Euglena

The species Euglena gracilis has been widely used in the laboratory as a model organism{4}.

Most species of Euglena have photosynthetic chloroplasts in the cell body, allowing them to feed autotrophically, like plants. However, they can also feed heterotrophically, like animals. Because Euglena have both animal and plant characteristics, early taxonomists, working within Linnaeus' two-kingdom biological classification system, had difficulty classifying them{5}. It was the question of where to place these "unclassifiable" creatures that prompted Ernst Haeckel to add a third living kingdom (a fourth kingdom in toto) to Linnaeus' Animale, Vegetabile (and Lapideum meaning Mineral): the kingdom Protista{7}.

Form and function

When feeding as a heterotroph, Euglena absorbs nutrients by osmotrophy and can survive without light by feeding on organic matter, such as beef extract, peptone, acetate, ethanol or carbohydrates{8}{9}. When there is sufficient light for it to feed phototrophically, it uses chloroplasts containing the pigments chlorophyll a and chlorophyll b to produce sugars by photosynthesis{10}. The chloroplasts of Euglena are surrounded by three membranes, whereas those of plants and green algae (among which early taxonomists often placed Euglena) have only two membranes. This fact has been taken as morphological evidence that the chloroplasts of Euglena evolved from a eukaryotic green alga{11}. Thus, the similarities between Euglena and plants are not due to kinship but to secondary endosymbiosis. Molecular phylogenetic analysis has provided support for this hypothesis, which is now generally accepted{12}{13}.

Diagram of Euglena.

The chloroplasts of Euglena contain pyrenoids, which are used in the synthesis of paramylon, a form of starch energy storage that allows Euglena to survive periods of light deprivation. The presence of pyrenoids is used as an identifying characteristic of the genus, separating it from other euglenoids, such as Lepocinclis and Phacus.{14}

Euglena[/i] 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 under light microscopy. In some species, such as Euglena mutabilis, both flagella are "non-emergent", i.e. entirely confined within the cell's reservoir, and therefore cannot be seen under the light microscope{15}{16} In species that have a long emergent flagellum, this can be used to help the organism swim{17} The surface of the flagellum is covered with about 30,000 extremely fine filaments called mastigonemes{18}.

Like other euglenoids, Euglena has a red ocelli, an organelle composed of granules of carotenoid pigments. The red spot itself does not appear to be photosensitive. Instead, it filters sunlight that falls on a light-sensing structure at the base of the flagellum (a bulge, called a paraflagellar body), allowing only certain wavelengths of light to reach it. When the cell rotates relative to the light source, the ocelli partially blocks the source, allowing the Euglena to find the light and approach it (a process known as phototaxis){19}.

Spiral film strips

Euglena[/i] does not have a cell wall. Instead, it has a pellicle consisting of a protein layer supported by a substructure of microtubules, arranged in spiral bands around the cell. The action of these film bands sliding over each other, known as metabolism, gives Euglena its exceptional flexibility and contractility{19}. The mechanism of this Euglenoid movement is not understood, but its molecular basis may be similar to that of amoeboid movement{20}.

In conditions of low humidity, or when food is scarce, Euglena forms a protective wall around itself and remains dormant as a resting cyst until environmental conditions improve.

Reproduction

Euglena[/i] reproduce asexually by binary fission, a form of cell division. Reproduction begins with mitosis of the cell nucleus, followed by division of the cell itself. Euglena[/i] divide longitudinally, starting at the front end of the cell, with duplication of the flagellar processes, gosier and stigma. At present a cleavage is formed in the anterior part, and a V-shaped bifurcation gradually moves towards the posterior part, until the two halves are entirely separated{21}.

Sexual conjugation relationships are rare and have not been proven{22}.

Historical background and first classification

Cercaria viridis (= E. viridis) from O.F. Müller's Animalcula Infusoria. 1786

The Euglena species were among the first protists to be observed under the microscope.

In 1674, in a letter to the Royal Society, the Dutch microscopy pioneer Antoni van Leeuwenhoek wrote that he had taken samples of water from an inland lake, in which he found "animalcules" that were "green in the middle, and white in front and behind." Clifford Dobell considers it "almost certain" that this is Euglena viridis, whose "peculiar arrangement of chromatophores... gives the flagellate this appearance at low magnification"{23}.

