Amoeba

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This article is about the cellular body type. For the genus, see Amoeba (genus). For other uses, see Amoeba (disambiguation).

An amoeba (also ameba, amœba or amoeboid) is a type of cell or organism which has the ability to alter its shape, primarily by extending and retracting pseudopods.[1] Amoebae do not form a single taxonomic group, but are found in every major lineage of eukaryotic organisms (domain Eukaryota). Amoeoboid cells occur not only among the protozoa, but also fungi, algae and animals.[2][3][4]

Among microbiologists, the terms "amoeboid" and "amoebae" are often used interchangeably for any organism that exhibits amoeboid movement.[5][6]

The best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which are widely cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" Dictyostelium discoideum.

Shape, movement and nutrition[edit]

Amoebae move and eat by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell.[7] Groups of amoebae are distinguished by the appearance and internal structure of their pseudopods. Amoebozoan species typically have bulbous pseudopods, rounded at the ends and roughly tubular in cross-section (lobose). Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminiferan emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolaria and the amoeboids loosely called Heliozoa, have stiff, needle-like, radiating actinopods supported from within by bundles of microtubules.[2][8]

Shell of the testate amoeba Difflugia acuminata, made up of mineral particles.

Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (lacking any hard covering). Testate amoebae shells are composed various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.[9]

To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.[10] This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. Marine amoebae do not usually possess a contractile vacuole, because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.[11]

The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other protists. Some are detritivores and eat dead organic material. Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, and there is no fixed place on the cell at which phagocytosis normally occurs.[12] Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.[13]

Amoebae in multicellular organisms: animals and slime molds[edit]

Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.[14]

Amoeboid stages also occur in the multicellular fungus-like protists, the so-called slime molds. Both the plasmodial slime molds, currently classified in the class Myxogastria, and the cellular slime molds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage. The cells of the former form a giant multinucleate amoeboid organism,[15] while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating slug which functions as a single organism.[5]

Classification[edit]

Amoebae are no longer classified together in a single group, since any taxon that included all of the traditional amoebae would be a polyphyletic group, containing many species that are not closely related.

In older classification systems, most amoebae were placed in the class or subphylum Sarcodina. Within the Sarcodina, amoebae were divided into morphological categories, on the basis of the form and structure of their pseudopods. Amoebae with pseudopods supported by regular arrays of microtubules were called actinopods, whereas those with unsupported pseudopods were classified as rhizopods. The Rhizopoda were further subdivided into lobose, filose, and reticulose amoebae, according to the morphology of their pseudopods.

When molecular (genetic) analyses confirmed that Sarcodina was a polyphyletic group, the old scheme was abandoned and the amoebae were dispersed among many other groups. The majority of traditional "Sarcodines" are now placed in two eukaryote supergroups (unranked taxa above the Kingdom level): Amoebozoa and Rhizaria. The rest have been distributed among the excavates, opisthokonts, and stramenopiles. Some, like the Centrohelida, have yet to be placed in any supergroup.[16]

Whereas older classifications were based mainly on morphology, modern schemes are based upon cladistics. Phylogenetic analyses place amoeboid genera into the following groups:

Grouping Genera Morphology
Amoebozoa
  • Lobose pseudopods (Lobosea): Lobose pseudopods are blunt, and there may be one or several on a cell, which is usually divided into a layer of clear ectoplasm surrounding more granular endoplasm.
Rhizaria
  • Filose pseudopods (Filosa): Filose pseudopods are narrow and tapering. The vast majority of filose amoebae, including all those that produce shells, are placed within the Cercozoa together with various flagellates that tend to have amoeboid forms. The naked filose amoebae also includes vampyrellids.
  • Reticulose pseudopods (Endomyxa): Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Foraminifera, a large group of marine protists that generally produce multi-chambered shells. There are only a few sorts of naked reticulose amebas, notably the gymnophryids, and their relationships are not certain.
  • Radiolarians are a subgroup of actinopods that are now grouped with rhizarians.
Excavata
Chromalveolate Heterokont: Hyalodiscus, Labyrinthula
Alveolata: Pfiesteria
Nucleariid Micronuclearia, Nuclearia
Ungrouped/
unknown
Adelphamoeba, Astramoeba, Cashia, Dinamoeba, Flagellipodium, Flamella, Gibbodiscus, Gocevia, Hollandella, Iodamoeba, Malamoeba, Nollandia, Oscillosignum, Paragocevia, Parvamoeba, Pernina, Pontifex, Protonaegleria, Pseudomastigamoeba, Rugipes, Striamoeba, Striolatus, Subulamoeba, Theratromyxa, Trienamoeba, Trimastigamoeba, Vampyrellium

