View source for Euglena

{{Short description|Genus of unicellular flagellate eukaryotes}}
{{Automatic taxobox
| image = Euglena - 400x (8999902391).jpg
| image_caption = ''Euglena'' sp.
| taxon = Euglena
| authority = [[Christian Gottfried Ehrenberg|Ehrenberg]], 1830
| type_species = ''[[Euglena viridis]]''
| type_species_authority = (O.F.Müller) Ehrenberg<ref name=AlgaeBase>{{AlgaeBase genus | id=43651 | title=''Euglena'' Ehrenberg, 1830 | access-date=2025-05-01 }}</ref>
}}

'''''Euglena''''' is a genus of [[Unicellular organism|single-celled]], [[flagellate]] [[eukaryote]]s. 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.<ref>{{Cite web|url = http://euglena.msu.edu/taxalist.shtml|title = The Euglenoid Project: Alphabetic Listing of Taxa|access-date = Sep 20, 2014|website = The Euglenoid Project|publisher = Partnership for Enhancing Expertise in Taxonomy|archive-url = https://web.archive.org/web/20170223055757/http://euglena.msu.edu/taxalist.shtml|archive-date = February 23, 2017}}</ref><ref>{{Cite web|url = http://euglena.msu.edu/Teachers/Frameset.htm|title = The Euglenoid Project for Teachers|access-date = Sep 20, 2014|website = The Euglenoid Project for Teachers|publisher = Partnerships for Enhancing Expertise in Taxonomy|archive-url = https://web.archive.org/web/20170223060520/http://euglena.msu.edu/Teachers/Frameset.htm|archive-date = February 23, 2017}}</ref> 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 (''[[Euglena viridis|E.&nbsp;viridis]]'') or red (''[[Euglena sanguinea|E.&nbsp;sanguinea]]'').<ref name="Wolosski-2002">{{cite book |last=Wolosski |first=Konrad |chapter-url=https://books.google.com/books?id=Sc4897dfM_MC&pg=PA144 |title=The Freshwater Algal Flora of the British Isles: an Identification Guide to Freshwater and Terrestrial Algae |editor1-first=David M. |editor1-last=John |editor2-first=Brian A. |editor2-last=Whitton |editor3-first=Alan J. |editor3-last=Brook |isbn=978-0-521-77051-4 |page=144 |chapter=Phylum Euglenophyta|date=2002-04-25 |publisher=Cambridge University Press }}</ref>

The species ''[[Euglena gracilis]]'' has been used extensively in the laboratory as a [[model organism]].<ref>{{cite journal |doi=10.1093/nar/gki574 |title=Unusual features of fibrillarin cDNA and gene structure in Euglena gracilis: Evolutionary conservation of core proteins and structural predictions for methylation-guide box C/D snoRNPs throughout the domain Eucarya |year=2005 |last1=Russell |first1=A. G. |journal=Nucleic Acids Research |volume=33 |issue=9 |pages=2781–91 |pmid=15894796 |last2=Watanabe |first2=Y |last3=Charette |first3=JM |last4=Gray |first4=MW |pmc=1126904}}</ref>

Most species of ''Euglena'' have photosynthesizing [[chloroplast]]s within the body of the cell, which enable them to feed by [[autotrophy]], like plants. However, they can also take nourishment [[heterotroph]]ically, like animals. Since ''Euglena'' have features of both animals and plants, early taxonomists, working within the [[Linnaean taxonomy|Linnaean]] two-kingdom system of biological classification, found them difficult to classify.<ref>{{cite book |first1=Lynn |last1=Margulis |author1-link=Lynn Margulis |editor1-first=Lynn |editor1-last=Margulis |editor2-first=Dorion |editor2-last=Sagan |chapter=Power to the Protoctists |chapter-url=https://books.google.com/books?id=XccH9fgT8AgC&pg=PA29 |pages=29–35 |title=Dazzle Gradually: Reflections on the Nature of Nature |publisher=Chelsea Green |location=White River Junction |year=2007 |isbn=978-1-60358-136-3}}</ref><ref>{{cite book |url=https://archive.org/details/plantanimalsstud00keeb |last1=Keeble |first1=Frederick |title=Plant-animals: a study in symbiosis |publisher=Cambridge University Press |location=London |year=1912 |pages=103–4 |oclc=297937639}}</ref> It was the question of where to put such "unclassifiable" creatures that prompted [[Ernst Haeckel]] to add a third living kingdom (a fourth kingdom ''in toto'') to the ''Animale'', ''Vegetabile'' (and ''Lapideum'' meaning ''Mineral'') of [[Carl Linnaeus|Linnaeus]]: the Kingdom [[Protista]].<ref>{{cite book |editor1-first=Eldra Pearl |editor1-last=Solomon |editor2-first=Linda R. |editor2-last=Berg |editor3-first=Diana W. |editor3-last=Martin |chapter=Kingdoms or Domains? |chapter-url=https://books.google.com/books?id=qBOPoEc-zu4C&pg=PA421 |pages=421–7 |title=Biology |publisher=Brooks/Cole Thompson Learning |location=Belmont |year=2005 |edition=7th |isbn=978-0-534-49276-2}}</ref>

