Introduction
F unctional health feeds are a pillar of modern aquaculture. Inactivated yeast cell wall (YCW) fractions are widely documented to conditionally confer health and/or performance benefits across a diversity of species . However , only few dose-response studies have been reported and none in marine species limiting the optimal application of this biotechnology . The aim of this study was to document the animal performance and mucosal health response to graded intake of a specific YCW paraprobiotic with the view to inform the strategic use of this ingredient. The specific YCW tested in this study was previously selected based on its specific parietal features and promising functional benefits on the intestinal barrier and immune homeostasis (YCW 2; Rawling et al., In Press).
Materials and Methods
The 9-week trial (Riasearch Lda ; Murtosa, Portugal ) tested 4 diet s in quadruplicate tanks (500 L tanks flow-through indoor , pumped-ashore natural seawater ~ 22 °C; ~36 ppt ) in juvenile European seabass (BWi = 11.3 g ; 45 fish/tank) reared under ideal conditions (43 days) then chronically exposed to environmental and handling stress (20 days) (Fig 1 ). The basal diet was formulated to the species requirements and according to current commercial practices ( 55% plant meal , 20% fish meal; 15% LAPs; 6% fish oil, 6.5% rapeseed oil ; 46.3% crude protein; 16,4% crude lipid; gross energy 21.1 MJ/kg). Test diets were prepared by incorporating graded level of a selected, proprietary YCW ingredient (Lallemand Animal Nutrition, Blagnac, France) “ on-top” the basal recipe a t 0 (Control), 1.5; 3.0 or 4.5 kg/T feed and manufactured by traditional hot extrusion. Test d iets were hand-fed to apparent satiation 4 times daily over the trial’s duration.
Growth and feed performance were assessed over phase I, phase II and the overall trial’s duration based on total tank biomass, individual body-weight (15 fish/tank) and apparent daily feed intake per tank. Tissues were sampled at T1 and T2 as follow: crude skin mucus quantity (4 fish/tank, surgical absorbent sponges) , skin mucus , faeces , gill, gut and skin (3 fish/tank). Skin mucus and plasma were assayed for immune biomarkers; faeces for intestinal health markers ; mucosae histology focussed on morphometry and goblet cell population and mucosal gene expression was performed using tissue-specific panels of 20 genes (immune, cellular and oxidative stress, epithelium barrier function) . D ata were analysed by 2-way ANOVA and post hoc analysis with significance accepted at P < 0.05.
Results
Growth performance increas ed with increasing YCW level (Fig 2a, b). M ean body-weight was significantly higher in YCW 4.5 compared to the Control at the end of Phase I and II, with YCW 1.5 and 3.0 showing intermediary values. Similar trends were evident for SGR although differences were significant over the full trial period only for YCW 3.0 (+4.5%) and YCW 4.5 (+ 6.8%) compared to the Control.
There was a significant diet effect on FCR which was increasingly improved with increasing YCW intake over the whole trial’s duration (Fig 2c). FCR overall improve d by up to 8.3 % in YCW 4.5 compared to Control with, interestingly, an a pparent mitigation of its degradation over the period of chronic-stress exposure (Phase II) in this group.
Skin mucus excretion was significantly higher in YCW 4.5 compared to the C ontrol at the end of the trial (Fig 2d) with numerical increases at lower YCW intakes. S imilar trends were measured at the end of the Phase I (ideal conditions).
Further tissues analysis will be presented.
Conclusion
There was a positive dose relationship between dietary intake of YCW and zootechnical performance as well as skin mucus excretion . In particular, t he highest level of YCW tested (4.5 kg/T feed) appeared to mitigate the degradation of the zootechnical performances but also to boost skin mucus excretion during chronic-stress exposure .
Taken together, this YCW paraprobiotic as the potential to support the performance and robustness of European seabass juveniles. The feed supplementation strategy can be tailored to the levels of challenge expected and of benefits targeted.
Rawling M, Schiavone M, Merrifield DL, Apper A, Castex M, Leclercq E, Foey A 2023. Yeast cell wall extracts from Saccharomyces cerevisiae varying in structure and composition differentially shape the innate immunity and mucosal tissue responses of the intestine of Zebrafish (Danio rerio). Front. Immunol. Vol 14, doi: 10.3389/fimmu.2023.1158390.