Twenty-two years later, John Harris published a short series of "Microscopic Observations" in which he reported examining "a small drop of the green surface of a puddle of water" and finding that it was "composed entirely of animals of many shapes and sizes". Among them were "oval creatures, the central part of which was grass-green, but each end of which was clear and transparent", which "contracted and expanded, tumbled several times together, and then darted away like fishes"{24}.

In 1786, O.F. Müller gave a more complete description of the organism, which he named Cercaria viridis, noting its distinctive colour and changing body shape. Müller also provided a series of illustrations, accurately describing the undulatory and contractile movements (metabolism) of the Euglena's body.{25}

Euglena from Félix Dujardin's Histoire Naturelle des Zoophytes, 1841

In 1830, C. G. Ehrenberg renamed Müller's Cercaria Euglena viridis and placed it, in accordance with the ephemeral classification system he invented, among the Polygastrica of the family Astasiaea: creatures with several stomachs without alimentary canal, with variable body shape but without pseudopods or lorica{26}. Using the newly invented achromatic microscope{28}, Ehrenberg was able to see Euglena's ocelli, which he correctly identified as a "rudimentary eye" (although he mistakenly thought this meant that 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-" (good, good) and gl?n? (eyeball, joint cavity){29}.

Ehrenberg, however, did not notice the flagella of Euglena. The first to publish a record of this feature was Félix Dujardin, who added "flagelliform filament" to the descriptive criteria of the genus in 1841{30} Subsequently, the class Flagellata (Cohn, 1853) was created for creatures, like Euglena, possessing one or more flagella. Although "Flagellata" is no longer used as a taxon, the idea of using flagella as a phylogenetic criterion remains vigorous{31}.

Recent phylogeny and classification

Eugenoid movement, known as metabolism

In 1881, Georg Klebs made a primary taxonomic distinction between green and colourless flagellate organisms, separating photosynthetic euglenoids from heterotrophs. The latter (largely colourless, shape-changing uniflagellates) were divided between the Astasiaceae and Peranemaceae, while the flexible green euglenoids were generally assigned to the genus Euglena.{32}

In 1948, Pringsheim argued that the distinction between green and colourless flagellates had no taxonomic justification, although he recognised its practical value. He proposed a sort of compromise, placing colourless and saprotrophic euglenoids in the genus Astasia, while allowing some colourless euglenoids to share a genus with their photosynthesising cousins, provided they had structural features proving a common ancestry. Among the green euglenoids themselves, Pringsheim recognised the close kinship of some species of Phacus and Lepocinclis with some species of Euglena{32}.

The idea of classifying euglenoids according to their mode of feeding was finally abandoned in the 1950s, when A. Holland published a major revision of the phylum, grouping the organisms according to common structural features, such as the number and type of flagella{34}. 34] If any doubt remained, it was removed in 1994, when genetic analysis of the non-photosynthetic euglenoid Astasia longa confirmed that this organism retains DNA sequences inherited from an ancestor that must have had functional chloroplasts{35}.

In 1997, a morphological and molecular study of eugenozoans placed Euglena gracilis in close relationship with the species Khawkinea quartana, with Peranema trichophorum basal to both. Two years later, molecular analysis showed that E. gracilis was, in fact, more closely related to Astasia longa than to some of the other recognised species such as Euglena. In 2015, Dr Ellis O'Neill and Professor Rob Field sequenced the transcriptome of Euglena gracilis, which provides information about all the genes the organism actively uses. They found that Euglena gracilis has a whole range of new, unclassified genes capable of making new forms of carbohydrates and natural products{37}{38}.

The venerable Euglena viridis was found to be genetically closer to Khawkinea quartana than to any of the other Euglena species studied. Recognising the polyphyletic nature of the genus Euglena, Marin et al. (2003) revised it to include some members traditionally placed in Astasia and Khawkinea.{14}

Human consumption

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 an ingredient in other foods to make them healthier{39}.

Feedstock for biofuel production

The lipid content of Euglena (mainly wax esters) is considered a promising feedstock for biodiesel and jet fuel production{40} Under the guidance of Itochu, a start-up called Euglena Co, Ltd completed a refining plant in Yokohama in 2018, with a production capacity of 125 kilolitres of biofuel and biodiesel per year{41}{42}.