==History of knowledge==
===Early observations===
[[File:Mullers cercaria viridis detail.jpg|thumb|left|300px|''Cercaria viridis'' (= ''E. viridis'') from [[Otto Friedrich Müller|O.F. Müller]]'s ''Animalcula Infusoria''. 1786]]
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 [[Antonie 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."<ref>{{cite book |last=Dobell |first=Clifford |title=Antony van Leeuwenhoek and his 'Little Animals' |url=https://archive.org/details/antonyvanleeuwen00clif |url-access=registration |location=New York |publisher=Dover |orig-date=1932 |year=1960 |page=[https://archive.org/details/antonyvanleeuwen00clif/page/111 111] |isbn=978-0-486-60594-4}}</ref> Twenty-two years later, [[John Harris (writer)|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."<ref>{{cite journal |bibcode=1695RSPT...19..254H |jstor=102304 |doi=10.1098/rstl.1695.0036 |url=https://archive.org/details/philtrans01804684 |title=Some Microscopical Observations of Vast Numbers of Animalcula Seen in Water by John Harris, M. A. Rector of Winchelsea in Sussex, and F. R. S |year=1695 |last1=Harris |first1=J. |journal=Philosophical Transactions of the Royal Society of London |volume=19 |issue=215–235 |pages=254–9|doi-access=free }}</ref>

In 1786, [[Otto Friedrich Müller|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 the cell body.<ref>{{cite book |url=https://archive.org/details/animalculainfuso00ml |last1=Müller |first1=Otto Frederik |last2=Fabricius |first2=Otto |title=Animalcula Infusoria, Fluvia Tilia et Marina |year=1786 |publisher=Hauniae, Typis N. Mölleri |pages=126, 473}}</ref> In 1830, [[Christian Gottfried Ehrenberg|C. G. Ehrenberg]] 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.<ref>[https://www.biodiversitylibrary.org/bibliography/2077#/summary Ehrenberg, C. Organisation, Systematik und geographisches Verhältnifs der Infusionsthierchen. Vol. II. Berlin, 1830. pp 58-9]</ref><ref>{{cite book |url=https://archive.org/details/historyofinfusor00pritrich |last=Pritchard |first=Andrew |title=A history of Infusoria, living and fossil: arranged according to 'Die Infusionsthierchen' of C.G. Ehrenberg |publisher=Whittaker |location=London |year=1845 |page=86 |hdl=2027/uc2.ark:/13960/t5fb4z64c}}</ref> By making use of the newly invented achromatic microscope,<ref>{{cite journal|journal=Notes and Queries|date=July–December 1855|volume=12|issue=13|page=459|url=https://books.google.com/books?id=gwAjAQAAIAAJ&q=Notes+and+queries+ehrenberg+microscope&pg=PA459|title=Notes and Queries}}</ref> Ehrenberg was able to see ''Euglena'''s 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).<ref>{{cite encyclopedia |url = http://www.merriam-webster.com/dictionary/euglena |title = Merriam-Webster online dictionary |encyclopedia = Encyclopædia Britannica |access-date= 6 July 2005 }}</ref>

[[File:Dujardin euglena.jpg|thumb|200px|''Euglena'' from [[Félix Dujardin]]'s ''Histoire Naturelle des Zoophytes'', 1841]]

