Abstract
Selenium (Se) supplementation either as inorganic or organic form was evaluated in Atlantic salmon post-smolt in vivo and in vitro. The basal diet was formulated to be low in fish meal and contain 0.24 mg Se kg−1; six other diets with Se inclusion of 0.15, 0.4, 0.7 or 1.1 mg kg−1 as sodium selenite (SS) and 0.15 or 0.4 mg kg−1 as L-selenomethionine (SM) were formulated from the basal diet. The diets were fed to Atlantic salmon post-smolt (mean initial weight, 216 ± 27 g) in triplicate groups (35 fish tank−1) and reared in flow-through seawater (33 ppt) at 10–12 °C for 9 weeks. At the end of the feeding trial, whole fish and tissues were sampled for in vivo assessment; whereas, liver cells and head kidney leukocytes (HKL) were isolated and their primary cultures used for in vitro assessment following exposure to hydrogen peroxide (H2O2), lipopolysaccharide (LPS) or poly I:C (PIC). Growth, feed intake, feed conversion ratio, specific growth rate, hepato-somatic index, proximate composition and mineral concentration of the whole fish (except for Se) were unaffected by dietary Se (p > .05). Hematocrit was significantly higher in fish fed the 0.4 mg Se supplemented feeds, irrespective of the Se source (p = .02). The Se concentration in whole body, liver, muscle, plasma, kidney and liver/kidney Se ratio increased with increasing dietary Se concentration (p < .0001). Level of oxidised glutathione (GSSG) in liver and head kidney followed a quadratic function (p < .05) indicating that the concentrations were lower at intermediate SS supplementation of 0.4 and 0.65 mg Se kg−1 (total Se, 0.65 and 0.87 mg Se kg−1). Impact of Se sources on glutathione redox status was similar. Slope-ratio analysis revealed SM to be more efficient than SS in improving apparent availability, whole body or tissues Se status, Se retention and reducing Se loss to the environment. In vitro, the mRNA expression of p38mapk and aif, in liver cells were affected by the impact of dietary Se, but not by the treatment of H2O2 (p < .05). In the HKL, the LPS and PIC induced pro-inflamatory action of il-1β, cox2, nfkβ and viperin were attenuated by SM supplementation, but not by SS (p < .001). Dietary Se supplementation required to the basal diet containing 0.24 mg Se kg−1 was 0.41 mg Se as SS (total 0.65 mg kg−1) or 0.17 mg Se as SM (total 0.41 mg kg−1) based on body Se homeostasis or tissue Se status. SM inclusion at 0.4 mg kg−1 diet (total, 0.65 mg kg−1) attenuated LPS or PIC induced pro-inflammatory responses in vitro. Overall, Se requirement of Atlantic salmon post smolt was 0.27 mg kg−1 diet, on availale basis. Dietary Se level required to maintain body Se homeostasis and improved health status of Atlantic salmon fed plant-based diets (0.65 mg kg−1 diet) exceed the existing EU maximum limit of 0.5 mg Se kg−1 diet. SM as the Se source in salmon feeds has the potential to improve salmon health and reduce Se emissions from Norwegan salmon farming by 60 to 70%.
Highlights
Selenium (Se) is an essential nutrient exerting its role as selenoproteins (SeCys) and selenium containing proteins (SeMet) in numerous biological processes involving free radical and antioxidant metabolism, immune and inflammatory responses (Regina, 2015)
Experimental design The study followed a regression design with 5 graded levels of dietary Se supplemented as sodium selenite (SS) to determine the dietary Se level in plant-based diets at which Se requirement is satisfied in Atlantic salmon post-smolt
The Se levels were selected based on the range of dietary Se level at which tissue saturation occured in Atlantic salmon post-smolt, 0.6 to 0.9 mg kg−1 diet following the inclusion of a multinutrient package, including Se (Antony Jesu Prabhu et al, 2019)
Summary
Selenium (Se) is an essential nutrient exerting its role as selenoproteins (SeCys) and selenium containing proteins (SeMet) in numerous biological processes involving free radical and antioxidant metabolism, immune and inflammatory responses (Regina, 2015). Diet is the major route of Se supply to fish (Janz, 2012) and the dietary Se level essential to satisfy the requirement to fish ranges between 0.15 and 1.85 mg kg−1 diet, across different fish species (Antony Jesu Prabhu et al, 2016). An indication of a dietary Se range (0.6 to 0.9 mg kg−1 diet) to satisfy requirement in post-smolt Atlantic salmon was obtained recently from a multi-nutrient regression study (Antony Jesu Prabhu et al, 2019). The fish meal based salmonid feeds often contain close to 1 mg Se kg−1 diet and Se supplementation was not considered essential to meet the minimal Se requirements (Bell and Cowey, 1989; Lorentzen et al, 1994). The importance of Se supplementation in plant ingredient based fish feeds gained significance ever more and with it, the bioavailability of different Se forms
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