| Euglena | |
|---|---|
.jpg) | |
| Euglena sp. | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Discoba |
| Phylum: | Euglenozoa |
| Class: | Euglenida |
| Clade: | Euglenophyceae |
| Order: | Euglenales |
| Family: | Euglenaceae |
| Genus: | Euglena Ehrenberg, 1830 |
| Type species | |
| Euglena viridis (O.F.Müller) Ehrenberg[1] | |
Euglena is a genus ofsingle-celled,flagellateeukaryotes. It is the best-known and most widely studied member of the classEuglenoidea, a diverse group containing some 54genera and at least 200 species.[2][3] Species ofEuglena 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).[4]
The speciesEuglena gracilis has been used extensively in the laboratory as amodel organism.[5]
Most species ofEuglena have photosynthesizingchloroplasts within the body of the cell, which enable them to feed byautotrophy, like plants. However, they can also take nourishmentheterotrophically, like animals. SinceEuglena have features of both animals and plants, early taxonomists, working within theLinnaean two-kingdom system of biological classification, found them difficult to classify.[6][7] It was the question of where to put such "unclassifiable" creatures that promptedErnst Haeckel to add a third living kingdom (a fourth kingdomin toto) to theAnimale,Vegetabile (andLapideum meaningMineral) ofLinnaeus: the KingdomProtista.[8]
Species ofEuglena were among the first protists to be seen under the microscope. In 1674, in a letter to the Royal Society, the Dutch pioneer of microscopyAntonie 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 wereEuglena viridis, whose "peculiar arrangement of chromatophores...gives the flagellate this appearance at low magnification."[9] 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."[10]
In 1786,O.F. Müller gave a more complete description of the organism, which he namedCercaria 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.[11] In 1830,C. G. Ehrenberg renamed Müller'sCercariaEuglena 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.[12][13] By making use of the newly invented achromatic microscope,[14] Ehrenberg was able to seeEuglena'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).[15]
Ehrenberg did not noticeEuglena's flagella, however. The first to publish a record of this feature wasFélix Dujardin, who added "filament flagelliforme" to the descriptive criteria of the genus in 1841.[16] Subsequently, the class Flagellata (Cohn, 1853) was created for creatures, likeEuglena, 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.[17]
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 thePeranemaceae, while flexible green euglenoids were generally assigned to the genusEuglena.[18]
As early as 1935, it was recognized that this was an artificial grouping, however convenient.[19] 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 genusAstasia, 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 ofPhacus andLepocinclis with some species ofEuglena.[18]
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.[20] In the 1970s, it was hypothesized that photosynthetic euglenoids derived their chloroplasts by engulfing an algal cell and took its photosynthetic machinery. Thissecondary endosymbiosis hypothesis was later confirmed through molecular evidence, and it appears that the photosynthetic euglenoids are grouped into one clade.[21] However, genetic analysis of the non-photosynthesizing euglenoidAstasia 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.[22] Recognizing the non-monophyletic nature of the genusEuglena, Marin et al. (2003) have revised it to include certain members traditionally placed inAstasia andKhawkinea.[23]
Throughout its taxonomic history,Euglena served as a "holding bag" for species that did not morphologically fit into other groups. This madeEuglena a heterogeneous assemblage, and made correct species identification very difficult. Some researchers proposed intra-generic groups withinEuglena; 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.[24] However, molecular phylogenetics once again showed that these groups did not always correspond to evolutionary lineages.[25] To revise this, taxonomists have transferred species out ofEuglena and into other genera, includingLepocinclis,[23]Phacus,[26] and the newly proposed generaDiscoplastis,Euglenaria, andEuglenaformis.[24]
When feeding as a heterotroph,Euglena takes in nutrients byosmotrophy, and can survive without light on a diet of organic matter, such asbeef extract,peptone,acetate,ethanol orcarbohydrates.[27][28] When there is sufficient sunlight for it to feed byphototrophy, it uses chloroplasts containing the pigmentschlorophyll a andchlorophyll b to produce sugars byphotosynthesis.[29]Euglena's chloroplasts are surrounded by three membranes, while those of plants and thegreen algae (among which earlier taxonomists often placedEuglena) have only two membranes. This fact has been taken as morphological evidence thatEuglena's chloroplasts evolved from aeukaryotic green alga.[30] Thus, the similarities betweenEuglena and plants would have arisen not because of kinship but because of a secondaryendosymbiosis. Molecular phylogenetic analysis has lent support to this hypothesis, and it is now generally accepted.[31][32]
Euglena chloroplasts containpyrenoids, used in the synthesis ofparamylon, a form of starch energy storage enablingEuglena 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 asLepocinclis andPhacus.[23] 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.[21]
Chloroplast morphology inEuglena is diverse, and can be broadly divided into four groups. The first group consists ofE. archaeoplastidiata, which has a single, parietal chloroplast with two diplopyrenoids. The second group (e.g.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.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.[21]
Euglena have two flagella rooted inbasal 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 asEuglena 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.[33][34] In species that possess a long, emergent flagellum, it may be used to help the organism swim.[35] The surface of the flagellum is coated with about 30,000 extremely fine filaments calledmastigonemes.[36]
Like other euglenoids,Euglena possess a redeyespot, an organelle composed ofcarotenoid pigment granules. The red spot itself is not thought to bephotosensitive. 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 theEuglena to find the light and move toward it (a process known asphototaxis).[37]
Euglena lacks acell wall. Instead, it has apellicle made up of a protein layer supported by a substructure ofmicrotubules, arranged in strips spiraling around the cell. The action of these pellicle strips sliding over one another, known asmetaboly, givesEuglena its exceptional flexibility and contractility.[37] The mechanism of this euglenoid movement is not understood, but its molecular basis may be similar to that ofamoeboid movement.[38] 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 asneutral red.[21]
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.
Euglena reproduce asexually throughbinary fission, a form ofcell division. Reproduction begins with themitosis of thecell 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 theanterior, and a V-shaped bifurcation gradually moves toward theposterior, until the two halves are entirely separated.[39]
Reports ofsexual conjugation are rare, and have not been substantiated.[40]
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 chloroplastgenome sequenced, and the chloroplast ofEuglena has been extensively studied in the fields of biochemistry, cell biology and molecular biology.[24] In 2015, Ellis O'Neill and Professor Rob Field have sequenced the transcriptome ofEuglena gracilis, which provides information about all of the genes that the organism is actively using. They found thatEuglena gracilis has a whole host of new, unclassified genes which can make new forms ofcarbohydrates and natural products.[41][42] In addition,Euglena is commonly used in classrooms to demonstrate important biological processes, such as photosynthesis,[43] orpopulation growth.[44]
The taste of powderedEuglena 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.[45] Kemin Industries sells a euglena nutraceutical supplement ingredient featuring driedEuglena gracilis with high levels ofbeta glucan.[46]
The lipid content ofEuglena (mainly wax esters) is seen as a promising feedstock for production of biodiesel andjet fuel.[47] Under the aegis ofItochu, 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.[48][49]
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