Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae,

ICBN The ''International Code of Nomenclature for algae, fungi, and plants'' (ICN) is the set of rules and recommendations dealing with the formal botanical names that are given to plants, fungi and a few other groups of organisms, all those "trad ...

) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum

Euglenophyta Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typic ...

and their cell structure is typical of that group. They are commonly found in freshwater, especially when it is rich in organic materials, with a few marine and

endosymbiotic An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...

members. Many euglenids feed by

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

, or strictly by

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...

. A monophyletic group consisting of the

mixotroph A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...

ic Rapaza viridis (1 species) and the two groups Eutreptiales (24 species) and Euglenales (983 species) have chloroplasts and produce their own food through

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

. This group is known to contain the carbohydrate

paramylon Paramylon is a carbohydrate similar to starch. The chloroplasts found in ''Euglena'' contain chlorophyll which aids in the synthesis of carbohydrates to be stored as starch granules and paramylon. Paramylon is made in the pyrenoid Pyrenoids a ...

. Euglenids split from other Euglenozoa more than a billion years ago. The plastids in all extant photosynthetic species is the result from secondary endosymbiosis between a phagotrophic eukaryovorous euglenid and a

Pyramimonas ''Pyramimonas'' is a genus of green algae in the order Pyramimonadales.See the NCBI The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institute ...

-related green alga.

Structure

Euglenoids are distinguished mainly by the presence of a type of cell covering called a pellicle. Within its taxon, the pellicle is one of the euglenoids' most diverse morphological features. The pellicle is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral microtubules. This varies from rigid to flexible, and gives the cell its shape, often giving it distinctive striations. In many euglenids, the strips can slide past one another, causing an inching motion called

metaboly ''Euglena'' is a genus of single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. Species of ''Euglena'' are found i ...

. Otherwise, they move using their flagella.

Classification

The euglenids were first defined by Otto Bütschli in 1884 as the flagellate order Euglenida, as an animal. Botanists subsequently created the algal division Euglenophyta; thus, they were classified as both animals and plants, as they share characteristics with both. Conflicts of this nature are exemplary of why the kingdom

Protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...

a was adopted. However, they retained their double-placement until the flagellates were split up, and both names are still used to refer to the group. Their chlorophylls are not masked with accessory pigments.

Nutrition

The classification of euglenids is still variable, as groups are being revised to conform with their molecular

phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spe ...

. Classifications have fallen in line with the traditional groups based on differences in nutrition and number of flagella; these provide a starting point for considering euglenid diversity. Different characteristics of the euglenids' pellicles can provide insight into their modes of movement and nutrition. As with other Euglenozoa, the primitive mode of nutrition is

. Prey such as

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

and smaller flagellates is ingested through a

cytostome A cytostome (from ''cyto-'', cell and ''stome-'', mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuol ...

, supported by microtubules. These are often packed together to form two or more rods, which function in ingestion, and in ''Entosiphon'' form an extendable siphon. Most

phagotrophic Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is c ...

euglenids have two flagella, one leading and one trailing. The latter is used for gliding along the substrate. In some, such as ''

Peranema ''Peranema'' is a genus of free-living phagotrophic euglenids (Euglenida; Euglenozoa; Excavata). There are more than 20 nominal species, varying in size between 8 and 200 micrometers. ''Peranema'' cells are gliding flagellates found in freshwat ...

'', the leading flagellum is rigid and beats only at its tip.

Osmotrophic euglenoids

Osmotrophic euglenids are euglenids which have undergone

osmotrophy Osmotrophy is a feeding mechanism involving the movement of dissolved organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic microorganisms use osmotrophy to derive some of their energy. ...

. Due to a lack of characteristics that are useful for

taxonomical In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...

purposes, the origin of osmotrophic euglenids is unclear, though certain morphological characteristics reveal a small fraction of osmotrophic euglenids is derived from phototrophic and phagotrophic ancestors. A prolonged absence of light or exposure to harmful chemicals may cause atrophy and absorption of the chloroplasts without otherwise harming the organism. A number of species exists where a chloroplast's absence was formerly marked with separate genera such as ''Astasia'' (colourless ''

Euglena ''Euglena'' is a genus of single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. Species of ''Euglena'' are found in f ...

'') and '' Hyalophacus'' (colourless ''

Phacus ''Phacus'' is a genus of unicellular excavates, of the phylum Euglenozoa (also known as Euglenophyta), characterized by its flat, leaf-shaped structure, and rigid cytoskeleton known as a pellicle. These eukaryotes are mostly green in colour, an ...

