Introduction
The reduction in the use of marine ingredients in aquafeeds raises concerns regarding the supply of key functional nutrients, such as taurine. This is particularly relevant as plant-derived ingredients, commonly used in fish diet formulations, are virtually devoid of taurine. Taurine plays key roles in osmoregulation, antioxidant defence and immune function, and its deficiency may compromise fish resilience to environmental stressors. Thermal stress is a relevant challenge under climate change scenarios, yet the interaction between taurine nutrition and stress response remains poorly understood. This study evaluated whether taurine supplementation modulates physiological, immune and metabolic responses to thermal stress in Senegalese sole (Solea senegalensis) fed plant-based diets.
Material and Methods
Three isonitrogenous and isolipidic diets (59% crude protein and 8% crude fat, dry matter basis) were formulated using practical ingredients. A control diet (CTRL) contained 69% marine ingredients, while in the PLANT diet marine ingredients were reduced to 11%. The PLANTTAU diet was the PLANT formulation supplemented with 1% taurine.
Senegalese sole juveniles (41 ± 1 g) were distributed into 18 tanks in a recirculating aquaculture system and maintained at 19.7 ± 0.5 ��C during a four-week conditioning period. Fish were then sampled after 24 h fasting (n = 3 tanks per diet). The remaining fish were subjected to a thermal challenge by increasing water temperature by 5 ��C for 12 days, followed by a second sampling.
Growth performance, hepatosomatic index (HSI), plasma stress indicators, innate immune parameters and hepatic free amino acids, including metabolites related to methionine metabolism, were analysed.
Results and Discussion
Growth performance and plasma cortisol and glucose were not affected by diet or thermal challenge. However, thermal stress modulated innate immunity, decreasing lysozyme and antiprotease activities while increasing nitric oxide levels.
A significant diet × stress interaction was observed for peroxidase activity. Post-stress peroxidase activity was highest in CTRL fish, while PLANTTAU reached comparable values, unlike the unsupplemented PLANT diet. This suggests that taurine supplementation supported the maintenance of innate oxidative immune response under thermal stress.
HSI decreased after thermal challenge, suggesting mobilisation of hepatic reserves. Hepatic free amino acid profiles showed marked diet- and stress-dependent changes. The PLANT diet strongly depleted hepatic taurine, whereas supplementation restored taurine levels close to CTRL before stress. Thermal challenge reduced hepatic taurine in CTRL and PLANTTAU fish, suggesting increased taurine utilisation under stress.
Beyond taurine, methionine-related metabolism was also modulated by diet and stress. Hepatic methionine decreased after thermal challenge in all dietary groups, suggesting increased utilisation under stress. CTRL fish showed the highest pre-stress methionine levels and, after stress, a marked increase in homocysteine, cystathionine and cysteine, indicating activation of the methionine–transsulfuration pathway. By contrast, PLANT fish showed lower methionine availability and consistently low homocysteine levels, while cystathionine and cysteine increased after stress. In PLANTTAU fish, methionine was also reduced after stress, but cystathionine and cysteine remained stable, suggesting that taurine supplementation altered the stress-induced adjustment of sulphur amino acid metabolism rather than restoring a CTRL-like profile.
Diet-dependent responses were also observed for amino acids linked to nitrogen metabolism and redox balance. Glutamine decreased after stress in CTRL, increased in PLANT and remained stable in PLANTTAU. Glycine increased after stress in CTRL and PLANTTAU, whereas it decreased in PLANT. Together, these results suggest that taurine supplementation did not simply restore the overall hepatic amino acid profile of CTRL fish, but contributed to stabilising selected metabolic pools under thermal stress.
Conclusion
Taurine supplementation in plant-based diets for Senegalese sole did not affect growth but modulated selected immune and metabolic responses to thermal stress. While hepatic taurine levels were restored, broader sulphur amino acid metabolism was not fully normalised. Taurine supplementation stabilised specific metabolites and supported post-stress peroxidase activity, highlighting its role in promoting resilience under challenging conditions.
Acknowledgment
This research received funding by national funds through FCT - Funda����o para a Ci��ncia e a Tecnologia, I.P., and by the European Commission's Recovery and Resilience Facility, within the scope of UID/04326/2025 and UID/04423/2025 (https://doi.org/10.54499/UID/04326/2025; https://doi.org/10.54499/UID/04423/2025), UID/PRR/04326/2025 and UID/PRR/04423/2025 (https://doi.org/10.54499/UID/PRR/04326/2025; https://doi.org/10.54499/UID/PRR/04423/2025), and LA/P/0101/2020 (https://doi.org/10.54499/LA/P/0101/2020). MC also received funding from FCT through 2021.06497.BD.