When first introduced byGeorg Goldfuss, in 1818, the taxon Protozoa was erected as aclass within the Animalia,[3] with the word 'protozoa' meaning "first animals", because they often possessanimal-like behaviours, such asmotility andpredation, and lack acell wall, as found in plants and manyalgae.[4][5][6]
This classification remained widespread in the 19th and early 20th century,[7] and even became elevated to a variety of higher ranks, includingphylum,subkingdom,kingdom, and then sometimes included within the paraphyleticProtoctista orProtista.[8]
By the 1970s, it became usual to require that all taxa bemonophyletic (all members being derived from one common ancestor that is itself regarded as belonging in the taxon), andholophyletic (containing all of the known descendants of that common ancestor). The taxon 'Protozoa' fails to meet these standards, so grouping protozoa with animals, and treating them as closely related, became no longer justifiable.
The term continues to be used in a loose way to describe single-celled protists (that is, eukaryotes that are not animals,plants, orfungi) that feed byheterotrophy.[9] Traditional textbook examples of protozoa areAmoeba,Paramecium,Euglena andTrypanosoma.[10]
The word "protozoa"(singularprotozoon) was coined in 1818 byzoologistGeorg August Goldfuss (=Goldfuß), as the Greek equivalent of the GermanUrthiere, meaning "primitive, or original animals" (ur- 'proto-' +Thier 'animal').[11] Goldfuss created Protozoa as aclass containing what he believed to be the simplest animals.[3] Originally, the group included not only single-celledmicroorganisms but also some "lower"multicellular animals, such asrotifers,corals,sponges,jellyfish,bryozoans andpolychaete worms.[12] The termProtozoa is formed from theGreek wordsπρῶτος (prôtos), meaning "first", andζῷα (zôia), plural ofζῷον (zôion), meaning "animal".[13][14]
In 1848, with better microscopes andTheodor Schwann andMatthias Schleiden'scell theory, the zoologistC. T. von Siebold proposed that the bodies of protozoa such asciliates andamoebae consisted of single cells, similar to those from which themulticellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only suchunicellular forms, to the exclusion of allMetazoa (animals).[15] At the same time, he raised the group to the level of aphylum containing two broad classes of microorganisms:Infusoria (mostlyciliates) andflagellates (flagellated protists andamoebae). The definition of Protozoa as a phylum or subkingdom composed of "unicellular animals" was adopted by the zoologistOtto Bütschli—celebrated at his centenary as the "architect of protozoology".[16]
John Hogg's illustration of the Four Kingdoms of Nature, showing "Primigenal" as a greenish haze at the base of the Animals and Plants, 1860
As a phylum under Animalia, the Protozoa were firmly rooted in a simplistic "two-kingdom" concept of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany.[17]
Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algaeEuglena andDinobryon havechloroplasts forphotosynthesis, like plants, but can also feed on organic matter and aremotile, like animals. In 1860,John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms or living beings, are animals or plants."[18] As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae, which he combined under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the kingdom Primigenum.[18][19][20]
In 1866,Ernst Haeckel proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in later work, he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds oftissues.[21]
Frederick Chapman's The foraminifera: an introduction to the study of the protozoa (1902)
Despite these proposals, Protozoa emerged as the preferred taxonomic placement forheterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such asHerbert Copeland,Robert H. Whittaker andLynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi.[17] A variety ofmulti-kingdom systems were proposed, and the kingdoms Protista and Protoctista became established in biology texts and curricula.[22][23][24]
By 1954, Protozoa were classified as "unicellular animals", as distinct from the "Protophyta", single-celled photosynthetic algae, which were considered primitive plants.[25] In the system of classification published in 1964 by B.M. Honigsberg and colleagues, the phylum Protozoa was divided according to the means of locomotion, such as by cilia or flagella.[26]
