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In 1786, Műller described small green flagellate he called Cercaria viridis. Subsequently, Ehrenberg (1830) renamed C. viridis to Euglena viridis, which is now recognized as the type species of the genus Euglena. There are about 65 species in the genus Euglena, and E. viridis and E. gracilis are most studied among them.
Euglena spp. belong to a large taxonomic group of unicellular organisms Euglenozoa that contain colorless and pigmented organisms. Among those are osmotrophs that lack a feeding apparatus and are able to absorb molecules directly from eutrophic environments, e.g., Distigma and Rhabdomonas, parasites such as Trypanosoma, Leishmania, Giardia , and phagotrophs that hunt and ingest particulate food, including bacteria and other unicellular organisms living in these environments. Among phagotrophs there are bacterivores that feed on bacteria (Petalomonas, Ploeotia and Entosiphon) and eukaryovores, e.g., Dinema, Peranema and Urceolus) that feed on eukaryotes. Many are also be capable of photosynthesis.
Typical Euglena are fusiform (have spindle-like shape of the cell that is wide in the middle and tapers at both ends) or cylindrical. They have two flagella (hence their common name Flagellates), with the second flagellum often being rudimentary. They also have one or more chloroplasts and feed osmotrophically and/or autotrophically (by photosynthesis, i.e., food is produced from inogranic substances and light).
For hundreds of years, zoologists claimed these fascinating organisms to be animals, while botanists regarded them as plants.
Photosynthetic euglenids are a clade of chimeric cells that are derived from a relatively recent endosymbiosis with green algal prey cells.
Photosynthetic euglenids adopted distinct flagellar beat patterns (e.g. a lasso or figure-eight beat pattern that pulls the cell forward), allowing them to stay in the water column above the substrate.
Photosynthetic euglenids have an enlarged flagellar pocket (reservoir), a photosensory swelling at the base of dorsal (emergent) flagellum and a carotenoid-based structure involved in phototaxis (stigma or eyespot).
Brief description of euglenoid flagellatesEuglena is a genus of single-celled, free living flagellated microorganisms that show both plant- (photosynthesis) and animal-like (absence of cell wall) characteristics. The majority of Euglena species are green due to the presence of chloroplasts containing chlorophylls a and b. Members of the genus Euglena are found widely in nature, inhabiting freshwater pools, ponds and lakes. Euglena are able to use photosynthesis and heterotrophic oxidative assimilation as interchangeable and apparently equivalent sources of carbon and energy. No sexual reproduction has been detected in Euglena.
Brief description of E. viridisBody is fusiform, rounded anteriorly, gradually tapering posteriorly forming a tip. Dimensions: 40 to 60 μm long and 10 to 18 μm in diameter. Flagellum is of the same length as the body, or slightly longer. There are 1 to 15 chloroplasts (literature sources contradict) that are star-shaped (stellar). Paramylon bodies are ovoid to short rod-shaped, up to 5 μm long, and are mostly arranged around the central portion of the grouped chloroplasts, and other are scattered.
EcologyVarious species of the genus Euglena are found in freshwater eutrophic (rich in mineral and organic nutrients) pools and ditches. Thriving proliferating organisms may render the water green and soup-like appearance. Euglenoids are sometimes the dominant forms on the surface of thick bottom deposits of ponds, especially if the organic content is high.
Optimum growth (E. gracilis) appears in the range of 25°-30°C with doubling time being about 10 hours.
Nutrition [under revision]Euglena is a mixotroph (or facultative photoautotroph): it is a photoautotrophic organism because it can utilize carbon dioxide as a carbon source in the presence of light through photosynthesis in chloroplasts and ammonium and nitrate as nitrogen sources; and it is also a heterotrophic organism because it can utilize dissolved organic compounds as carbon source.
Many species of Euglena are obligatory photomesotrophs, i.e. organisms that apparently cannot grow in inorganic media and require certain amino acids as a source of nitrogen.
Growth of most euglenoids is accelerated by certain lower organic acids (for example, acetate) and in some species by lower alcohols.
