This study explored the sustainable valorization of fruit waste extracts from sugarcane bagasse (SB), banana peel (BP), and watermelon rind (WR) for Euglena gracilis biomass and β-1,3-glucan production. The extracts were prepared using water extraction (WE), high-temperature and pressure treatment (HTP), and dilute sulfuric acid treatment (DSA). The DSA-treated extracts consistently yielded the best results. E. gracilis cultured in SB-DSA showed the highest cell density with a 2.08-fold increase compared to the commercial HUT medium, followed by BP-DSA (1.35-fold) and WR-DSA (1.70-fold). Photosynthetic pigment production increased significantly, with chlorophyll a yield being highest in SB-DSA (1.90-fold increase). The chlorophyll a/b ratio and total carotenoid content also improved, indicating enhanced light-harvesting capacity and photoprotection. Photosynthetic efficiency, measured by chlorophyll fluorescence, notably improved. The maximum quantum yield of PSII (Fv/Fm) increased by up to 25.88% in SB-DSA, suggesting reduced stress and improved overall photosynthetic health. The potential photochemical efficiency (Fv/F0) showed even greater improvements: up to 40.53% in SB-DSA. Cell morphology analysis revealed larger cell aspect ratios, implying a more active cellular physiological state. β-1,3-glucan yield also increased by 23.99%, 12.92%, and 23.38% in SB-DSA, BP-DSA, and WR-DSA, respectively. This study demonstrates the potential of pretreated fruit waste as a cost-effective and sustainable medium for E. gracilis cultivation, offering the dual benefits of waste valorization and high-value compound production. These findings contribute to the development of more efficient biorefinery processes and align with the circular economy principles in food biotechnology.

Euglena gracilis’s mixotrophic metabolism offers biotechnological potential. This study investigated how glucose, sodium acetate, ethanol, and propanetriol regulate its growth, photosynthesis, and paramylon production. All carbon sources boosted paramylon yield versus photoautotrophic controls. Ethanol and glucose were both highly effective, supporting the highest biomass accumulation (5.71 and 4.42-fold increases, respectively) and paramylon content without a significant difference between them. Ethanol supplementation enhanced chlorophyll b via coupled TCA cycle/glyoxylate shunt activity, while glucose showed the strongest tendency for high paramylon and the highest carotenoid content (13.36-fold higher). Sodium acetate triggered alkaline stress (pH 8.5), suppressing pigments and inducing spherical cells. Propanetriol reduced biomass but enhanced PSII efficiency (Fv/Fm). These results demonstrate carbon source-driven metabolic partitioning: ethanol and glucose both excel in promoting growth and storage, while additionally directing carbon toward chlorophyll b or carotenoids, respectively. These findings enable targeted bioprocess optimization: selection between ethanol or glucose can be based on the value of co-products, advancing E. gracilis as a sustainable cell factory.

Microalgae, as functional food ingredients with high nutritional value and sustainability, have garnered significant attention for their content of proteins, polyunsaturated fatty acids, vitamins, and antioxidant components