Ehrenberg did not notice ''Euglena''{{'}}s flagella, however. The first to publish a record of this feature was [[Félix Dujardin]], who added "filament flagelliforme" to the descriptive criteria of the genus in 1841.<ref>{{cite book |url=https://archive.org/details/histoirenaturell00duja |last1=Dujardin |first1=Félix |title=Histoire Naturelle des Zoophytes. Infusoires, comprenant la Physiologie et la Classification de ces Animaux, et la Manière de les Étudier a l'aide du Microscope |location=Paris |year=1841 |page=358}}</ref> 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.<ref>{{cite journal |doi=10.1078/143446103322454112 |title=Phylogeny and Classification of Phylum Cercozoa (Protozoa) |year=2003 |last1=Cavalier-Smith |first1=Thomas |last2=Chao |first2=Ema E.-Y. |s2cid=26079642 |journal=Protist |volume=154 |issue=3–4 |pages=341–58 |pmid=14658494}}</ref>

===Recent classification===
[[File:Euglenoid movement.jpg|thumb|left|300px|Euglenoid movement, known as [[metaboly]]]]
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 [[Peranema]]ceae, while flexible green euglenoids were generally assigned to the genus ''Euglena''.<ref name="Pringsheim-1948">{{cite journal |doi=10.1111/j.1469-185X.1948.tb00456.x |title=Taxonomic Problems in the Euglenineae |year=1948 |last1=Pringsheim |first1=E. G. |journal=Biological Reviews |volume=23 |pages=46–61 |pmid=18901101 |issue=1|s2cid=33439406 }}</ref>

As early as 1935, it was recognized that this was an artificial grouping, however convenient.<ref>{{cite journal |doi=10.1002/jobm.19670070220 |title=F. E. Fritsch, the Structure and Reproduction of the Algae Vol. I/II. XIII und 791, XIV und 939 S., 245 und 336 Abb., 2 und 2 Karten. Cambridge 1965 (reprinted): Cambridge University Press 90 S je Band |year=2007 |last1=Schwartz |first1=Adelheid |journal=Zeitschrift für Allgemeine Mikrobiologie |volume=7 |issue=2 |pages=168–9}}</ref> In 1948, Pringsheim affirmed that the distinction between green and colorless flagellates had little taxonomic justification, although he acknowledged its practical appeal. He proposed something of a compromise, placing colorless, [[saprotrophic]] 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''.<ref name="Pringsheim-1948" />

By the 1950s, when A. Hollande published a major revision of the phylum, organisms were classified by shared structural features, such as the number and type of flagella.<ref name="Linton-1999">{{cite journal |doi=10.1111/j.1550-7408.1999.tb04606.x |title=A Molecular Study of Euglenoid Phylogeny using Small Subunit rDNA |year=1999 |last1=Linton |first1=Eric W. |last2=Hittner |first2=Dana |last3=Lewandowski |first3=Carole |last4=Auld |first4=Theresa |last5=Triemer |first5=Richard E. |journal=The Journal of Eukaryotic Microbiology |volume=46 |issue=2 |pages=217–23 |pmid=10361741|s2cid=31420687 }}</ref> In the 1970s, it was hypothesized that photosynthetic euglenoids derived their chloroplasts by engulfing an algal cell and took its photosynthetic machinery. This [[secondary endosymbiosis]] hypothesis was later confirmed through molecular evidence, and it appears that the photosynthetic euglenoids are grouped into one clade.<ref name=Triemer>{{cite book |editor-first1=John D.|editor-last1=Wehr|editor-first2=Robert G.|editor-last2=Sheath|editor-first3=J. Patrick|editor-last3=Kociolek |date= 2014 |edition=2 |title= Freshwater Algae of North America: Ecology and Classification |last1= Triemer|first1=Richard E. |last2= Zakryś |first2=Bożena |chapter= Chapter 10. Photosynthetic Euglenoids |url= |location= |publisher= Elsevier Inc. |page= |isbn=978-0-12-385876-4 }}</ref> However, 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; therefore, some once-photosynthetic lineages must have later lost the chloroplasts.<ref>{{cite journal |doi=10.1007/BF00309557 |title=Genes for components of the chloroplast translational apparatus are conserved in the reduced 73-kb plastid DNA of the nonphotosynthetic euglenoid flagellate Astasia longa |year=1994 |last1=Gockel |first1=Gabriele |last2=Hachtel |first2=Wolfgang |last3=Baier |first3=Susanne |last4=Fliss |first4=Christian |last5=Henke |first5=Mark |journal=Current Genetics |volume=26 |issue=3 |pages=256–62 |pmid=7859309|s2cid=8082617 }}</ref> Recognizing the non-monophyletic nature of the genus ''Euglena,'' Marin et al. (2003) have revised it to include certain members traditionally placed in ''Astasia'' and ''Khawkinea''.<ref name="Marin-2003" />