E. gracilis is well known for its high capacity for physiological adaptation to diverse culture conditions. Organisms grown in the dark lose their chlorophyll, and their chloroplasts regress to form proplastids. Upon exposure to light in inorganic medium, they are able to re-differentiate chloroplasts.
Reports of ingestion of particulate food (holozoic nutrition) by Euglena are not confirmed.
Movements of euglenoidsTypes: (1) swimming; (2) contraction; (3) crawling; (4) gliding. Contraction is also known as euglenoid movement or metaboly, results from cells becoming spherical from an end-to-end contraction followed by quick expansion or otherwise actively contorting themselves in a variety of shapes through various peristaltic-like motions. Metaboly depends on elasticity of pellicle.
Culturing EuglenaEuglena requires culture medium rich in organic nutrients including vitamins B1 and B12 (auxotrophic species).
See Culturing Euglena (.pdf) from FLINN scientific, inc.
Cell size varies widely within genus. Length ranges are 34-78 and width 5-24 μm.
- Pellicle also called periplast; complex cell region comprised of the plasma membrane, supportive proteinaceous strips, subtending microtubules, and tubular cisternae of endoplasmic reticulum that subtends the plasma membrane and runs along the length of the cell; the strips are associated with cell plasticity (metaboly); Euglena spp. have many strips (more than 14) that are arranged helically; these helical pellicles commonly permit the cell to distort its shape via the sliding strip model and produce wriggling movements termed euglenoid movement
- Pyrenoid center of carbon dioxide fixation within the chloroplast 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).
- Mucocytes also called muciferous bodies, pellicle pores; uniform in size, spherical or spindle-shaped, small subpellicular bodies, with openings located between periplast folds, which excrete mucus outside the cell
- Stigma also called eyespot; a prominent, red body near the flagellar basis at the anterior cell pole; composed of 30-50 carotenoid-containing granules embedded in a colorless matrix; plays role in phototaxis (locomotory movement, that occurs when an organism moves in response to the light)
- Paraflagellar body a swelling at the union of flagella (emergent and rudimentary); together with stigma it plays role in light sensitivity; it is made up of a paracrystalline material containing flavins, pterins, and rhodopsin; it is believed that paraflagellar body may be actually responsible for light detection and that stigma serves as a shield, which depending on the orientation, may prevent light from reaching the swelling
- Reservoir also called flagellar pocket or pouch; a deep and usually conspicuous, permanently inflated invagination at the anterior end of a cell; the old term gullet is mostly avoided now because the pocket has not been found to be involved in the direct uptake of macroparticulate food, although pinocytotic (plasma membrane) uptake of large protein molecules within the reservoir has been demonstrated in Euglena
- Contracile vacuole a single vacuole, which may be conspicuous at the time it is near its maximal expansion and about to contract; through periodic contraction, the contractile vacuole eliminates osmotically collected water and then is reformed by a progressive coalescence of smaller vacuoles; the expansion-contraction cycle occurs once every 20-30 sec depending mostly on the temperature; a contractile vacuole is always in a fixed area near the stigma and empties into reservoir
- Flagellum locomotory structure; in Euglena the emergent flagellum has two helical arrangements of hairs on its surface: tufts of three to four fine hairs, 2-3 μm long, form one continuous row; and tufts of finer, more numerous and shorter hairs form another continuous row next to it; the total number of these hairs is estimated at 30,000 per flagellum
- Paramylon also called paramylum; the typical carbohydrate of the euglenoids and apparently is not found in other orders of flagellates or algae; paramylon bodies assume a variety of shapes (flat disks, concavo-convex disks, rods, rings, etc.); the shape and size may undergo considerable variation with the state of nutrition of the cell, but the larger characteristically shaped bodies are usually persistent and are used for the differentiation of species; a term was first suggested by Gottleib (1851) because of the similarity in chemical composition to amylon (starch)
- Mitochondrion a membrane-enclosed organelle found in most eukaryotic cells; mitochondria have a double membrane; the outer membrane is smooth while the inner membrane is convoluted, forming folds called cristae where food (sugar) is combined with oxygen to produce ATP - the primary energy source for the cell; small spherical or bacilliform granules identified as mitochondria are usually scattered in Euglena's cytoplasm
a membrane-enclosed organelle found in eukaryotic cells; it contains most of the cell's genetic material, organized as
multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes;
the nucleus of the euglenoids contains centrally located endosome (or nucleolus)
- Nucleolus also called endosome; intranuclear, RNA-containing organelle where ribosomal RNA (rRNA) is transcribed and assembled; the nucleolus divides during mitosis but does not contribute toward the formation of the chromosomes
High resolution posters of this image for a classroom or office are available at GeoChemBio shop
There are no confirmed reports of sexual reproduction (production of haploid gametes and their union) in Euglena.
