EUGLENOPHYTA: CHARACTERISTICS, REPRODUCTION, NUTRITION, CLASSIFICATION - BIOLOGY - 2025

Euglenophyta is a division of the kingdom Protista that includes green and colorless flagellate protozoan organisms. The euglenidae, and therefore the euglenophytes, belong to the supergroup Excavata and to the phylum Euglenozoa, which is a very diverse phylum, especially in terms of its nutritional characteristics.

The first euglenophytes were described in the 1830s by Ehrenberg and since then they have been extensively studied, mainly thanks to their relatively large cell size, ease of culture and collection.

General sketch of a Euglena (Source:

Claudio Miklos via Wikimedia Commons)

The Protista kingdom is a polyphyletic kingdom whose members are characterized by being, for the most part, unicellular eukaryotic organisms with heterotrophic and autotrophic representatives. Within this kingdom, in addition to the euglenidae, are the kinetoplasts, apicomplexes, chlorophytes and others.

It is worth mentioning that Euglenophyta is the term used to define a robust phylogenetic clade that groups the photoautotrophic forms that have plastids, while the term “euglenid” is used to name all the organisms of the Euglenozoa phylum, both photoautotrophs and heterotrophs.

Most of the organisms of the euglenophyte group are freshwater, although there are reports of some saltwater species. These were the first protists discovered and described in detail and their name derives from the genus Euglena, whose species were the first euglenidae described in the 17th century.

characteristics

Euglenophytes have a wide variety of shapes: they can be elongated, oval or spherical, and even leaf-shaped. However, phylogenetic studies indicate that the spindle cell form is the most common within this group.

Inside they have a large network of protein bands connected under the plasma membrane that make up a structure known as a film.

They have a single branched mitochondrion that is distributed throughout the cell body. Most species have an ocellus or "eye spot" that makes them capable of detecting different wavelengths.

Flagella

They normally have two flagella as organs of locomotion. These flagella arise anteriorly within a cellular invagination consisting of a tubular canal. The base of the flagella is supported on the wall of the invagination.

The emergent portion of each flagellum has a unilateral row of hairs. The photoreceptor organ is located in a thickening located at the base of the flagellum.

Plastids

The different genera of euglenophytes have some differences in terms of the morphology of the chloroplasts, as well as their position in the cell, their size, number and shape. Different authors agree on the fact that euglenophytes have plastids of secondary origin.

Paramil

The main reserve substance of the eugleneids, including the euglenophytes, is paramyl. This is a starch-like macromolecule consisting of glucose residues linked by β-1,3 bonds and deposited as solid granules with a helical organization.

Paramyl can be found as granules in the cytoplasm or associated with chloroplasts forming what some authors call "paramyl centers". The size and shape of the granules is very diverse and often depends on the species considered.

Core

Euglenophytes, as well as the other members of the phylum, have a single chromosomal nucleus and their nuclear membrane is not a continuation of the endoplasmic reticulum. The division of the nucleus occurs as an intranuclear mitosis without the participation of the centrioles.

Reproduction

Asexual reproduction

The reproduction of euglenophytes is primarily asexual. Mitosis in these organisms is somewhat different from what has been observed in animals, plants, and even other protists.

The beginning of cell division is marked by the migration of the nucleus towards the base of the flagella. During division, neither the nuclear envelope nor the nucleoli disappear in these organisms.

When they reach the proper position, both structures elongate at the same time that the chromosomes move to the center of the nucleus and form a metaphase plate in the shape of a thread. The center of the plate is penetrated by the nucleoli.

Unlike the rest of the eukaryotes, the nucleus in the euglenids initially elongates perpendicular to the length of the cell axis, thus separating the sister chromatids. Only after the elongation of the nucleus is complete do the spindle fibers shorten and the chromosomes move towards the poles.

When cells reach telophase, the nucleus is stretched throughout the entire cell. The strangulation of the nuclear membrane ends with the division of the nucleolus and the separation of the daughter nuclei.

Cytokinesis occurs by the formation of a division groove that forms in the anterior region of the cell and moves towards the posterior region until the two new cells separate.

Sexual reproduction

For a long time it was thought that flagellate euglenoid species lacked sexual reproduction, however, recent studies have shown that many of them present some type of meiosis throughout their life cycle, although the reports are not very clear about it.

Nutrition

Euglenophytes are readily available in freshwater bodies with abundant deposits of decomposing organic matter.

