DIETARY POLYPHENOLIC EXTRACTS HAVE ALTERNATIVE PATHS TO MODULATE REDOX AND METABOLIC RESPONSES IN TURBOT POSTLARVAE
Introduction
Nutritional modulation occurring during critical windows of development, may have long-term consequences on somatic structures, physiological functions or metabolic status of the organism (. To identify optimum dietary formulations, applying dietary modulation concepts to fish nutrition offers opportunities to tailor specific metabolic pathways and physiological functions in farmed fish species . There has been a growing interest in dietary inclusion of plant-derived extracts containing a wide spectrum of bioactive molecules that act as appetite enhancers, growth promoters and immunostimulants in fish (. Green tea (Camellia sinensis L.) and grape seed (Vitis vinifera L. ssp. sativa) are two plants known for their abundant content and diversity of polyphenols. Several biotic (age, species) and abiotic factors (temperature, diet) can modulate the antioxidant defences and as a consequence, the oxidative status of the animals. Therefore, the present study was aimed to evaluate if dietary polyphenolic extracts (green tea and grape seed) supplementation modulates growth performance, antioxidant status and metabolic responses of Scophthalamus maximus postlarvae and help fish to cope with a stressful event.
Materials & Methods
Turbot (Scophthalmus maximus) postlarvae (initial dry weight: 22.6 8.0 mg) were reared in a recirculating aquaculture system (RAS) in triplicate. From 29 to 51 days after hatching (DAH), fish were fed one of three diets: a control (CTRL), and the CTRL supplemented with either green tea extract (GTE) or grape seed extract (GSE). At 50 DAH, post-larvae were subjected to a 24-hour acute thermal challenge (+5°C) to assess resilience. Growth performance was recorded at 50 DAH. Biochemical markers, including superoxide dismutase (SOD) and catalase (CAT) activities, as well as lipid peroxidation (LPO), were analysed alongside the expression of genes involved in antioxidant defence (sod2, cat, gpx4, nrf2), cellular stress (hsp60, hsp70, sirt3), digestive capacity (tryp, pept1, lat1), lipid metabolism (aco, fasn, fabp2, cpt1), gut integrity (cldn15, tjp1, muc2), mitochondrial regulation (atp5f1a, ppargc1a, opa1, ucp2), and nitrogen metabolism (gdh, glul, got2) under both standard and challenged conditions.
Results & Discussion
Dietary treatments did not affect growth (352.7 11.3 mg; 46.8 0.18 mm). However, survival was lower in GTE and GSE groups (69.0%) compared to CTRL (77.7%), suggesting that although bioactive, inclusion levels may require refinement for post-larvae.
Distinct molecular strategies emerged between extracts. Under standard conditions, GTE-fed larvae showed a "sparing effect" on endogenous systems, with significantly lower expression of cat, nrf2, and sirt3. Conversely, GSE-fed larvae exhibited "molecular readiness", significantly upregulating genes related to gut integrity (tjp1), mitochondrial dynamics (opa1), and amino acid transport (lat1).
Following the thermal challenge, GTE larvae exhibit lower LPO levels than CTRL, likely through direct radical-scavenging rather than enzymatic activation. In contrast, GSE-fed larvae displayed a robust, proactive response to stress, with significant upregulation of antioxidant (cat, gpx4), chaperoning (hsp70), digestive (tryp), and metabolic (atp5f1a, got2, gdh) pathways compared to CTRL. These results indicate that while GTE acts as a passive protector, GSE functions as a potent priming agent, enhancing the metabolic and redox machinery required to meet increased physiological demands during environmental stress.
Conclusion
Dietary polyphenolic extracts induced distinct physiological strategies in turbot post-larvae without affecting growth. Under thermal stress, GTE-fed larvae relied primarily on direct radical scavenging, effectively mitigating oxidative damage while sparing endogenous defences. Conversely, GSE promoted a broad molecular priming effect, activating a robust metabolic and antioxidant response to meet increased physiological demands. The lower survival rates indicate that while these extracts offer significant redox advantages, inclusion levels must be carefully balanced to avoid metabolic trade-offs or potential toxicity during early ontogeny.
Acknowledgment
This work is part of project E!4876 FlatFIRST_1171, supported by EUROSTARS-3 program, and by Portugal and the European Union through ERDF, Algarve 2030, and COMPETE 2030, in the framework of Portugal 2030 and Portuguese national funds from FCT - Foundation for Science and Technology through contracts UID/04326/2025, UID/PRR/04326/2025 and LA/P/0101/2020 (DOI:10.54499/LA/P/0101/2020), and from the operational programmes CRESC Algarve 2020 and COMPETE 2020 through contract EMBRC.PT ALG-01-0145-FEDER-022121 to CCMAR. Aqsa Sharif acknowledges financial support from FCT through the PhD grant 2025.07044.BD.
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