Euglenoids

Euglenoids are a diverse group of unicellular, flagellated protists in the phylum Euglenozoa. They are commonly found in freshwater and damp soils and include both photosynthetic and non-photosynthetic species; many are mixotrophic, capable of photosynthesis when light is available and of ingesting prey or organic particles when it is not.

Most euglenoids lack a rigid cell wall and instead have a flexible pellicle composed of proteinaceous strips. They typically possess one or two flagella, with the longer flagellum used for propulsion. A pigmented eyespot (stigma) helps orient the organism toward light. The cytoplasm contains a paramylon body, a storage polymer of beta-1,3-glucan, and in photosynthetic forms a chloroplast that is often conspicuous. Chloroplasts in euglenoids arise from secondary endosymbiosis with green algae and typically contain chlorophylls a and b.

Photosynthetic euglenoids store energy as paramylon rather than starch and frequently exhibit a pyrenoid within the chloroplast. They inhabit a range of aquatic and moist terrestrial environments, including ponds, streams, and damp soils, where they can be free-swimming or attached to surfaces. They photosynthesize in light but can switch to heterotrophy by ingesting bacteria or detritus when light is limited.

Reproduction is mainly asexual, occurring by longitudinal binary fission; some species may engage in sexual processes or form resting cysts under adverse conditions. Taxonomically, they are commonly referred to as euglenoids or euglenids within the broader group Euglenozoa. They play ecological roles as primary producers and consumers and are model organisms in studies of chloroplast evolution and phototaxis.