EFFECT OF THE DIETARY INCLUSION OF BLOOD HYDROLYSATES ON EUROPEAN SEABASS RESPONSE TO STRESS

Introduction

 In aquaculture farms, fish are subjected to several  stress situations , including high farming densities,  periodic  handling, routine procedures or even changes in temperature, oxygen and other environmental parameters .  All these stressors may induce oxidative stress  by causing an imbalance between the generation of reactive oxygen species (ROS) and the scavenging activity of antioxidants [1].  The accumulation of ROS may damage biomolecules such as proteins, lipids or nucleic acid, impairing growth, immune response and flesh quality [2] . Bioactive peptides ,  small amino acid chains with properties including antioxidant, mineral-binding, immunomodulatory or antimicrobial activities , can be obtained from protein-rich by-products, making them attractive ingredients for inclusion in aquafeeds within the context of a circular economy.  Thus, supplementation of diets with functional ingredients  able to modulate fish  oxidative stress has been considered in this work, using bioactive peptides included in swine blood hydrolysates (BH). W e  have investigat ed  the potential of the dietary inclusion of BH to  improve European seabass (Dicentrarchus labrax) oxidative status  after air exposure.

Methods

Three fractions of swine BH obtained  by autohydrolysis (AH) or enzymatically were selected. The  enzymatically  obtained  BH were further submitted to a micro- (MF) or nanofiltration (NF). Dried hydrolysates were then included in five isolipidic and isoproteic diets for European seabass: a fishmeal (FM) based diet (positive control, PC), a commercial-based diet where 50% of FM was replaced by  plant proteins (negative control, NC) and three diets where 3% of each BH was added to the NC. Diets were assigned to triplicate groups of 71 European seabass juveniles (initial weight 12.3 ±  1.4 g),  and  fed to apparent satiation in a recirculating saltwater system (RAS) . After 12 weeks , 9 fish per treatment were either immediately sampled or air-exposed for 1 minute and let to recover for 6 hours prior to sampling. Plasma and  liver were collected to evaluate oxidative stress indicators.

Results

a

Plasmatic cortisol and lactate were elevated for all diets after the air exposure although differences among diets were not found . Glucose  levels  were unaffected by either diet or stress.  Regarding  oxidative stress  markers in the liver , lipid peroxidation (LPO), measured by TBARS,  tended to  increase after air exposure. Considering the non-stressed fish,  MF  diet  led to the  lowest  LPO  levels,  being significantly lower than the AH, but without differing significantly from the remaining diets.  In stressed fish, the dietary impact on LPO levels was limited.  Protein oxidation, evaluated by carbonyl content, decreased in stressed fish, irrespectively of the dietary treatment .  Liver catalase was significantly lower  in the NC, NF and AH compared to the PC , and increased  after stress in all treatments .  Glutathione peroxidase was unaffected by diets or stress condition .  Fish fed MF had a significantly higher  superoxide dismutase  activity than those fed  PC and  AH diets and was reduced in  all  stressed fish compared to  the non-stressed ones .

Conclusions

 The stress response triggered by air exposure involved  an increase in  cortisol levels, which was followed by an  increment in plasma lactate ; however, the tested BHs could not enhance  plasma response  to stress . Regarding  liver  oxidation markers, the  stress  challenge increased LPO, as ROS can accumulate in the liver under oxidative stress. Carbonyls decreased post-stress, likely due to a feedback interaction with the radicals produced during LPO, which acted to reduce protein oxidation.  The NF, composed of smaller peptides than the other BH , may modulate  European seabass  antioxidant  defences by lowering catalase levels without increasing LPO,  suggesting this  diet could be providing  exogenous antioxidants to counteract ROS-induced oxidative stress .

References

1.                        Hanke, I., et al., Chronic stress under commercial aquaculture conditions: Scale cortisol to identify and quantify potential stressors in milkfish (Chanos chanos) mariculture. Aquaculture, 2020. 526: p. 735352.

2.                        Abdel-Tawwab, M., et al., Dietary garlic and chitosan enhanced the antioxidant capacity, immunity, and modulated the transcription of HSP70 and Cytokine genes in Zearalenone-intoxicated European seabass. Fish Shellfish Immunol, 2021. 113: p. 35-41.

Acknowledgments

Work supported by Project MOBFOOD, POCI-01-0247-FEDER-024524·LISBOA-01-0247-FEDER-024524, cofounded by PORTUGAL2020, Lisb@a2020, COMPETE 2020 and the EU. DR thanks FCT, SANFEED and SenseTest© for her PhD grant (PD/BDE/150524/2019). RP thanks FCT for his PhD grant ( SFRH/BD/144631/2019).