''). Due to the lack of a developed cytostome, these forms feed exclusively by osmotrophic absorption.

Reproduction

Although euglenids share several common characteristics with animals, which is why they were originally classified as so, no evidence has been found of euglenids ever using

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...

. This is one of the reasons they could no longer be classified as animals. For euglenids to reproduce, asexual reproduction takes place in the form of

binary fission Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...

, and the cells replicate and divide during mitosis and

cytokinesis Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and mei ...

. This process occurs in a very distinct order. First, the

basal bodies A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor W ...

and flagella replicate, then the

and microtubules (the feeding apparatus), and finally the nucleus and remaining

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is com ...

. Once this occurs, the organism begins to cleave at the basal bodies, and this cleavage line moves towards the center of the organism until two separate euglenids are evident. Because of the way that this reproduction takes place and the axis of separation, it is called longitudinal

cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...

or longitudinal binary fission.

Gallery

File:Euglena.gracilis.jpg, ''

'' sp. (Euglenales) File:Phacus pleuronectes - 400x (13263445584).jpg, ''

'' sp. (Euglenales) File:Trachelomonas sp.jpg, ''

Trachelomonas ''Trachelomonas'' is a genus of swimming, free-living euglenoids characterized by the presence of a shell-like covering called a lorica. Details of lorica structure determine the classification of distinct species in the genus. The lorica can e ...

'' sp. (Euglenales) File:Bioconvection Euglena Concentration series Colour.jpg, Euglenoid cultures in Petri dishes File:Euglena scheme no arrows.svg, Cell diagram FMIB 40826 Astasia contorta--Two Aspects.jpeg, ''

Astasia Astasis is a lack of motor coordination marked by an inability to stand, walk or even sit without assistance due to disruption of muscle coordination. The term ''astasia'' is interchangeable with ''astasis'' and is most commonly referred to as '' ...

'' sp. (Euglenales) File:Album général des Cryptogames, Pl. 23.jpg, ''Euglena'', ''Astasia'' and ''Phacus'' spp. (Euglenales) File:Algen I. (Schizophyceen, Flagellaten, Peridineen) (1910) (17762559370).jpg, ''Euglena, Phacus'' and ''Lepocinclis'' spp. (Euglenales) File:Algen I. (Schizophyceen, Flagellaten, Peridineen) (1910) (17947077272).jpg, '' Anisonema,

Petalomonas ''Petalomonas'' is a genus of phagotrophic, flagellated euglenoids.Guiry, M. D.; Guiry, G. M. (2002). “Petalomonas F.Stein 1859”. Retrieved February 10, 2019, fro/ref> Phagotrophic euglenoids are one of the most important forms of flagellates ...

, Notosolenus, Scytomonas'' and '' Tropidoscyphus'' spp. (Sphenomonadales); ''

Heteronema ''Heteronema'' is a genus of phagotrophic, flagellated euglenoids that are most widely distributed in fresh water environments.Guiry, M. D.; Guiry, G. M. (2012). “Heteronema Dujardin, 1841”. Retrieved February 14, 2019, fro/ref> This genus c ...

Dinema ''Dinema'' is a genus of orchids. It is represented by a single currently accepted species, ''Dinema polybulbon'', native to Mexico, Central America, and the Caribbean. Description They are epiphytes or lithophytes; with pseudobulbs 10 mm ...

'' and '' Entosiphon'' spp. (Heteronematales)

References

Bibliography

* Ciugulea, I. & Triemer, R. E. (2010) ''A Color Atlas of Photosynthetic Euglenoids''. Michigan State University Press, East Lansing, MI, 204 p.

* Leander, B. S., Triemer, R. E., & Farmer, M. A. (2001). Character evolution in heterotrophic euglenids. ''European Journal of Protistology'', 37(3), 337-356

* Leander, B.S., Lax, G., Karnkowska, A., Simpson, A.G.B. (2017). Euglenida. In: Archibald, J.M., Simpson, A.G.B., Slamovits, C. (Eds.). ''Handbook of the Protists''. Springer, pp. 1–42. * Leedale, G. F. (1978). Phylogenetic criteria in euglenoid flagellates. ''BioSystems'' 10: 183–187

* Wołowski, K & Hindák, F. (2005). ''Atlas of Euglenophytes''. Cracow: VEDA Publishing House of the Slovak Academy of Sciences, 136 p.

External links

*
The Euglenoid Project

Tree of Life: Euglenida
{{Taxonbar, from1=Q134863, from2=Q18916511, from3=Q2356588 Algal taxonomy Euglenozoa Extant Ypresian first appearances