Pathogenic interactions with other organisms[edit]

Some amoebae can infect other organisms pathogenically, causing disease:

References[edit]

  1. ^ SIngleton, Paul (2006). Dictionary of Microbiology and Molecular Biology, 3rd Edition, revised. Chichester, UK: John Wiley & Sons. p. 32. ISBN 978-0-470-03545-0. 
  2. ^ a b http://tolweb.org/notes/?note_id=51
  3. ^ http://www.bms.ed.ac.uk/research/others/smaciver/amoebae.htm
  4. ^ http://www.microscopy-uk.org.uk/mag/wimsmall/sundr.html
  5. ^ a b Marée, Athanasius FM, and Paulien Hogeweg. "How amoeboids self-organize into a fruiting body: multicellular coordination in Dictyostelium discoideum." Proceedings of the National Academy of Sciences 98.7 (2001): 3879-3883.
  6. ^ Mackerras, M. J., and Q. N. Ercole. "Observations on the action of paludrine on malarial parasites." Transactions of the Royal Society of Tropical Medicine and Hygiene 41.3 (1947): 365-376.
  7. ^ Alberts et al. Eds. (2007). Molecular Biology of the Cell 5th Edition. New York: Garland Science. p. 1037. ISBN 9780815341055. 
  8. ^ Margulis, Lynn (2009). Kingdoms and Domains. Academic Press. pp. 206–7. ISBN 978-0-12-373621-5. 
  9. ^ Ogden, C. G. (1980). An Atlas of Freshwater Testate Amoeba. Oxford, London, Glasgow: Oxford University Press, for British Museum (Natural History). pp. 1–5. ISBN 0198585020. 
  10. ^ Alberts et al. Eds. (2007). Molecular Biology of the Cell 5th Edition. New York: Garland Science. p. 663. ISBN 9780815341055. 
  11. ^ Kudo, Richard Roksabro. "Protozoology." Protozoology 4th Edit (1954). p. 83
  12. ^ Thorp, James H. (2001). Ecology and Classification of North American Freshwater invertebrates. San Diego: Academic. p. 71. ISBN 0-12-690647-5.
  13. ^ Jeon, Kwang W. (1973). Biology of Amoeba. New York: Academic Press. p. 100. 
  14. ^ Friedl, Peter, Stefan Borgmann, and Eva-B. Bröcker. "Amoeboid leukocyte crawling through extracellular matrix: lessons from the Dictyostelium paradigm of cell movement." Journal of leukocyte biology 70.4 (2001): 491-509.
  15. ^ Nakagaki et al. (2000). "Intelligence: Maze-solving by an amoeboid organism". Nature 407 (6803): 470. doi:10.1038/35035159. PMID 11028990. Retrieved Sep 14, 2014. 
  16. ^ Jan Pawlowski: The twilight of Sarcodina: a molecular perspective on the polyphyletic origin of amoeboid protists. Protistology, Band 5, 2008, S. 281–302. (pdf, 570 kB)
  17. ^ a b Park, J. S.; Simpson, A. G. B.; Brown, S.; Cho, B. C. (2009). "Ultrastructure and Molecular Phylogeny of two Heterolobosean Amoebae, Euplaesiobystra hypersalinica gen. Et sp. Nov. And Tulamoeba peronaphora gen. Et sp. Nov., Isolated from an Extremely Hypersaline Habitat". Protist 160 (2): 265–283. doi:10.1016/j.protis.2008.10.002. PMID 19121603.  edit

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