Throughout its taxonomic history, ''Euglena'' served as a "[[wastebasket taxon|holding bag]]" for species that did not morphologically fit into other groups. This made ''Euglena'' a heterogeneous assemblage, and made correct species identification very difficult. Some researchers proposed intra-generic groups within ''Euglena''; for example Pringsheim (1956) named five groups (Rigidae, Lentiferae, Catilliferae, Radiatae, Serpentes) based on cell shape and chloroplast morphology, while Zakryś (1986) named three subgenera (Euglena, Calliglena and Discoglena) based on chloroplast and paramylon morphology.<ref name=Schwartzbach/> However, molecular phylogenetics once again showed that these groups did not always correspond to evolutionary lineages.<ref>{{cite journal|doi=10.1099/00207713-51-3-773 |title=Phylogenetic analysis of chloroplast small-subunit rRNA genes of the genus Euglena Ehrenberg |date=2001 |last1=Milanowski |first1=R. |last2=Zakryś |first2=B. |last3=Kwiatowski |first3=J. |journal=International Journal of Systematic and Evolutionary Microbiology |volume=51 |issue=3 |pages=773–781 |pmid=11411697 }}</ref> To revise this, taxonomists have transferred species out of ''Euglena'' and into other genera, including ''Lepocinclis'',<ref name="Marin-2003" /> ''Phacus'',<ref>{{cite journal|doi=10.1016/j.protis.2010.02.002 |title=Reconstructing Euglenoid Evolutionary Relationships using Three Genes: Nuclear SSU and LSU, and Chloroplast SSU rDNA Sequences and the Description of ''Euglenaria'' gen. nov. (Euglenophyta) |date=2010 |last1=Linton |first1=Eric W. |last2=Karnkowska-Ishikawa |first2=Anna |last3=Kim |first3=Jong Im |last4=Shin |first4=Woongghi |last5=Bennett |first5=Mathew S. |last6=Kwiatowski |first6=Jan |last7=Zakryś |first7=Bożena |last8=Triemer |first8=Richard E. |journal=Protist |volume=161 |issue=4 |pages=603–619 |pmid=20434949 }}</ref> and the newly proposed genera ''[[Discoplastis]]'', ''[[Euglenaria]]'', and ''[[Euglenaformis]]''.<ref name=Schwartzbach/>