The following methods of asexual reproduction are reported in euglenoids:
- Reproduction is usually by longitudinal binary fission of the flagellated stage.
- Division of non-flagellated cells contained within reproductive or division cysts. In E. gracilis and E. viridis these cysts may contain up to 32 or even 64 cells.
- Division of cells in pallmeloid stage (non-motile cells covered with mucilage). Documented for E. viridis, E. stellata, E. schmitzii, E. pisciformis and others.
- Free-swimming flagellated form
- Palmella non-motile cells of Euglena that become covered with mucilage and undergo repeated division in this condition are said to be in palmelloid stage; palmelloid cells move in a creeping manner before becoming flagellated again
- Protective Thick-walled closed cysts.
- Reproductive Thin walled; cell division occurs in them.
- Temporary Also, transitory or resting; wall is thick but not completely closed, cell usually flagellated and free to move about in cavity; usually formed in response to strong sunlight
- Passarelli et al. Euglena gracilis photoreception interpreted by microspectroscopy. European Journal of Protistology, Volume 39, Number 4, December 2003 , pp. 404-409(6).
- B.W. Wilson BW, Levedahl BH. Synthetic and division rates of Euglena gracilis grown in batch cultures ☆Synthetic and division rates of Euglena gracilis grown in batch cultures. Experimental Cell Research. Volume 35, Issue 1, June 1964, Pages 69–76.
- Tannrenther GW. Nutrition and reproduction in Euglena Zoological Laboratory, University of Missouri, 1922
- Ratcliffe HL. Mitosis and cell division in Euglena spyrogyra EHRENBERG Department of Medical Zoology, John Hopkins University, 1927
- James TW, Crescitelli F, Loew ER, McFarland WN. The eyespot of Euglena gracilis: a microspectrophotometric study. Vision Res. 1992 Sep;32(9):1583-91.
- Leander BS, Esson HJ, Breglia SA. Macroevolution of complex cytoskeletal systems in euglenids. Bioessays. 2007 Oct;29(10):987-1000. (free article, very innformative)
- Jahn TL. The euglenoid flagellates. The Quarterly Review of Biology © 1946 The University of Chicago Press
- Regnault A, Piton F, Calvayrac R. Growth, proteins and chlorophyll in Euglena adapted to various C/N balances. Phytochemistry. Volume 29, Issue 12, 1990, Pages 3711–3715
- Kosmala S. Phylogeny and systematics of Euglena (Euglenaceae) species with axial, stellate chloroplasts based on morphological and molecular data—new taxa, emended diagnoses, and epitypifications. Journal of Phycology. Volume 45, Issue 2, pages 464–481, April 2009
- Shin W and Triemer R. Phylogenetic analysis of the genus Euglena (Euglenophyceae) with particular reference to the type species Euglena viridis. J. Phycol. 40, 759–771 (2004) (.pdf)
- Ellen M. Euglena Viridis (Ehrenberg.) American Midland Naturalist. Vol. 8, No. 6, Nov., 1922
- Buetow DE. Euglena Citable reviews in the life sciences (2011)
- From Freshwater Algae of North America: Ecology and Classification. By John D. Wehr, Robert G. Sheath 2003 - Nature