Tissue localization of selenium of parental or dietary origin in rainbow trout (Oncorhynchus mykiss) fry using LA-ICP MS bioimaging

Effects of dietary and/or parental supplementation with selenium and mercury on their localisation in rainbow trout fry tissues (Oncorhynchus mykiss) by quantitative LA-ICP MS imaging

The increasing use of alternative feeds for sustainable aquaculture leads to a reduced selenium content requiring the use of supplements to maintain the antioxidant status of farmed fish. The introduction of tuna by-products in fish feed is attractive for their selenium content, but the presence of mercury limits their use as it may negatively impact fish health. Indeed, selenium and mercury metabolism, in relation to their interaction and biological effects in fish, is far from being fully understood. This study aims to assess the impact of parental and dietary supplementation with selenomethionine and methylmercury on selenium and mercury localisation and concentrations in tissues of 3-week rainbow trout fry. Six diets were used. Broodstock were fed a plant-based diet Bpc (with 0.2 μg g-1 selenium and 0 μg g-1 mercury) or a tuna by-product-based diet Bt (with 4 μg g-1 selenium and basal mercury at 0.2 μg g-1) during 6 months, supplemented or not with 4 μg g-1 of selenomethionine for the plant-based diet Bpo and/or with 2 μg g-1 of methylmercury for diets Bph, Bpho and Bth. Their offspring were fed the plant-based diet Fpc, supplemented or not with the same levels of selenomethionine and methylmercury, until their third week. Mercury and selenium concentrations were determined in fry whole-body by ICP MS and quantitative images of selenium and mercury were produced by LA-ICP MS from fry whole-body thin sections. Dietary supplementation of fry with selenium and/or mercury increased their concentration in all tissues, especially in liver, kidney, muscle and intestine. Parental selenomethionine supplementation had a more pronounced effect on fry selenium levels than tuna by-products diet. Mercury transfer from broodstock to their offspring was weak. Mercury concentrations decreased in the kidney of fry supplemented with both selenium and mercury, which could be related to the presence of selenium.

Maternal supplementation of nano-selenium in a plant-based diet improves antioxidant competence of female Arabian yellowfin sea bream (Acanthopagrus arabicus) breeders and their progeny

Antioxidants such as selenium (Se) play vital roles in reproduction success and larval development in fish. A three-month feeding experiment was conducted to examine the impact of enriching a plant-based diet (60% of fishmeal was substituted with a blend of plant ingredients) with nano-selenium (nano-Se) on antioxidant metabolism in female brooders and the progeny of Arabian yellowfin sea bream (Acanthopagrus arabicus). At this point, the plant-based diet was supplemented with graded levels of nano-Se at 0, 0.5, 1, 2, and 4 mg/Kg diet. Moreover, a fishmeal-based diet served as a positive control (FMD-Control). Broodfish were randomly distributed into eighteen 10 m³ rectangular concrete tanks (8 males and 8 females in each tank). Each experimental diet was subjected to three replications. Selenium retention increased in the serum, liver, ovary, eggs, and three-day larvae with increasing dietary nano-Se levels (P < 0.05). Supplementing the plant-based diet with 2-4 mg nano-Se/Kg significantly enhanced normal embryogenesis, fertilization, hatching, and larval survival rates. Generally, fish fed on plant-based diets with lower nano-Se supplementation (0-0.5 mg/Kg) had higher catalase and superoxide dismutase activities in the liver, ovaries, eggs, and larvae compared to the other groups. Glutathione peroxidase activity and total antioxidant capacity markedly increased, whereas lipid peroxidation decreased in the liver, ovary, serum, eggs, and progeny of broodfish fed with nano-Se supplemented diets (P < 0.05). In conclusion, supplementation of 2-4 mg nano-Se /Kg in a plan-based feed is recommended for the improvement of antioxidant defense in female A. arabicus brooders and their progeny.

Long-term effect of parental selenium supplementation on the one-carbon metabolism in rainbow trout (Oncorhynchus mykiss) fry exposed to hypoxic stress

This study evaluated how different forms of selenium (Se) supplementation into rainbow trout broodstock diets modified the one-carbon metabolism of the progeny after the beginning of exogenous feeding and followed by hypoxia challenge. The progeny of three groups of rainbow trout broodstock fed either a control diet (Se level: 0·3 µg/g) or a diet supplemented with inorganic sodium selenite (Se level: 0·6 µg/g) or organic hydroxy-selenomethionine (Se level: 0·6 µg/g) was cross-fed with diets of similar Se composition for 11 weeks. Offspring were sampled either before or after being subjected to an acute hypoxic stress (1·7 mg/l dissolved oxygen) for 30 min. In normoxic fry, parental Se supplementation allowed higher glutathione levels compared with fry originating from parents fed the control diet. Parental hydroxy-selenomethionine treatment also increased cysteine and cysteinyl-glycine concentrations in fry. Dietary Se supplementation decreased glutamate-cysteine ligase (cgl) mRNA levels. Hydroxy-selenomethionine feeding also lowered the levels of some essential free amino acids in muscle tissue. Supplementation of organic Se to parents and fry reduced betaine-homocysteine S-methyltransferase (bhmt) expression in fry. The hypoxic stress decreased whole-body homocysteine, cysteine, cysteinyl-glycine and glutathione levels. Together with the higher mRNA levels of cystathionine beta-synthase (cbs), a transsulphuration enzyme, this suggests that under hypoxia, glutathione synthesis through transsulphuration might have been impaired by depletion of a glutathione precursor. In stressed fry, S-adenosylmethionine levels were significantly decreased, but S-adenosylhomocysteine remained stable. Decreased bhmt and adenosylmethionine decarboxylase 1a (amd1a) mRNA levels in stressed fry suggest a nutritional programming by parental Se also on methionine metabolism of rainbow trout.