Despite awareness that the traditional Protozoa was not aclade, a natural group with a common ancestor, some authors have continued to use the name, while applying it to differing scopes of organisms. In a series of classifications byThomas Cavalier-Smith and collaborators since 1981, the taxon Protozoa was applied to certain groups of eukaryotes, and ranked as a kingdom.[27][28][29] A scheme presented by Ruggiero et al. in 2015, placed eight not closely related phyla within kingdom Protozoa:Euglenozoa,Amoebozoa,Metamonada,Choanozoasensu Cavalier-Smith,Loukozoa,Percolozoa,Microsporidia andSulcozoa.[10] This approach excludes several major groups traditionally placed among the protozoa, such as theciliates,dinoflagellates,foraminifera, and the parasiticapicomplexans, which were moved to other groups such asAlveolata andStramenopiles, under the polyphyleticChromista. The Protozoa in this scheme wereparaphyletic, because it excluded some descendants of Protozoa.[10]
The continued use by some of the 'Protozoa' in its old sense[30] highlights the uncertainty as to what is meant by the word 'Protozoa', the need for disambiguating statements such as "in the sense intended by Goldfuß", and the problems that arise when new meanings are given to familiar taxonomic terms. Some authors classify Protozoa as a subgroup of mostly motile protists.[31] Others class any unicellular eukaryotic microorganism as protists, and make no reference to 'Protozoa'.[32] In 2005, members of the Society ofProtozoologists voted to change its name to the International Society ofProtistologists.[33]
In the system of eukaryote classification published by the International Society of Protistologists in 2012, members of the old phylum Protozoa have been distributed among a variety of supergroups.[34]
Protists are distributed across all major groups of eukaryotes, including those that contain multicellular algae, green plants, animals, and fungi. If photosynthetic and fungal protists are distinguished from protozoa, they appear as shown in the phylogenetic tree of eukaryotic groups.[35][36] TheMetamonada are hard to place, being sister possibly toDiscoba, possibly toMalawimonada.[37]
Many parasitic Protozoa reproduce both asexually andsexually.[38] However, sexual reproduction is rare among free-living protozoa and it usually occurs when food is scarce or the environment changes drastically.[40] Bothisogamy andanisogamy occur in Protozoa, anisogamy being the more common form of sexual reproduction.[41]
Protozoans, as traditionally defined, range in size from as little as 1micrometre to severalmillimetres, or more.[42] Among the largest are the deep-sea–dwellingxenophyophores, single-celled foraminifera whose shells can reach 20 cm in diameter.[43]
The ciliateSpirostomum ambiguum can attain 3 mm in length
Free-living protozoa are common and often abundant in fresh, brackish and salt water, as well as other moist environments, such as soils and mosses. Some species thrive in extreme environments such as hot springs[54] and hypersaline lakes and lagoons.[55] All protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by formingresting cysts that enable them to remain dormant until conditions improve.[56]
All protozoa areheterotrophic, deriving nutrients from other organisms, either by ingesting them whole byphagocytosis or taking up dissolved organic matter or micro-particles (osmotrophy).Phagocytosis may involve engulfing organic particles withpseudopodia (asamoebae do), taking in food through a specialized mouth-like aperture called acytostome, or using stiffened ingestion organelles[57]
Parasitic protozoa use a wide variety of feeding strategies, and some may change methods of feeding in different phases of their life cycle. For instance, the malaria parasitePlasmodium feeds bypinocytosis during its immaturetrophozoite stage of life (ring phase), but develops a dedicated feedingorganelle (cytostome) as it matures within a host's red blood cell.[58]