The chloroplasts of euglenophytes are surrounded by three membranes and their thylakoids are stacked in a trio. These organisms use as photosynthetic pigments, in addition to chlorophylls a and b, phycobilins, β-carotenes, and the xanthophylls neoxanthin and diadinoxanthin.

Despite their auxotrophy, certain euglenophytes need to acquire some vitamins such as vitamin B1 and vitamin B12 from their environment, since they are not able to synthesize it themselves.

Classification

The Euglenozoa phylum is a monophyletic phylum composed of the Euglenida, Kinetoplaste, DIplonemea and Symbiontida groups. Euglenids are characterized by the presence of a film-like cytoskeleton and include phototrophic, heterotrophic, and mixotrophic organisms.

The group of euglenophytes is divided into three orders and 14 genera in total. The orders are represented by Rapaza, the Eutrepiales and the Euglenales. The Rapaza order contains only one marine species, R. viridis, which is characterized by having mixotrophic cells and a different alimentary apparatus from the species of the other orders.

The Eutrepiales possess certain characteristics that suggest that these organisms are ancestral, among them the capacity to adapt to marine aquatic environments and the presence of two emergent flagella. Within the order of the Eutrepiales are the genera Eutreptia and Eutreptiella.

Both genera have phototrophic or photoautotrophic cells with a flexible cytoskeleton and lack of alimentary apparatus.

The Euglenales are a more diverse group and have a single emergent flagellum, and are believed to be exclusively freshwater. This order encloses phototrophic and heterotrophic species with films or rigid cytoskeletons.

The order is divided into two families of monophyletic origin: Euglenaceae and Phacaceae.

The Euglenaceae family contains eight genera: Euglena (polyphyletic group), Euglenaria, Euglenaformis, Cryptoglena, Monomorphina, Colacium, Trachelomonas, and Strombomonas. They vary greatly with respect to the shape, position and number of plastids and general cell morphology.

The Phacaceae family includes three genera: Phacus (paraphyletic group), Lepocinclis and Discoplastis. Members of the Phacus and Lepocinclis families have a flattened rigid film that gives them a helical shape.

Examples of species

The most representative genus of euglenophytes is undoubtedly the genus Euglena. Within this genus is the species Euglena gracilis.

This organism has been used to carry out photosynthetic studies, since it presents a photosynthesis typical of higher plants and is capable of using various organic compounds to grow in the dark, making it a model phototropic organism for research.

Organisms of this species and others of the same genus have also been used for biotechnological purposes, since their chloroplasts and cytoplasm are sites of abundant synthesis of different compounds with biotechnological importance such as vitamin E, paramilon, wax esters, polyunsaturated fatty acids, biotin and some amino acids.

References

1. Bicudo, CEDM, & Menezes, M. (2016). Phylogeny and Classification of Euglenophyceae: A Brief Review. Frontiers in Ecology and Evolution, 4 (March), 1–15.
2. Brusca, R., & Brusca, G. (2005). Invertebrates (2nd ed.). Madrid: McGraw-Hill Interamericana of Spain.
3. Cavalier-Smith, T. (2016). Higher Classification and Phylogeny of Euglenozoa. European Journal of Protistology, 1–59.
4. Cramer, M., & Myers, J. (1952). Growth and Photosynthetic Characteristics of Euglena gracilis. Für Mikrobiologie, 17, 384–402.
5. Karnkowska, A., Bennet, M., Watza, D., Kim, J., Zakrys, B., & Triemer, R. (2014). Phylogenetic Relationships and Morphological Character Evolution of Photosynthetic Euglenids (Excavata) Inferred from Taxon-rich Analyzes of Five Genes. Journal of Eukaryotic Microbiology, 62 (3), 362–373.
6. Krajcovic, J., Vesteg, M., & Shawartzbach, S. (2014). Euglenoid flagellates: A multifaceted biotechnology platform. Journal of Biotechnology.
7. Leedale, G. (1966). Euglenida / euglenophytai. Rev. Microbiol.
8. Sansón, M., Reyes, J., Hernández-Díaz, C., & Braun, J. (2005). Green tides caused by Eutreptiella sp. in Playa de San Marcos (N of Tenerife, Canary Islands) (Eutreptiales, Euglenophyta). Museums of Tenerife - Vieraea, 33.
9. Triemer, RE, & Zakry, B. (2015). Photosynthetic Euglenoids in Freshwater Algae of North America (pp. 459–483).
10. Vanclová, AMG, Hadariová, L., & Hampl, V. (2017). Secondary Plastids of Euglenophytes. Advances in Botanical Research, 84, 321–358.