Introduction
Marine fish larvae are particularly vulnerable to oxidative stress and metabolic imbalances during early ontogeny. The intensification of marine larviculture needs innovative strategies to enhance larval robustness and resilience. Green tea extract, rich in polyphenols such as epigallocatechin gallate (EGCG), has emerged as a promising phytobiotic due to its potent antioxidant and health-promoting properties. In species like turbot (Scophthalmus maximus), maintaining gut integrity and metabolic efficiency is crucial for successful weaning and subsequent performance. Incorporating botanical extracts may provide a non-invasive tool to prime the larval physiological system, potentially improving the response to common acute stressors. While the antioxidant benefits of green tea are well-documented in adult fish, its role in modulating the metabolic and molecular pathways during the fast-growing postlarval stage remains poorly understood. This study aims to evaluate the potential of green tea extract as a functional dietary supplement to strengthen turbot postlarvae, assessing its impact on growth performance, redox physiology, and the molecular regulation of key metabolic pathways before and after an acute thermal challenge.
Material and MethodsThe trial was conducted using turbot (Scophthalmus maximus) postlarvae reared in a Recirculating Aquaculture System (RAS) from 29 to 51 days after hatching (DAH). Three experimental diets were tested in triplicate: a commercial-like control (CTRL) and two diets supplemented with green tea extract at two inclusion levels, LOW and HIGH (the latter being double the former). At 51DAH, larvae were subjected to an acute thermal shock of +5°C above the rearing temperature, followed by a second sampling. Growth performance KPIs, including dry weight, survival, total length, and relative growth rate (RGR), were recorded. Physiological redox status was assessed alongside molecular analyses. Gene expression related to redox regulation (sod2, gpx4, cat, nrf2), nitrogen metabolism (gdh, glul, got2), amino acid transporters (pept1, lat1), digestive capacity (tryp), lipid metabolism (aco, fasn, fabp2), gut integrity (cldn15, tjp1, muc2), mitochondrial biogenesis (atp5f1a, ppargc1a, opa1), and cellular stress (hsp60, hsp70, sirt3) was quantified.
Results
No significant differences were observed in survival, dry weight, total length, or RGR across treatments. Before the thermal challenge, SOD activity was lower in green tea-fed groups compared to the CTRL. Following the temperature stressor, the gdh gene was significantly upregulated in the LOW treatment relative to the CTRL, although glul and got2 remained unaffected. Regarding the redox response post-challenge, sod2 and gpx4 showed no differences, while cat and nrf2 expression was lower in the LOW group compared to the CTRL. Other markers for cellular stress, digestive capacity, lipid metabolism, gut integrity, and mitochondrial biogenesis were not significantly affected by the diets or the stressor.
Conclusions
Dietary inclusion of green tea extract did not alter turbot growth performance but successfully modulated key molecular pathways. The upregulation of gdh under thermal stress suggests a higher nitrogen metabolism adjustment during environmental challenges, with further mobilization of amino acids as energy source. Crucially, the downregulation of nrf2 and cat likely reflects a direct ROS scavenging effect by green tea polyphenols, exerting a 'sparing effect' on the endogenous antioxidant system by reducing the need for gene activation. Overall, green tea extract shows potential to bolster metabolic resilience and physiological priming in marine larviculture.
Acknowledgments
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 This study received Portuguese national funds from FCT - Foundation for Science and Technology through contracts UID/04326/2025 (DOI https://doi.org/10.54499/UID/04326/2025), UID/PRR/04326/2025 (DOI https://doi.org/10.54499/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.