==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]], [[peptone]], [[acetate]], [[ethanol]] or [[carbohydrate]]s.<ref>{{Cite book |url=https://books.google.com/books?id=TMpJo2rXAT4C&q=leadbetter+flagellates+unity&pg=PP1 |title=Flagellates: Unity, Diversity and Evolution |last1=Leadbeater |first1=Barry S. C. |last2=Green |first2=John C. |date=2002-09-11 |publisher=CRC Press |isbn=978-0-203-48481-4 |language=en}}</ref><ref>{{Cite journal |last1=Pringsheim |first1=E. G. |last2=Hovasse |first2=R. |date=1948-06-01 |title=The Loss of Chromatophores in Euglena Gracilis |journal=New Phytologist |language=en |volume=47 |issue=1 |pages=52–87 |doi=10.1111/j.1469-8137.1948.tb05092.x|doi-access=free }}</ref> 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]].<ref>{{cite book |last=Nisbet |first=Brenda |title=Nutrition and Feeding Strategies in Protozoa |year=1984 |isbn=978-0-7099-1800-4 |page=[https://archive.org/details/nutritionfeeding0000nisb/page/73 73] |publisher=Croom Helm |url=https://archive.org/details/nutritionfeeding0000nisb/page/73 }}</ref> ''Euglena's'' chloroplasts are surrounded by three membranes, while those of plants and the [[green algae]] (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]] green alga.<ref>{{cite journal |doi=10.1139/b78-345 |title=The chloroplasts of ''Euglena'' may have evolved from symbiotic green algae |year=1978 |last1=Gibbs |first1=Sarah P. |journal=Canadian Journal of Botany |volume=56 |issue=22 |pages=2883–9}}</ref> Thus, the similarities between ''Euglena'' and plants would have arisen not because of kinship but because of a secondary [[endosymbiosis]]. Molecular phylogenetic analysis has lent support to this hypothesis, and it is now generally accepted.<ref>{{cite journal |bibcode=1995PNAS...92.9122H |jstor=2368422 |title=A Nuclear Gene of Eubacterial Origin in Euglena gracilis Reflects Cryptic Endosymbioses During Protist Evolution |last1=Henze |first1=Katrin |last2=Badr |first2=Abdelfattah |last3=Wettern |first3=Michael |last4=Cerff |first4=Rudiger |last5=Martin |first5=William |volume=92 |year=1995 |pages=9122–6 |journal=Proceedings of the National Academy of Sciences of the United States of America |doi=10.1073/pnas.92.20.9122 |pmid=7568085 |issue=20 |pmc=40936|doi-access=free }}</ref><ref>{{cite journal |doi=10.1046/j.1529-8817.2003.02075.x |title=Phylogeny of euglenophyceae based on small subunit rDNA sequences: Taxonomic implications |year=2003 |last1=Nudelman |first1=Mara Alejandra |last2=Rossi |first2=Mara Susana |last3=Conforti |first3=Visitacin |last4=Triemer |first4=Richard E. |journal=Journal of Phycology |volume=39 |issue=1 |pages=226–35|s2cid=85275367 }}</ref>

''Euglena'' chloroplasts contain [[pyrenoid]]s, 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]]''.<ref name="Marin-2003">{{cite journal |pmid=12812373 |year=2003 |last1=Marin |first1=B |last2=Palm |first2=A |last3=Klingberg |first3=M |last4=Melkonian |first4=M |title=Phylogeny and taxonomic revision of plastid-containing euglenophytes based on SSU rDNA sequence comparisons and synapomorphic signatures in the SSU rRNA secondary structure |volume=154 |issue=1 |pages=99–145 |journal=Protist |doi=10.1078/143446103764928521}}</ref> Pyrenoids may be surrounded by a single paramylon cap (these pyrenoids are called haplopyrenoids), a bilateral paramylon cap (these are called diplopyrenoids), or a cluster of small paramylon grains (called a paramylon center), or may be "naked" and lack associated paramylon bodies.<ref name=Triemer>{{cite book |editor-first1=John D.|editor-last1=Wehr|editor-first2=Robert G.|editor-last2=Sheath|editor-first3=J. Patrick|editor-last3=Kociolek |date= 2014 |edition=2 |title= Freshwater Algae of North America: Ecology and Classification |last1= Triemer|first1=Richard E. |last2= Zakryś |first2=Bożena |chapter= Chapter 10. Photosynthetic Euglenoids |url= |location= |publisher= Elsevier Inc. |page= |isbn=978-0-12-385876-4 }}</ref>

Chloroplast morphology in ''Euglena'' is diverse, and can be broadly divided into four groups. The first group consists of ''[[Euglena archaeoplastidiata|E. archaeoplastidiata]]'', which has a single, parietal chloroplast with two diplopyrenoids. The second group (e.g. ''[[Euglena viridis|E. viridis]]'') has axial, stellate chloroplasts with paramylon center. The third group has parietal, lobed chloroplasts, each with a naked, haplo- or diplopyrenoid; this group is very diverse and identification may be difficult. The fourth group (e.g. ''[[Euglena sanguinea|E. sanguinea]]'') has plate-like, parietal chloroplasts each with a single diplopyrenoid. The chloroplast centers are located deep within the cell, and the chloroplasts are deeply dissected into long bands, which follow the spiral contours of the cell.<ref name=Triemer/>

