughtonii under hypoxia stress including feeding rate, respiratory metabolism and enzyme activity were studied for the first time. It was found that hypoxic training could effectively improve the body's response to hypoxic stress, enriching the research data related to hypoxic adaptation, and providing data for exploring the stress adaptation mechanism of S. broughtonii.

Keywords

• scapharca broughtonii
• hypoxic preconditioning
• hypoxia stress
• physiological and biochemical characteristics