euglenids

Euglenids, or euglenoids, are a diverse group of unicellular flagellates within the phylum Euglenozoa. They are found in freshwater and damp soils worldwide, with some marine species. Many euglenids are photosynthetic, containing chloroplasts; others are heterotrophic, and some can switch between nutrition modes (mixotrophy).

Morphology: Most have a flexible, elongated cell body covered by a pellicle of protein ribbons. They possess two flagella at the anterior end, typically one longer than the other. A light-sensitive stigma (eyespot) helps orient toward or away from light. Chloroplasts are derived from secondary endosymbiosis of green algae, enabling photosynthesis during daylight; stored carbohydrate is paramylon. The cytoplasm contains a nucleus, mitochondria, and contractile vacuoles.

Feeding and behavior: In light they photosynthesize, using photosystems; in darkness they can feed by phagocytosis—taking in bacteria and small particles through a cytostome and oral helical groove. This mixotrophic ability allows adaptation to fluctuating environments. They move with their flagella and can glide with the pellicle.

Reproduction and ecology: Reproduction is mainly asexual by longitudinal binary fission; some species show meiotic or sexual processes under certain conditions, but such reproduction is not common. Ecologically, euglenids contribute to primary production in freshwater ecosystems and can form blooms under favorable light and nutrient conditions.

Taxonomy: Historically grouped with euglenoids in the class Euglenophyceae or phylum Euglenozoa; modern systems place euglenids within Euglenozoa with relationships to other flagellates. They are not pathogenic to humans.