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''Euglena'' is a genus of
single cell Single cell and similar can mean: Biology * Single-cell organism *Single-cell protein * Single-cell recording, a neuro-electric monitoring technique * Single-cell sequencing **Single cell epigenomics *Single-cell transcriptomics Other *Single-cell ...
flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and thei ...
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
s. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
and at least 200 species. Species of ''Euglena'' are found in fresh water and salt water. They are often abundant in quiet inland waters where they may bloom in numbers sufficient to color the surface of ponds and ditches green (''E. viridis'') or red ('' E. sanguinea''). The species ''
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 ...
'' has been used extensively in the laboratory as a
model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
. Most species of ''Euglena'' have photosynthesizing
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...
s within the body of the cell, which enable them to feed by
autotrophy An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...
, like plants. However, they can also take nourishment
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
ically, like animals. Since ''Euglena'' have features of both animals and plants, early taxonomists, working within the Linnaean two-kingdom system of biological classification, found them difficult to classify. It was the question of where to put such "unclassifiable" creatures that prompted
Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist and artist. He discovered, described and named thousands of new s ...
to add a third living kingdom (a fourth kingdom ''in toto'') to the ''Animale'', ''Vegetabile'' (and ''Lapideum'' meaning ''Mineral'') of
Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...
: the Kingdom
Protista 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 e ...
.


Form and function

When feeding as a heterotroph, ''Euglena'' takes in nutrients by osmotrophy, and can survive without light on a diet of organic matter, such as
beef extract Meat extract is highly concentrated meat stock, usually made from beef or chicken. It is used to add meat flavour in cooking, and to make broth for soups and other liquid-based foods. Meat extract was invented by Baron Justus von Liebig, a Ger ...
,
peptone Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...
,
acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...
,
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
or
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s. When there is sufficient sunlight for it to feed by phototrophy, it uses chloroplasts containing the pigments chlorophyll a and chlorophyll b to produce sugars by
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 in ...
. ''Euglena's'' chloroplasts are surrounded by three membranes, while those of plants and the
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/ Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga ...
(among which earlier taxonomists often placed ''Euglena'') have only two membranes. This fact has been taken as morphological evidence that ''Euglena's'' chloroplasts evolved from a
eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
green alga. Thus, the similarities between ''Euglena'' and plants would have arisen not because of kinship but because of a secondary
endosymbiosis 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 ...
. Molecular phylogenetic analysis has lent support to this hypothesis, and it is now generally accepted. ''Euglena'' chloroplasts contain pyrenoids, used in the synthesis of paramylon, a form of starch energy storage enabling ''Euglena'' to survive periods of light deprivation. The presence of pyrenoids is used as an identifying feature of the genus, separating it from other euglenoids, such as '' Lepocinclis'' and '' Phacus''. ''Euglena'' have two flagella rooted in basal bodies located in a small reservoir at the front of the cell. Typically, one flagellum is very short, and does not protrude from the cell, while the other is long enough to be seen with light microscopy. In some species, such as ''Euglena mutabilis'', both flagella are "non-emergent"--entirely confined to the interior of the cell's reservoir--and consequently cannot be seen in the light microscope. In species that possess a long, emergent flagellum, it may be used to help the organism swim. The surface of the flagellum is coated with about 30,000 extremely fine filaments called mastigonemes. Like other euglenoids, ''Euglena'' possess a red eyespot, an organelle composed of
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
pigment granules. The red spot itself is not thought to be photosensitive. Rather, it filters the sunlight that falls on a light-detecting structure at the base of the flagellum (a swelling, known as the paraflagellar body), allowing only certain wavelengths of light to reach it. As the cell rotates with respect to the light source, the eyespot partially blocks the source, permitting the ''Euglena'' to find the light and move toward it (a process known as phototaxis). ''Euglena'' lacks a
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...
. Instead, it has a pellicle made up of a protein layer supported by a substructure of
microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27  nm and have an inner diameter between 1 ...
s, arranged in strips spiraling around the cell. The action of these pellicle strips sliding over one another, known as metaboly, gives ''Euglena'' its exceptional flexibility and contractility. The mechanism of this euglenoid movement is not understood, but its molecular basis may be similar to that of amoeboid movement. In low moisture conditions, or when food is scarce, ''Euglena'' forms a protective wall around itself and lies dormant as a resting cyst until environmental conditions improve.


Reproduction

''Euglena'' reproduce asexually through binary fission, a form of
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 ...
. Reproduction begins with the
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
of the
cell nucleus The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
, followed by the division of the cell itself. ''Euglena'' divide longitudinally, beginning at the front end of the cell, with the duplication of flagellar processes, gullet and stigma. Presently, a cleavage forms in the
anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
, and a V-shaped bifurcation gradually moves toward the posterior, until the two halves are entirely separated. Reports of
sexual conjugation Isogamy is a form of sexual reproduction that involves gametes of the same morphology (indistinguishable in shape and size), found in most unicellular eukaryotes. Because both gametes look alike, they generally cannot be classified as male or ...
are rare, and have not been substantiated.


