Yuping Sun

The effects of selenium supplementation in Tachysurus fulvidraco (T. fulvidraco) on low-temperature stress are not known. In this study, 280 healthy T. fulvidraco were divided into two groups, the G0 group (a control group) and the T0 group (a selenium treatment group on a 0.22 mg/kg diet), for a 6-week feeding time. Then, low-temperature stress (water temperature dropped from 26 to 13 °C, with a rate of 1 °C/h) was administered after that. The feeding results showed that selenium increased the percent weight gain (PWG), specific growth rate (SGR), and survival rate (SR) of T. fulvidraco and decreased the feed conversion rate (FCR), but these differences were not significant (p > 0.05). Under low temperatures, selenium still has no significant effects on antioxidant indexes such as glutathione peroxidase (GSH-Px) activity, superoxide dismutase (SOD) activity, and malondialdehyde (MDA) content in serum (p > 0.05). However, metabolomic analysis revealed that selenium caused changes in lipids and lipid-like molles, organic acids and their derivatives, and fatty acyls. Choline, linoleic acid, and glycerophospholipid metabolism pathways; d-arginine and d-ornithine metabolism; valine, leucine, and isoleucine degradation; and biosynthesis pathways, as well as pyrimidine metabolism pathways, were activated to produce these metabolites to combat against this stress. In addition, selenium increased the diversity of intestinal microbes in T. fulvidraco and decreased the relative abundance of Plesiomonas. However, the combined analysis showed the intestinal microbe changes did not affect metabolite production. In summary, selenium activated lipid, carbohydrate, and amino acid metabolism for energy substance provision, reduced the oxidation and production of other harmful substances, and increased the intestinal microbe diversity of T. fulvidraco to improve resistance to low-temperature stress.

Selenium is an essential trace element in animals, which can improve growth performance and anti-stress ability. Temperature is a crucial environmental factor affecting the health of fish, while Pelteobagrus fulvidraco (P. fulvidraco) can not tolerate low temperature. Does dietary selenium affect the resistance to low temperature stress of this fish? Metabolomics and microbiome analysis were used to investigate it. Feeding results showed selenium supplementation increased the weight gain rate (WGR), specific growth rate (SGR), and survival rate (SR) of P. fulvidraco, and decreased the feed conversion rate (FCR) (P > 0.05), under low temperature stress, selenium also enhanced the glutathione peroxidase (GSH-Px) activity, reduced the malondialdehyde (MDA) content in serum, but these differences were not significant (P > 0.05). Metabonomics analysis revealed lipids and lipid-like molles, organic acids and derivatives, fatty acyls were the mainly differential metabolites caused by selenium, among these, indoles and derivatives, steroids and steroid derivatives, glycerolipids and sphingolipids were significantly up-regulated, while erythrina alkaloids, organic oxygen compounds, organic nitrogen compounds and purine nucleosides were significantly down-regulated. Choline, linoleic acid and glycerophospholipid metabolism pathways, d-arginine and d-ornithine metabolism, valine, leucine, and isoleucine degradation and biosynthes pathways as well as pyrimidine metabolism pathways produced these metabolites. In addition, selenium supplementation also increased the diversity of intestinal microbes in P. fulvidraco and decreased the relative abundance of Plesiomonas under low temperature stress, however, the intestinal microbes change did not affect the metabolites production. In summary, selenium supplementation actived lipid, carbohydrate and amino acid metabolism for energy substances provision, meanwhile reduced oxidation and other harmful substances production, increased the intestinal microbes diversity of P. fulvidraco to improve the low temperature stress resistance.

Conservation methods have been shown to affect forage nutrient composition and value, but little information is available about the effect of forage conservation on plant condensed tannins (CT). The objective of this study was to assess the effects of conservation method on the concentration, chemical composition and biological activity of CT. Whole-plant purple prairie clover (PPC, Dalea purpurea Vent.) was harvested at full flower and conserved as freeze-dried forage (FD), hay (HAY) or silage (SIL). Concentration of CT in conserved PPC was determined by the butanol-HCl-acetone method. Structural composition, protein-precipitation capacity and anti-bacterial activity of CT isolated from conserved forage were determined by in situ thiolytic degradation followed by HPLC-MS analysis, a protein precipitation assay using bovine serum albumin and ribulose 1,5-disphosphate carboxylase as model proteins and by an Escherichia coli (E. coli) growth test, respectively. Conservation method had no effect on concentration of total CT, but ensiling decreased (p < 0.001) extractable CT and increased (p < 0.001) protein- and fiber-bound CT. In contrast, hay-making only increased (p < 0.01) protein-bound CT. Regardless of conservation method, epigallocatechin (EGC), catechin (C) and epicatechin (EC) were the major flavan-3-ol units, and gallocatechin (GC) was absent from both terminal and extension units of PPC CT. The SIL CT had the lowest (p < 0.001) EGC, but the highest (p < 0.01) EC in the extension units. Similarly, SIL CT exhibited a lower (p < 0.001) mean degree of polymerization (mDP), but higher (p < 0.001) procyanidins (PC) than FD or HAY CT. The protein-precipitating capacity of CT in conserved PPC ranked (p < 0.001) as FD > HAY > SIL. E. coli growth n M9 medium was inhibited by 25–100 µg/mL of CT isolated from FD, HAY and SIL (p < 0.05), but preservation method had no effect on the ability of CT to inhibit bacterial growth. The results demonstrated that ensiling decreased the extractability and protein-precipitating capacity of CT by increasing the proportions of PC. Purple prairie clover conserved as hay retained more biologically active CT than if it was conserved as silage.