Effects of light intensity on metabolism and antioxidant defense in Haliotis discus hannai Ino

Abstract

We examined the effects of light intensity on the metabolism, antioxidant enzymes, and expression of related genes in Haliotis discus hannai Ino, to determine the optimal lighting conditions for aquaculture of these animals. Succinate dehydrogenase activity was significantly lower after 48h in animals exposed to light intensities of 30 and 60μmol/m2/s, while lactate dehydrogenase activity and lactic acid content at 96h were significantly higher compared with animals in the dark or exposed to 5 or 15μmol/m2/s (both P<0.05). There was no significant difference in reactive oxygen species (ROS) contents between animals in the dark and those exposed to 5 or 15μmol/m2/s (P>0.05) throughout the experiment. However, ROS contents under light intensities of 30 and 60μmol/m2/s peaked at 12h, when expression levels of genes encoding catalase (CAT), thioredoxin peroxidase (TPX), sigma-glutathione-s-transferase (GSTS), and mu-glutathione-s-transferase (GSTm) also began to increase, and CAT and glutathione peroxidase (GPX) activities were significantly higher than in animals in the dark or exposed to 5 or 15μmol/m2/s (P<0.05). After a further 12h, the ROS content at 30μmol/m2/s began to decrease, and GPX activity and malondialdehyde (MDA) content also gradually decreased, followed by increases in ROS and MDA 48h later, to reach respective peaks at 96h. Gene expression levels of TPX, GSTS, GSTm, heat shock protein 70 (HSP70), and CAT began to decrease 72h later, suggesting that excessive accumulation of ROS may have caused oxidative damage to the tissues. At a light intensity of 60μmol/m2/s, expression levels of TPX, GSTS, GSTm, and HSP70 genes, total antioxidant capacity, and GPX activity peaked at 48h, and the ROS content was significantly higher than in any other group 48h later (P<0.05). However, gene expression levels of TPX, GSTS, GSTm, HSP70, HSP90, and CAT, and reduced glutathione content began to decrease, indicating that the antioxidant system had been unable to perform its normal physiological functions to withstand the long-term stress of adverse environmental factors. These results suggest that the light intensity should be controlled at 5–15μmol/m2/s during H. discus hannai aquaculture, to maintain the organism's normal physiological metabolism. Statement of relevanceThe disc abalone Haliotis discus hannai is an economically important shellfish in China, with demand growing for this high-protein and low-fat seafood. In 2014, 11,5397 tons of H.discushannai were harvested from aquacultural sources, but supply cannot currently meet consumer demand. In recent years, the natural habitat of H. discus hannai has declined as a result of overfishing, marine reclamation, water pollution, among others, leading to a sharp decrease in the numbers of H. discus hannai in the wild. Thus, the development of an aquacultural system for farming H. discus hannai, as well as for its protection in the wild, are a focus of current research.Light, including photoperiod, quality and intensity, is one of the key environmental factors influencing the growth, culture and survival of aquatic organisms. Over evolutionary time, organisms have evolved both physiological and behavioral mechanisms that enable them to adapt to diurnal fluctuations in light. In Chinese traditional abalone aquaculture, a sunshade net is usually used because of the photophobic nature of abalone. In particular, the aquaculturist usually provides a darkened setting for adult abalones to increase their food intake rate, facilitate their growth and promote gonadal development. For abalones in their natural environment, the light intensity in the water area is relatively stable, suggesting that there is a key regulative effect of light intensity on their growth and development.Gao et al. (2016a,b) found that a dark environment not only adversely affected the daily aquaculture production, but was also associated with a lower growth rate compared with animals reared under higher light intensities, because of lower food conversion efficiency and greater energy losses through excretion and feces. Physiological metabolism and antioxidant defense systems may therefore be key indicators for measuring the suitability of light intensity for aquaculture production. We therefore examined the effects of light intensity on the metabolism and antioxidant defense system, and on the expression of related genes, in H. discus hannai using a light-emitting diode (LED) to replace the conventional fluorescent light source. The results of this study will improve our understanding of the physiological tolerance and antioxidant defense characteristics of abalone under different light intensities, and enable the optimization of environmental light conditions in abalone aquaculture.