Protozoa may also live asmixotrophs, combining a heterotrophic diet with some form ofautotrophy. Some protozoa form close associations with symbiotic photosynthetic algae (zoochlorellae), which live and grow within the membranes of the larger cell and provide nutrients to the host. The algae are not digested, but reproduce and are distributed between division products. The organism may benefit at times by deriving some of its nutrients from the algal endosymbionts or by surviving anoxic conditions because of the oxygen produced by algal photosynthesis. Some protozoans practicekleptoplasty, stealingchloroplasts from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliateMesodinium rubrum retains functioningplastids from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. The symbionts may be passed along to dinoflagellates of the genusDinophysis, which prey onMesodinium rubrum but keep the enslaved plastids for themselves. WithinDinophysis, these plastids can continue to function for months.[59]
Organisms traditionally classified as protozoa are abundant inaqueous environments andsoil, occupying a range oftrophic levels. The group includesflagellates (which move with the help of undulating and beatingflagella).Ciliates (which move by using hair-like structures calledcilia) andamoebae (which move by the use of temporary extensions of cytoplasm calledpseudopodia). Many protozoa, such as the agents of amoebic meningitis, use both pseudopodia and flagella. Some protozoa attach to the substrate or form cysts, so they do not move around (sessile). Most sessile protozoa are able to move around at some stage in the life cycle, such as after cell division. The term 'theront' has been used for actively motile phases, as opposed to 'trophont' or 'trophozoite' that refers to feeding stages.[citation needed]
Unlike plants, fungi and most types of algae, most protozoa do not have a rigid externalcell wall but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoa, such as the ciliates andeuglenozoans, the outer membrane of the cell is supported by a cytoskeletal infrastructure, known as apellicle.[60] The pellicle gives shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. Inciliates andApicomplexa, the pellicle includes a layer of closely packed vesicles called alveoli. Ineuglenids, the pellicle is formed fromprotein strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are theeuglenoids and the ciliateParamecium. In some protozoa, the pellicle hostsepibiotic bacteria that adhere to the surface by theirfimbriae (attachment pili).
Some protozoa live withinloricas – loose fitting but not fully intact enclosures. For example, many collar flagellates (Choanoflagellates) have an organic lorica or a lorica made from silicous sectretions. Loricas are also common among some green euglenids, various ciliates (such as thefolliculinids, various testate amoebae andforaminifera. The surfaces of a variety of protozoa are covered with a layer of scales and or spicules. Examples include theamoebaCochliopodium, many centrohelidheliozoa,synurophytes. The layer is often assumed to have a protective role. In some, such as the actinophryid heliozoa, the scales only form when the organism encysts. The bodies of some protozoa are supported internally by rigid, often inorganic, elements (as inAcantharea,Pylocystinea,Phaeodarea – collectively the 'Radiolaria', andEbriida).
Protozoa mostly reproduce asexually bybinary fission or multiple fission. Many protozoa also exchange genetic material by sexual means (typically, throughconjugation), but this is generally decoupled from reproduction.[61]Meiotic sex is widespread amongeukaryotes, and must have originated early in their evolution, as it has been found in many protozoan lineages that diverged early in eukaryotic evolution.[62]
In the well-studied protozoan speciesParamecium tetraurelia, the asexual line undergoes clonal aging, loses vitality and expires after about 200 fissions if the cells fail to undergo autogamy or conjugation. The functional basis for clonal aging was clarified bytransplantation experiments of Aufderheide in 1986.[63] These experiments demonstrated that the macronucleus, and not the cytoplasm, is responsible for clonal aging.
Additional experiments by Smith-Sonneborn,[64] Holmes and Holmes,[65] and Gilley and Blackburn[66] showed that, during clonal aging,DNA damage increases dramatically.[67] Thus, DNA damage in the macronucleus appears to be the principal cause of clonal aging inP. tetraurelia. In this single-celled protozoan, aging appears to proceed in a manner similar to that of multicellulareukaryotes (seeDNA damage theory of aging).