''Euglena'' have two flagella rooted in [[basal body|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.<ref>{{Cite book|last1=Ciugulea|first1=Ionel|title=A Color Atlas of Photosynthetic Euglenoids|last2=Triemer|first2=Richard|publisher=Michigan State University Press|year=2010|isbn=978-0-87013-879-9|location=East Lansing|pages=17 & 38}}</ref><ref>{{Cite journal|last1=Häder|first1=Donat-P.|last2=Melkonian|first2=Michael|date=1983-08-01|title=Phototaxis in the gliding flagellate, Euglena mutabilis|journal=Archives of Microbiology|language=en|volume=135|issue=1|pages=25–29|doi=10.1007/BF00419477|s2cid=19307809|issn=1432-072X}}</ref>  In species that possess a long, emergent flagellum, it may be used to help the organism swim.<ref>{{Cite journal|last1=Rossi|first1=Massimiliano|last2=Cicconofri|first2=Giancarlo|last3=Beran|first3=Alfred|last4=Noselli|first4=Giovanni|last5=DeSimone|first5=Antonio|date=2017-12-12|title=Kinematics of flagellar swimming in Euglena gracilis: Helical trajectories and flagellar shapes|journal=Proceedings of the National Academy of Sciences|language=en|volume=114|issue=50|pages=13085–13090|doi=10.1073/pnas.1708064114|issn=0027-8424|pmid=29180429|pmc=5740643|doi-access=free}}</ref> The surface of the flagellum is coated with about 30,000 extremely fine filaments called [[mastigoneme]]s.<ref>{{Cite journal|date=1978-06-01|title=Surface organization and composition of Euglena. II. Flagellar mastigonemes|journal=The Journal of Cell Biology|volume=77|issue=3|pages=805–826|issn=0021-9525|pmc=2110158|pmid=98532|last1=Bouck|first1=G. B.|last2=Rogalski|first2=A.|last3=Valaitis|first3=A.|doi=10.1083/jcb.77.3.805}}</ref>

Like other euglenoids, ''Euglena'' possess a red [[eyespot apparatus|eyespot]], an organelle composed of [[carotenoid]] 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]]).<ref name="Schaechter-2011">{{cite book |last=Schaechter |first=Moselio |title=Eukaryotic Microbes |publisher=Elsevier/Academic Press |location=San Diego |year=2011 |isbn=978-0-12-383876-6 |page=315}}</ref>

[[File:Euglena pellicle 2.jpg |thumb|Spiral pellicle strips]]
''Euglena'' lacks a [[cell wall]]. Instead, it has a [[Protozoa#Walls, pellicles, scales, and skeletons|pellicle]] made up of a protein layer supported by a substructure of [[microtubule]]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.<ref name="Schaechter-2011" /> The mechanism of this euglenoid movement is not understood, but its molecular basis may be similar to that of [[amoeboid movement]].<ref>{{cite book |author=O'Neill, Ellis |title=An exploration of phosphorylases for the synthesis of carbohydrate polymers (PhD thesis) |publisher=University of East Anglia |date=2013 |url=http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.590735 |pages=170–171 |access-date=2017-10-01 |archive-date=2016-03-21 |archive-url=https://web.archive.org/web/20160321035730/http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.590735 }}</ref> Some species have mucocysts, which are membrane-bound bodies containing mucilaginous threads. Mucocysts are located underneath the pellicle in parallel rows following the arrangement of pellicle strips. Their presence or absence and shape (spherical or spindle-shaped) are an important diagnostic for species-level identification, but are mostly only visible after staining with a dye such as [[neutral red]].<ref name=Triemer/>

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.

[[File:2023 Eugenid.svg|center|thumb|upright=2|{{center|'''Euglenid Body Plan'''}}{{ordered list|Dorsal [[flagellum]]|
[[Axoneme]]|
Paraflagellar rod|
[[Mastigoneme|Mastigonemes]], "hairs" attached to flagellum|
Flagellar pocket vestibulum|
Feeding apparatus|
Paraxial swelling|
[[Eyespot apparatus|Eyespot]], photoreceptor used to sense light direction and intensity|
[[Contractile vacuole]], regulates the quantity of water inside a cell|
Ventral flagellum|
Ventral root|
[[Golgi apparatus]]; modifies [[protein]]s and sends them out of the cell|
[[Endoplasmic reticulum]], the transport network for molecules going to specific parts of the cell|
[[Phagosome]]|
[[Lysosome]], holds enzymes|
[[Cell nucleus|Nucleus]]|
[[Nucleolus]]|
[[Plastid]] membranes (3, secondary)|
[[Thylakoid|Thylakoids]], site of the [[light-dependent reactions]] of [[photosynthesis]]|
[[Pyrenoid]], center of [[carbon fixation]]|
[[Paramylon]] [[Granule (cell biology)|granules]]|
[[Protozoa#Walls,_pellicles,_scales,_and_skeletons|Pellicular]] strip|
Muciferous body/Mucocyst|
[[Mitochondria|Mitochondrion]], creates [[Adenosine triphosphate|ATP]] (energy) for the cell (discoid [[crista|cristae]])}}]]

