Euglena gracilis

Euglena gracilis is a freshwater species of single-celled alga in the genus Euglena. It has secondary chloroplasts, and is a mixotroph able to feed by photosynthesis or phagocytosis. It has a highly flexible cell surface, allowing it to change shape from a thin cell up to 100 μm long to a sphere of approximately 20 μm. Each cell has two flagella, only one of which emerges from the flagellar pocket (reservoir) in the anterior of the cell, and can move by swimming, or by so-called "euglenoid" movement across surfaces. E. gracilis has been used extensively in the laboratory as a model organism, particularly for studying cell biology and biochemistry.

Other areas of their use include studies of photosynthesis, photoreception, and the relationship of molecular structure to the biological function of subcellular particles, among others. Euglena gracilis is the most studied member of the Euglenaceae.

E. gracilis was discovered as an effective bioindicator for phenol pollution in freshwater ecosystems and drainage. Their brief generating duration and particular biological reactions make it optimal for measuring phenol concentrations in the natural environment. The reported morphological abnormalities and unusual cell division reveal important information about the biological impacts of phenol on marine organisms. Using E. gracilis as a bioindicator can determine the level of phenol exposure in marine ecosystems and adopt appropriate mitigation actions to protect water quality and biodiversity.

Taxonomy
A morphological and molecular study of the Euglenozoa put E. gracilis in close kinship with the species Khawkinea quartana, with Peranema trichophorum basal to both, although a later molecular analysis showed that E. gracilis was more closely related to Astasia longa than to certain other species recognized as Euglena.

The transcriptome of E. gracilis was sequenced, showing that E. gracilis has many unclassified genes which can make complex carbohydrates and natural products.

Morphology
The morphology is characterized by a spindle-shaped cell with a length ranging from 40 to 150 micrometers. The cell contains a pellicle which is a flexible outer covering made up of proteinaceous strips called pellicular strips. This pellicle provides shape and structure to the cell. The movement of the E. gracilis is primarily achieved by its flagellum that emerges from a flagellar pocket. It has forward and backwards movement, as well as changes in its direction. Additionally, E. gracilis contains a light-sensitive eyespot, or stigma, which enables it to exhibit phototaxis by moving towards light sources for photosynthesis. The cell also possesses a contractile vacuole responsible for osmoregulation, helping maintain proper water balance within the cell.

Energy storage
Paramylon is a unique storage polysaccharide found in Euglena gracilis, serving as a reserve carbohydrate for energy storage. Structurally, paramylon is a linear β-1,3-glucan, distinct from the storage polysaccharide starch of plants and some species of alga.

The origin of the middle plastid membrane
The plastids contain three membranes. These membranes are an evolutionary vestige of the secondary endosymbiotic event that occurred between a phagotrophic eukaryovorous euglenid and a Pyramimonas-related green alga. The plastids of Euglena are unusual since most secondary plastids are surrounded by four envelopes. The two inner ones are derived from the inner and outer chloroplast envelopes of the primary plastid of the alga that was taken up during the symbiotic event. The two outermost are derived from the plasma membrane of the alga (third) and the phagosome of the host (fourth).

Biofuels
Microalgae are considered a possible source for biodiesel production due to their high lipid content. Its lipids may be suitable for biodiesel production due to their saturation, such as fatty acyl-CoA reductase and wax synthase. These ratios vary on environmental and cultivation conditions.

As food
In industry, Euglena gracilis is genetically engineered to produce a flour used to manufacture various protein-rich, non-animal foods.