Selenium (Se) is a key essential trace mineral directly or indirectly involved in the maintenance of the body redox balance and antioxidant defense . Bein g a structural component of several selenoproteins, including glutathione peroxidase (GPx ), Se helps to regulate lipid and protein oxidation processes, thereby enhancing the meat quality and its nutritional value. Traditionally , inorganic sources such as sodium selenite have been used to supplement Se in fish diets. However, in fish, research has shown that organic forms of Se have higher bioavailability and effectiveness in regulating oxidative stress and tissue retention rates than inorganic forms. The optimal levels of Se required to maximize fish performance may vary depending on the source, and further exploration in aquatic species is needed . The primary objective of this study was to compare the performance, product quality, and stress resistance of gilthead seabream ( Sparus aurata ) fed different sources of selenium (inorganic and two different organic sources) while meeting EFSA maximum allowed supplementation of organic Se as an additive of 0.2 mg kg -1.
Materials and Methods
Three diets (CTRL, AVSe and ORGSe ) were formulated to be isoproteic (48 % DM) , isolipidic (17% DM) and to vary in their Se source. Diets were supplemented with 0.2ppm Se as selenite (CTRL), Zn-L-SeMet (AVSe) or OH-Se-Met (ORGSe ). All experimental diets were extruded and hand fed to apparent satiety, three times a day (9h, 12h, and 16h) for 137 days, to q uadruplicate groups of gilthead seabream juveniles (50 g initial body weight). The f ish were subjected to a 12-h light/12-h dark photoperiod regime and kept in 250L fiberglass tanks in a recirculating saltwater system (salinity 35‰, 22 ± 1 ºC) . At the end of the growth trial, all fish were individually weighed and measured, and sampled for whole-body composition (n=4), total lipid and mineral composition of dorsal muscle (n=12), evaluation of hepatic oxidative stress (n=12) , shelf-life evaluation over a period of 14 days (n=8 ), muscle evaluation of pH, WHC, color, texture and lipid peroxidation (LPO) at days 7 and 14 (n=8), and for sensory analysis (n=20). In order to determine fish metabolic and oxidative status , 12 additional fish per treatment were subjected to overcrowding (100 kg m-3) f or 5 min and air exposure for 1 min. After 1 hour of recovery, plasma and liver samples were collected from these fish.
A ll dietary treatments were equally accepted by fish regarding growth performance. However, nutrient utilization and fish whole-body composition were significantly affected by the diets. Fish fed AVSe had significantly higher whole body protein content compared to those fed the CTRL diet. These fish were the leanest but did not differ significantly from the CTRL . In terms of mineral utilization, increased Se gain and retention in fish fed AVSe and ORGSe diets, resulted in higher whole body Se content in these fish compared to the CTRL. It must be highlighted that only AVSe diet was able to significantly increase Se deposition in the muscle (Figure 1 ). The flesh quality parameters analyzed did not reveal major differences among the tested diets. The only exception was hardness, which was higher in fish fed ORGSe diet in comparison to CTRL. This difference could not be organoleptically confirmed by the sensory panel that gave a positive evaluation to all fish. Concerning stress tolerance, antioxidant enzymes’ activity was either reduced (GR, GPx) or induced (GST) by the applied stress, although antioxidant enzymatic mechanisms did not differ significantly among dietary treatments. The source of Se supplemented to the diets has affected non-enzymatic antioxidant mechanisms. I n fish exposed to the acute stress challenge, oxidized glutathione (GSSG) and oxidative stress index (OSI) were significantly higher with AVSe than with CTRL diet. Despite the increased levels of GSSG and OSI, no signs of oxidative damage were observed in the liver, as evidenced by similar LPO levels amongst dietary treatments and stress conditions.
Discussion and Conclusion
All the tested diets were equally effective in promoting the growth of fish and ensuring high feed efficiency. T here were no significant differences in the overall consumer acceptance between the different muscle samples from the various diets. S tress exposure was shown to decrease GR and GPX activities which led to an increased level of oxidized glutathione (GSSG), particularly fish fed AVSe. This suggests a quicker response by these fish in challenging situations, by direct reaction of GSH with H2O2 . The cause behind the lack of activation of the antioxidant enzyme response could not be completely addressed in the present study and should be considered and evaluated in future studies. T he present study suggests that supplementation with organic Se, particularly from AVSe leads to leaner gilthead seabream fillets fortified in Se . The use of organic sources of Se, in particular of Zn-L-SeMet , could also have a positive environmental impact by improving Se gain and retention and therefore reducing Se emissions into the water bodies.
This study was funded by the Zinpro Corporation, 10400 Viking Drive, Eden Prairie, MN 55344, United States.