Historical background and early classification

Species of ''Euglena'' were among the first protists to be seen under the microscope. In 1674, in a letter to the Royal Society, the Dutch pioneer of microscopy Antoni van Leeuwenhoek wrote that he had collected water samples from an inland lake, in which he found "animalcules" that were "green in the middle, and before and behind white." Clifford Dobell regards it as "almost certain" that these were ''Euglena viridis'', whose "peculiar arrangement of chromatophores...gives the flagellate this appearance at low magnification." Twenty-two years later, John Harris published a brief series of "Microscopical Observations" reporting that he had examined "a small Drop of the Green Surface of some Puddle-Water" and found it to be "altogether composed of Animals of several Shapes and Magnitudes." Among them, were "oval creatures whose middle part was of a Grass Green, but each end Clear and Transparent," which "would contract and dilate themselves, tumble over and over many times together, and then shoot away like Fish." In 1786, O.F. Müller gave a more complete description of the organism, which he named ''Cercaria viridis'', noting its distinctive color and changeable body shape. Müller also provided a series of illustrations, accurately depicting the undulating, contractile movements (metaboly) of ''Euglenas body. In 1830,
C. G. Ehrenberg Christian Gottfried Ehrenberg (19 April 1795 – 27 June 1876) was a German naturalist, zoologist, comparative anatomist, geologist, and microscopist. Ehrenberg was an evangelist and was considered to be of the most famous and productive scient ...
renamed Müller's ''Cercaria'' ''Euglena viridis'', and placed it, in keeping with the short-lived system of classification he invented, among the Polygastrica in the family Astasiaea: multi-stomached creatures with no alimentary canal, variable body shape but no pseudopods or lorica. By making use of the newly invented achromatic microscope, Ehrenberg was able to see ''Euglenas eyespot, which he correctly identified as a "rudimentary eye" (although he reasoned, wrongly, that this meant the creature also had a nervous system). This feature was incorporated into Ehrenberg's name for the new genus, constructed from the Greek roots "eu-" (well, good) and glēnē (eyeball, socket of joint). Ehrenberg did not notice ''Euglena''s flagella, however. The first to publish a record of this feature was
Félix Dujardin Félix Dujardin (5 April 1801 – 8 April 1860) was a French biologist born in Tours. He is remembered for his research on protozoans and other invertebrates. Biography In 1840 he was appointed professor of geology and mineralogy at the U ...
, who added "filament flagelliforme" to the descriptive criteria of the genus in 1841. Subsequently, the class Flagellata (Cohn, 1853) was created for creatures, like ''Euglena'', possessing one or more flagella. While "Flagellata" has fallen from use as a taxon, the notion of using flagella as a phylogenetic criterion remains vigorous.


Recent phylogeny and classification

In 1881, Georg Klebs made a primary taxonomic distinction between green and colorless flagellate organisms, separating photosynthetic from heterotrophic euglenoids. The latter (largely colorless, shape-changing uniflagellates) were divided among the Astasiaceae and the Peranemaceae, while flexible green euglenoids were generally assigned to the genus ''Euglena''. As early as 1935, it was recognized that this was an artificial grouping, however convenient. In 1948, Pringsheim affirmed that the distinction between green and colorless flagellates had no taxonomic justification, although he acknowledged its practical appeal. He proposed something of a compromise, placing colorless,
saprotrophic Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ...
euglenoids in the genus ''Astasia'', while allowing some colorless euglenoids to share a genus with their photosynthesizing cousins, provided they had structural features that proved common ancestry. Among the green euglenoids themselves, Pringsheim recognized the close kinship of some species of ''Phacus'' and ''Lepocinclis'' with some species of ''Euglena''. The idea of classifying the euglenoids by their manner of nourishment was finally abandoned in the 1950s, when A. Hollande published a major revision of the phylum, grouping organisms by shared structural features, such as the number and type of flagella. If any doubt remained, it was dispelled in 1994, when genetic analysis of the non-photosynthesizing euglenoid ''Astasia longa'' confirmed that this organism retains sequences of DNA inherited from an ancestor that must have had functioning chloroplasts. In 1997, a morphological and molecular study of the Euglenozoa put ''Euglena gracilis'' in close kinship with the species ''Khawkinea quartana'', with ''Peranema trichophorum'' basal to both. Two years later, a molecular analysis showed that ''E. gracilis'' was, in fact, more closely related to ''Astasia longa'' than to certain other species recognized as ''Euglena''. In 2015, Dr Ellis O'Neill and Professor Rob Field have sequenced the transcriptome of ''Euglena gracilis'', which provides information about all of the genes that the organism is actively using. They found that ''Euglena gracilis'' has a whole host of new, unclassified genes which can make new forms of
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s and natural products. The venerable ''Euglena viridis'' was found to be genetically closer to ''Khawkinea quartana'' than to the other species of ''Euglena'' studied. Recognizing the polyphyletic nature of the genus ''Euglena,'' Marin et al. (2003) have revised it to include certain members traditionally placed in ''Astasia'' and ''Khawkinea''.


Human consumption

The taste of powdered euglena is described as dried sardine flakes, and contains minerals, vitamins and docosahexaenoic, an omega-3 acid. The powder is used as ingredient in other foods. Kemin Industries sells a euglena nutraceutical supplement ingredient featuring dried ''Euglena gracilis'' with high levels of beta glucan.


Feedstock for biofuel production

The lipid content of ''Euglena'' (mainly wax esters) is seen as a promising feedstock for production of biodiesel and jet fuel. Under the aegis of
Itochu is a Japanese corporation based in Umeda, Kita-ku, Osaka and Aoyama, Minato, Tokyo. It is one of the largest Japanese '' sogo shosha'' (general trading companies). Among Japanese trading companies, it is distinguished by not being descend ...
, a start-up company called Euglena Co., Ltd. has completed a refinery plant in Yokohama in 2018, with a production capacity of 125 kiloliters of bio jet fuel and biodiesel per year.Video explanation lacks technical details but suggests degree of government commitment to solving problems of large-scale cultivation and infrastructure. CEO of Euglena Co. wears euglena-green necktie.


Video gallery


See also

* Elysia chlorotica * Kleptoplasty


References


External links


The Euglenoid ProjectTree of Life web project: EuglenidaProtist Images: EuglenaImages and taxonomy
* {{Taxonbar, from=Q236001 Euglenozoa genera Articles containing video clips Taxa named by Christian Gottfried Ehrenberg