==Reproduction==
''Euglena'' reproduce asexually through [[Fission (biology)|binary fission]], a form of [[cell division]].  Reproduction begins with the [[mitosis]] of the [[cell nucleus]], 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 [[Anatomical terms of location|anterior]], and a V-shaped bifurcation gradually moves toward the [[Anatomical terms of location|posterior]], until the two halves are entirely separated.<ref>{{cite journal |first=Mary |last=Gojdics |title=The Cell Morphology and Division of Euglena deses Ehrbg |journal=Transactions of the American Microscopical Society |volume=53 |issue=4 |year=1934 |pages=299–310 |jstor=3222381 |doi=10.2307/3222381}}</ref>

Reports of [[isogamy|sexual conjugation]] are rare, and have not been substantiated.<ref>{{cite book | last1 = Lee | first1 = John J. | title = An Illustrated Guide to the Protozoa: organisms traditionally referred to as protozoa, or newly discovered groups | volume = 2 | edition = 2nd | publisher = Society of Protozoologists | year = 2000 | location = Lawrence, Kansas | page = 1137  }}</ref>

==Uses==
[[File:Bioconvection Euglena Concentration series Colour.jpg|thumb|right|Cultures of ''Euglena'' in a petri dish]]
''Euglena'' has been used extensively as a model organism. It is capable of both heterotrophic and photosynthetic growth, meaning it can be grown in both light and dark conditions and it is thus relatively easy to cultivate. ''Euglena'' was one of the first photosynthetic organisms to have its chloroplast [[genome]] sequenced, and the chloroplast of ''Euglena'' has been extensively studied in the fields of biochemistry, cell biology and molecular biology.<ref name=Schwartzbach>{{cite book|doi=10.1007/978-3-319-54910-1 |title=Euglena: Biochemistry, Cell and Molecular Biology |series=Advances in Experimental Medicine and Biology |date=2017 |volume=979 |isbn=978-3-319-54908-8 |editor-last1=Schwartzbach|editor-first1=S.D.|editor-last2=Shigeoka|editor-first2=Shigeru}}</ref> In 2015, 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]]s and natural products.<ref>[https://www.sciencedaily.com/releases/2015/08/150814075751.htm The potential in your pond] published on August 14, 2015 by the "John Innes Centre"</ref><ref>{{cite journal |doi=10.1039/C5MB00319A |pmid=26289754 |title=The transcriptome of Euglena gracilis reveals unexpected metabolic capabilities for carbohydrate and natural product biochemistry |year=2015 | last1=O'Neill | first1=Ellis C. | last2=Trick | first2=Martin | last3=Hill|first3=Lionel|last4=Rejzek|first4=Martin |last5=Dusi|first5=Renata G.|last6=Hamilton|first6=Christopher J.|last7=Zimba|first7=Paul V.|last8=Henrissat|first8=Bernard|last9=Field|first9=Robert A.|journal=Molecular BioSystems |volume=11 |issue=10 |pages=2808–21|doi-access=free}}</ref> In addition, ''Euglena'' is commonly used in classrooms to demonstrate important biological processes, such as photosynthesis,<ref name="Edelmann">{{cite journal |last1=Edelmann |first1=Hans G. |last2=Martius |first2=Thilo |last3=Hahn |first3=Achim |last4=Schlüter |first4=Kirsten |last5=Nessler |first5=Stefan H. |title=Learning about the Nature of Science Using Algae |journal=School Science Review |volume=98 |issue=362 |pages=85–90 |date=September 2016}}</ref> or [[population growth]].<ref>{{Cite journal|last1=Anggraini|first1=R.|last2=Somakim|last3=Hapizah|date=February 2019|title=Students' understanding of logarithms using the growth of Euglena viridis context|journal=Journal of Physics: Conference Series|language=en|volume=1166|issue=1 |article-number=012037|doi=10.1088/1742-6596/1166/1/012037|bibcode=2019JPhCS1166a2037A |s2cid=92221946 |issn=1742-6596|doi-access=free}}</ref>

