Euglena

Euglena is a genus of unicellular, flagellated protists in the phylum Euglenozoa. Members are primarily aquatic and can be photosynthetic, heterotrophic, or mixotrophic. The best known species is Euglena gracilis, a widely used model organism in cell and molecular biology.

Morphology and physiology: The cell is covered by a flexible pellicle rather than a rigid cell wall. A reservoir near the base of the flagellum bears one or two flagella, typically with a long, whip-like one used for locomotion and a shorter, sometimes hidden one. An eyespot (stigma) provides light detection and helps the cell move toward favorable light conditions. Chloroplasts are present in photosynthetic individuals and contain chlorophylls a and b, though some lineages may lose plastids under certain conditions. The organism stores carbohydrates as paramylon, a beta-1,3-glucan. Mitochondria and a nucleus are present, along with other standard organelles.

Nutrition and behavior: Euglena can photosynthesize when light is available, but they can also feed on bacteria and organic particles through phagocytosis, allowing them to survive in darkness or nutrient-poor environments. This mixotrophic metabolism enables flexible responses to changing resources. The flagellum and stigma coordinate movement and phototaxis, guiding the organism toward optimal light for photosynthesis.

Habitat and reproduction: They are common in freshwater environments, as well as moist soils and some brackish systems. Reproduction is primarily asexual by binary fission. Under stress, some species may form resting cysts or exhibit sexual-like processes, though true sexual reproduction is not widely documented.

Ecology and significance: As mixotrophs, Euglena contribute to microbial food webs and primary production. Euglena gracilis is widely studied for its chloroplast function, metabolism, and cellular responses to environmental conditions, and it serves as a popular educational organism for protist diversity.