Euglena
Encyclopedia
Euglena is a genus of unicellular flagellate
protist
s. It is the best known and most widely studied member of the phylum Euglenozoa
(also known as Euglenophyta), a diverse group containing some 44 genera and at least 800 species. Species of 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).
The species Euglena gracilis, has been used extensively in the laboratory as a model organism
.
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 heterotrophically
, 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. Indeed, it was the question of where to put such "unclassifiable" creatures that prompted Ernst Haeckel
to add a third kingdom to the Animale and Vegetabile of Linnaeus: the Kingdom Protista.
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 algae. 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 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
.
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 the Euglena to find the light and and move toward it (a process known as phototaxis
).
Euglena lacks a cell wall (an outer membrane containing cellulose). Instead, it has a 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 gives Euglena its exceptional flexibility and contractility.
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.
In 1674, in a letter to the Royal Society
, the Dutch pioneer of microscopy Antony 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 Fishes."
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 (or metaboly) of Euglena's body.
In 1830, 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 . 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).
Ehrenberg did not notice Euglenas 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.
made a primary taxonomical distinction between green and colorless flagellate organisms, separating the photosynthesizing Euglenoids from those that live by phagotrophy. The latter (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 taxonomical 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.
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 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 Euglena gracilis was, in fact, more closely related to Astasia longa than to certain other species recognized as Euglena. Furthermore, 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.
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s. It is the best known and most widely studied member of the phylum Euglenozoa
Euglenozoa
The Euglenozoa are a large group of flagellate protozoa. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. There are two main subgroups, the euglenids and kinetoplastids...
(also known as Euglenophyta), a diverse group containing some 44 genera and at least 800 species. Species of 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).
The species Euglena gracilis, has been used extensively in the laboratory as a model organism
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
.
Most species of Euglena have photosynthesizing chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s within the body of the cell, which enable them to feed by autotrophy
Autotrophy
Autotrophy is the ability to be self-sustained by producing food from inorganic compounds. Some bacteria and some archaea have this ability. Inorganic compounds are oxidized directly without sunlight to yield energy. This metabolic mode also requires energy for CO2 reduction, like photosynthesis,...
, like plants. However, they can also take nourishment heterotrophically
Heterotroph
A heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts with autotrophs, such as plants and algae, which can use energy from sunlight or inorganic compounds to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon...
, like animals. Since Euglena have features of both animals and plants, early taxonomists, working within the Linnaean
Linnaean taxonomy
Linnaean taxonomy can mean either of two related concepts:# the particular form of biological classification set up by Carl Linnaeus, as set forth in his Systema Naturæ and subsequent works...
two-kingdom system of biological classification, found them difficult to classify. Indeed, it was the question of where to put such "unclassifiable" creatures that prompted Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...
to add a third kingdom to the Animale and Vegetabile of Linnaeus: the Kingdom Protista.
Form and function
When feeding as a heterotroph, the Euglena surrounds a particle of food and consumes it by phagocytosis. When there is sufficient sunlight for it to feed by phototrophy, it uses chloroplasts containing the pigments Chlorophyll aChlorophyll a
Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary...
and Chlorophyll b
Chlorophyll b
Chlorophyll b is a form of chlorophyll. Chlorophyll b helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll a in polar solvents because of its carbonyl group. Its color is yellow, and it primarily absorbs blue light....
to produce sugars by photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. Euglena's chloroplasts are surrounded by three membranes, while those of plants and the green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
(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 algae. 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 pyrenoid
Pyrenoid
In cell biology, pyrenoids are organelles, centers of carbon dioxide fixation within the chloroplasts of algae and hornworts. Pyrenoids are not membrane-bound, but specialized areas of the plastid that contain high levels of ribulose-1,5-bisphosphate carboxylase/oxygenase .RubisCO fixes carbon...
s, used in the synthesis of paramylon
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 pyrenoids of Euglena. The eugenoids have chlorophylls a and b and they store...
, 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
Phacus
Phacus is a genus of unicellular protists, of the phylum Euglenozoa ....
.
Like other Euglenoids, Euglena possess a red eyespot
Eyespot apparatus
The eyespot apparatus is a photoreceptive organelle found in the flagellate cells of green algae and other unicellular photosynthetic organisms such as euglenids. It allows the cells to sense light direction and intensity and respond to it by swimming either towards the light or away from the...
, an organelle composed of carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
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 and move toward it (a process known as phototaxis
Phototaxis
Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves in response to the stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis...
).
Euglena lacks a cell wall (an outer membrane containing cellulose). Instead, it has a pellicle
Pellicle
Pellicle may refer to:*Pellicle , a thin layer supporting the cell membrane in various protozoa*Pellicle mirror, a thin plastic membrane which may be used as a beam splitter or protective cover in optical systems...
made up of a protein layer supported by a substructure of microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s, arranged in strips spiraling around the cell. The action of these pellicle strips sliding over one another gives Euglena its exceptional flexibility and contractility.
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.
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
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
, the Dutch pioneer of microscopy Antony 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 Fishes."
In 1786, O.F. Müller
Otto Friedrich Müller
Otto Friedrich Müller, also Mueller was a Danish naturalist.-Biography:Müller was born in Copenhagen. He was educated for the church, became tutor to a young nobleman, and after several years' travel with him settled in Copenhagen in 1767, and married a lady of wealth.His first important works,...
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 (or metaboly) of Euglena's body.
In 1830, C. G. Ehrenberg
Christian Gottfried Ehrenberg
Christian Gottfried Ehrenberg , German naturalist, zoologist, comparative anatomist, geologist, and microscopist, was one of the most famous and productive scientists of his time.- Early collections :...
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 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).
Ehrenberg did not notice Euglenas flagella, however. The first to publish a record of this feature was Félix Dujardin
Félix Dujardin
-External sources:* @ Encyclopædia Britannica Online...
, 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 KlebsGeorg Klebs
Georg Albrecht Klebs was a German botanist from Neidenburg , Prussia. His brother was the historian Elimar Klebs.-Life:...
made a primary taxonomical distinction between green and colorless flagellate organisms, separating the photosynthesizing Euglenoids from those that live by phagotrophy. The latter (colorless, shape-changing uniflagellates) were divided among the Astasiaceae and the Peranemaceae
Peranema
Peranema is a genus of free-living flagellate protists, with about 20 species, varying in size between 8 and 200 micrometers. They are found in freshwater lakes, ponds and ditches, and are often abundant at the bottom of stagnant pools rich in decaying organic material...
, 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 taxonomical 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.
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 Euglenozoa, put Euglena gracilis in close kinship with the species Khawkinea quartana, with Peranema trichophorum basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
to both. Two years later, a molecular analysis showed that Euglena gracilis was, in fact, more closely related to Astasia longa than to certain other species recognized as Euglena. Furthermore, 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.