The taste of powdered ''Euglena'' is described as dried sardine flakes, and contains minerals, vitamins and docosahexaenoic acid, an omega-3 acid. The powder is used as ingredient in other foods.<ref>{{cite news|url=https://www.japantimes.co.jp/news/2014/01/23/national/tiny-euglena-latest-fad-in-eating-healthy/#.XPJ52eQ4VZU |title=Tiny euglena latest fad in eating healthy|work= The Japan Times|date=23 January 2014}}</ref> Kemin Industries sells a euglena nutraceutical supplement ingredient featuring dried ''Euglena gracilis'' with high levels of [[Beta-glucan|beta glucan]].<ref>{{Cite web|title=BetaVia™|url=https://www.kemin.com/content/kemin/eu/en/home/products/betavia|access-date=2021-09-16|website=Kemin|language=en}}</ref>

The lipid content of ''Euglena'' (mainly wax esters) is seen as a promising feedstock for production of biodiesel and [[jet fuel]].<ref name="Toyama-2019">{{cite journal |last1=Toyama |first1=Tadashi  |last2=Hanaoka |first2=Tsubasa  |date=2019 |title=Enhanced production of biomass and lipids by Euglena gracilis via co-culturing with a microalga growth-promoting bacterium |journal=Biotechnology for Biofuels |volume=12 |page=205 |doi= 10.1186/s13068-019-1544-2|pmid=31695747 |pmc=6822413 |doi-access=free }}</ref> Under the aegis of [[Itochu]], 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.<ref>{{cite news |last=Okutsu |first=Akane |date=2018 |title=Biotech company euglena teams with ANA to fuel green commercial flights |url=https://asia.nikkei.com/Business/Companies/Jet-biofuel-mass-production-to-begin-in-Japan |publisher=Nikkei |publication-date=November 2, 2018 |access-date=April 8, 2020 }}</ref><ref>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. {{cite web |url=https://www.japan.go.jp/technology/innovation/fuelingjet.html |title=Fueling Jet Aircraft With Microalgae: Growing biofuel without farmlands |website=JapanGov |publisher=The Government of Japan |access-date=April 8, 2020 }}</ref>

==Video gallery==
<gallery widths="200px" heights="160px">
File:Euglena sp.ogg|Red ''Euglena'' sp.
File:Euglena mutabilis.ogv|''Euglena mutabilis'', showing metaboly, paramylon bodies and chloroplasts
File:Euglena sanguinea.ogg|''Euglena sanguinea''
File:Euglena metaboly and swimming movement.ogv|''Euglena'', moving by metaboly and swimming
</gallery>

==See also==
* ''[[Elysia chlorotica]]'', an animal with chloroplasts derived from algae
* [[Kleptoplasty]]

==References==
{{Reflist}}

==External links==
{{Wikispecies}}
{{Wiktionary}}
*[https://web.archive.org/web/20130721115857/http://euglena.msu.edu/ The Euglenoid Project]
*[http://tolweb.org/Euglenida/97461 Tree of Life web project: Euglenida]
*[http://protist.i.hosei.ac.jp/PDB/Images/Mastigophora/Euglena/ Protist Images: Euglena]
*[http://www.pirx.com/droplet/gallery/euglena.html Euglena at Droplet - Microscopy of the Protozoa]
*[https://web.archive.org/web/20071007115729/http://starcentral.mbl.edu/microscope/portal.php?pagetitle=assetfactsheet&imageid=23340 Images and taxonomy]
*{{cite journal |first1=George |last1=Constantopoulos |first2=Konrad |last2=Bloch |year=1967 |journal=The Journal of Biological Chemistry |volume=242 |issue=15 |pages=3538–42 |url=http://www.jbc.org/content/242/15/3538.short |title=Effect of Light Intensity on the Lipid Composition of ''Euglena gracilis''|doi=10.1016/S0021-9258(18)95895-3 |doi-access=free }}

{{Discoba}}

{{Taxonbar|from=Q236001}}
{{Authority control}}

[[Category:Euglenozoa genera]]
[[Category:Articles containing video clips]]
[[Category:Taxa named by Christian Gottfried Ehrenberg]]