Effects of Dietary Euglena gracilis Powder on Growth and Physiological Responses in Common Carp (Cyprinus carpio L.)

Growing pressure to make aquaculture more sustainable has renewed attention on bioactive feed ingredients obtained from natural sources. In the present work, juvenile FFRC-strain common carp (Cyprinus carpio L.) were used to examine how graded dietary inclusion of Euglena gracilis powder (EP) influences growth, redox status, immune responsiveness and the underlying molecular regulation. Four hundred and fifty fish (2.23 ± 0.01 g) were distributed at random among five isonitrogenous and isolipidic diets supplying 0 (control), 0.25, 0.5, 1 or 2% EP, and were reared for eight weeks. Survival, growth and feed use were all improved by EP, most evidently at 0.25% and 0.5%. Within this window, both antioxidant capacity and the activity of nonspecific immune enzymes increased, and hepatic transcription of genes governing antioxidant defence and cytokine signalling was correspondingly altered, notably through genes linked to the Nrf2/Keap1 axis. Inclusion at 2%, by contrast, conferred no further advantage and was accompanied by poorer feed conversion, so that the overall response was clearly dose-dependent and non-linear. Taken together, EP acts in a biphasic manner in common carp: 0.25–0.5% improves growth and physiological regulation, whereas higher levels are counterproductive. The results underline the promise of Euglena biomass as a renewable, algae-derived functional ingredient for aquafeed formulation. Future work should validate their effects in larger-scale and longer-term feeding trials, assess the consistency of raw material quality and feed processing, and evaluate production costs and economic returns to determine their feasibility for commercial application.