Amoeba
An amoeba (
Microbiologists often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement.[8][9]
In older classification systems, most amoebae were placed in the
The best known amoeboid
Shape, movement and nutrition
Amoeba do not have cell walls, which allows for free movement. Amoeba move and feed by using pseudopods, which are bulges of
Free-living amoebae may be "
To regulate
Diet
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.[18]
Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.[19]
Size range
The size of amoeboid cells and species is extremely variable. The marine amoeboid
Species or cell type | Size in micrometers |
---|---|
Massisteria voersi[20] | 2.3–3 |
Naegleria fowleri[23] | 8–15 |
Neutrophil (white blood cell)[24] | 12–15 |
Acanthamoeba[25] | 12–40 |
Entamoeba histolytica[26] | 15–60 |
Arcella vulgaris[27] | 30–152 |
Amoeba proteus[28] | 220–760 |
Chaos carolinense[29]
|
700–2000 |
Pelomyxa palustris[30] | up to 5000 |
Syringammina fragilissima[22]
|
up to 200000 |
Amoebae as specialized cells and life cycle stages
Some
Amoeboid stages also occur in the multicellular fungus-like protists, the so-called
Other organisms may also present amoeboid cells during certain life-cycle stages, e.g., the gametes of some green algae (
Amoebae as taxa
Early history and origins of Sarcodina
The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus" ("the Little Proteus").[38] Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus.[39] The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.[40]
In 1822, the genus Amiba (from the
In 1841, Félix Dujardin coined the term "sarcode" (from Greek σάρξ sarx, "flesh," and εἶδος eidos, "form") for the "thick, glutinous, homogeneous substance" which fills protozoan cell bodies.[44] Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells.[10] Thirty years later, the Austrian zoologist Ludwig Karl Schmarda used "sarcode" as the conceptual basis for his division Sarcodea, a phylum-level group made up of "unstable, changeable" organisms with bodies largely composed of "sarcode".[45] Later workers, including the influential taxonomist Otto Bütschli, amended this group to create the class Sarcodina,[46] a taxon that remained in wide use throughout most of the 20th century.
Within the traditional Sarcodina, amoebae were generally divided into
Dismantling of Sarcodina
In the final decade of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a
Classification
Recent classification places the various amoeboid genera in the following groups:
Supergroups | Major groups and genera | Morphology |
---|---|---|
Amoebozoa |
|
|
Rhizaria |
|
|
Excavata |
|
|
Heterokonta
|
|
|
Alveolata
|
| |
Opisthokonta |
|
|
Ungrouped/ unknown |
|
Some of the amoeboid groups cited (e.g., part of
Pathogenic interactions with other organisms
Some amoebae can infect other organisms pathogenically, causing disease:[52][53][54][55]
- Entamoeba histolytica is the cause of amoebiasis, or amoebic dysentery.
- Naegleria fowleri (the "brain-eating amoeba") is a fresh-water-native species that can be fatal to humans if introduced through the nose.
- Acanthamoeba can cause amoebic keratitis and encephalitis in humans.
- granulomatous amoebic meningoencephalitis.
Amoeba have been found to harvest and grow the bacteria implicated in plague.[56] Amoebae can likewise play host to microscopic organisms that are pathogenic to people and help in spreading such microbes. Bacterial pathogens (for example, Legionella) can oppose absorption of food when devoured by amoebae.[57] The currently generally utilized and best-explored amoebae that host other organisms are Acanthamoeba castellanii and Dictyostelium discoideum.[58] Microorganisms that can overcome the defenses of one-celled organisms can shelter and multiply inside them, where they are shielded from unfriendly outside conditions by their hosts.
Meiosis
Recent evidence indicates that several Amoebozoa lineages undergo meiosis.
Orthologs of genes employed in meiosis of sexual eukaryotes have recently been identified in the Acanthamoeba genome. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh and Mlh.[59] This finding suggests that the ‘'Acanthamoeba'’ are capable of some form of meiosis and may be able to undergo sexual reproduction.
The meiosis-specific
Studies of
Dictyostelium discoideum in the supergroup Amoebozoa can undergo mating and sexual reproduction including meiosis when food is scarce.[62][63]
Since the Amoebozoa diverged early from the
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Further reading
- Walochnik, J. & Aspöck, H. (2007). Amöben: Paradebeispiele für Probleme der Phylogenetik, Klassifikation und Nomenklatur. Denisia 20: 323–350. (In German)
- Amoebae: Protists Which Move and Feed Using Pseudopodia at the Tree of Life web project
- Pawlowski, J. & Burki, F. (2009). Untangling the Phylogeny of Amoeboid Protists. Journal of Eukaryotic Microbiology 56.1: 16–25.
External links
- Siemensma, F. Microworld: world of amoeboid organisms.
- Völcker, E. & Clauß, S. Visual key to amoeboid morphotypes. Penard Labs.
- The Amoebae website of Maciver Lab of the University of Edinburgh, brings together information from published sources.
- Molecular Expressions Digital Video Gallery: Pond Life – Amoeba (Protozoa) – informative amoeba videos