Euglena gracilis is a freshwater single-celled alga within the genus Euglena. It possesses secondary chloroplasts and is classified as a mixotroph, capable of both photosynthesis and phagocytosis for nutrition. Its cell surface is highly flexible, allowing it to change shape from a long, thin cell up to 100 μm in length to a spherical form about 20 μm in diameter. Each cell features two flagella, with one emerging from a reservoir at the front, enabling movement by swimming or euglenoid motion along surfaces. E. gracilis is extensively studied in laboratories as a model organism, particularly in fields such as cell biology and biochemistry. It is also used to investigate topics like photosynthesis, photoreception, and the correlation between molecular structure and the function of subcellular components. Moreover, E. gracilis has been identified as a valuable bioindicator for phenol pollution in freshwater environments and drainage systems. Its rapid generation time and specific biological responses make it ideal for assessing phenol levels in natural settings. Observations of morphological abnormalities and unusual cell division in E. gracilis provide crucial insights into the biological impacts of phenol on aquatic organisms. This makes it possible to gauge phenol exposure levels in marine ecosystems and implement appropriate measures to safeguard water quality and biodiversity.