EUGLENA is a genus of single-celled 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 800 species.
Euglena are found in fresh and salt waters. 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 ).
Euglena gracilis has been used extensively in the
laboratory as a model organism .
Most species of
Euglena have photosynthesizing chloroplasts within
the body of the cell, which enable them to feed by autotrophy , like
plants. However, they can also take nourishment heterotrophically ,
like animals. Since
Euglena have features of both animals and plants,
early taxonomists, working within the Linnaean three-kingdom system of
biological classification, found them difficult to classify. It was
the question of where to put such "unclassifiable" creatures that
Ernst Haeckel to add a third living kingdom (a fourth kingdom
in toto) to the Animale, Vegetabile (and Lapideum meaning Mineral) of
Linnaeus : the Kingdom
* 1 Form and function
* 2 Reproduction
* 3 Historical background and early classification
* 4 Recent phylogeny and classification
* 5 Human consumption
* 6 Video gallery
* 7 See also
* 8 References
* 9 External links
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
carbohydrates . 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 . 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. Thus, the intriguing similarities between
Euglena and the 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.
Euglena chloroplasts contain pyrenoids , used in the synthesis of
paramylon , a form of starch energy storage enabling
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
All euglenoids have two flagella rooted in basal bodies located in a
small reservoir at the front of the cell. In Euglena, one flagellum is
very short, and does not protrude from the cell, while the other is
relatively long, and often easily visible with light microscopy. In
some species, the longer, emergent flagellum is used to help the
Like other euglenoids,
Euglena possess a red 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
Euglena to find the light and move toward it (a process known as
phototaxis ). Spiral pellicle strips
Euglena lacks a cell wall . Instead, it has a pellicle made up of a
protein layer supported by a substructure of microtubules , arranged
in strips spiraling around the cell. The action of these pellicle
strips sliding over one another, known as metaboly, gives
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.
Euglena reproduce asexually through binary fission , a form of cell
division . Reproduction begins with the mitosis of the cell nucleus ,
followed by the division of the cell itself.
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 , and a V-shaped bifurcation gradually
moves toward the posterior , until the two halves are entirely
Reports of sexual conjugation are rare, and have not been
HISTORICAL BACKGROUND AND EARLY CLASSIFICATION
Cercaria viridis (= E. viridis) from O.F. Müller's Animalcula
Euglena were among the first protists to be seen under the
In 1674, in a letter to the Royal Society, the Dutch pioneer of
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 Euglena's body.
Euglena from Félix
Dujardin's Histoire Naturelle des Zoophytes, 1841
In 1830, C. G. Ehrenberg renamed Müller's Cercaria
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
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).
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. 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
Euglena movement, known as metaboly
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
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
The idea of classifiying 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
Euglena gracilis in close kinship with the species Khawkinea quartana,
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 carbohydrates and
Euglena viridis was found to be genetically closer to
Khawkinea quartana than to the other species of
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.
Starting in 2005, Tokyo-based
Euglena Company has started marketing
Euglena-based food and beverage products, based on their provision of
both plant- and animal-based nutrients. While the fitness of euglena
for human consumption had long been surmised,
Euglena Co. was the
first to develop a technique to cultivate and farm the microorganism
in large enough quantities to be commercially viable. The company's
main production facility is located on
Ishigaki Island ,
Okinawa , due
to favorable climate conditions.
Euglena Company is also experimenting with the use of
Euglena as a
potential fuel source.
Play media Red
Euglena mutabilis, showing metaboly, paramylon
bodies and chloroplasts
Play media Euglena, moving by metaboly and swimming
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