Free-living protozoa are found in almost all ecosystems that contain free water, permanently or temporarily. They have a critical role in the mobilization of nutrients in ecosystems. Within themicrobial food web they include the most important bacterivores.[57] In part, they facilitate the transfer of bacterial and algal production to successivetrophic levels, but also they solubilize the nutrients within microbial biomass, allowing stimulation of microbial growth. As consumers, protozoa prey uponunicellular orfilamentous algae,bacteria,microfungi, and micro-carrion. In the context of older ecological models of themicro- andmeiofauna, protozoa may be a food source formicroinvertebrates.
Most species of free-living protozoa live in similar habitats in all parts of the world.[68][69][70]
A wide range of protozoa livecommensally in the rumens ofruminant animals, such as cattle and sheep. These include flagellates, such asTrichomonas, and ciliated protozoa, such asIsotricha andEntodinium.[72] The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms.[73]
Association between protozoan symbionts and their host organisms can bemutually beneficial. Flagellated protozoa such asTrichonympha andPyrsonympha inhabit the guts oftermites, where they enable their insect host to digest wood by helping to break down complexsugars into smaller, more easily digested molecules.[74]
^abGoldfuß (1818)."Ueber die Classification der Zoophyten" [On the Classification of Zoophytes].Isis, Oder, Encyclopädische Zeitung von Oken (in German).2 (6):1008–19. From p. 1008:"Erste Klasse. Urthiere. Protozoa." (First class. Primordial animals. Protozoa.) [Note: each column of each page of this journal is numbered; there are two columns per page.]
^Goldfuß, Georg August (1820).Handbuch der Zoologie [Handbook of Zoology. First Part.]. Handbuch der naturgeschichte ... Von dr. G. H. Schubert.3. Th. (in German). Vol. 1. Nürnberg: Johann Leonhard Schrag. pp. xi–xiv.
^Siebold (vol. 1); Stannius (vol. 2) (1848).Lehrbuch der vergleichenden Anatomie [Textbook of Comparative Anatomy] (in German). Vol. 1:Wirbellose Thiere (Invertebrate animals). Berlin: Veit & Co. p. 3.{{cite book}}: CS1 maint: numeric names: authors list (link) From p. 3:"Erste Hauptgruppe. Protozoa. Thiere, in welchen die verschiedenen Systeme der Organe nicht scharf ausgeschieden sind, und deren unregelmässige Form und einfache Organisation sich auf eine Zelle reduziren lassen." (First principal group. Protozoa. Animals, in which the different systems of organs are not sharply separated, and whose irregular form and simple organization can be reduced to one cell.)
^Dobell, C. (April 1951). "In memoriam Otto Bütschli (1848–1920) 'architect of protozoology'".Isis; an International Review Devoted to the History of Science and Its Cultural Influences.42 (127):20–22.doi:10.1086/349230.PMID14831973.S2CID32569053.
^Scamardella, J. M. (December 1999). "Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista".International Microbiology.2 (4):207–16.PMID10943416.
^(Haeckel, 1866), vol. 1, pp. 215 ff. From p. 215:"VII. Character des Protistenreiches." (VII. Character of the kingdom of Protists.) From p. 216:"VII. B. Morphologischer Character des Protistenreiches. Ba. Character der protistischen Individualitäten. Der wesentliche tectologische Character der Protisten liegt in der sehr unvollkommenen Ausbildung und Differenzirung der Individualität überhaupt, insbesondere aber derjenigen zweiter Ordnung, der Organe. Sehr viele Protisten erheben sich niemals über den morphologischen Werth von Individuen erster Ordnung oder Plastiden." (VII. B. Morphological character of the kingdom of protists. Ba. "Character of the protist Individualities. The essentialtectological character of protists lies in the very incomplete formation and differentiation of individuality generally, however particularly of those of the second order, the organs. Very many protists never rise above the morphological level of individuals of the first order or plastids.")
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^El-Bawab, F. 2020.Invertebrate Embryology and Reproduction, Chapter 3 – Phylum Protozoa. Academic Press, pp 68–102.doi:10.1016/B978-0-12-814114-4.00003-5
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^Holmes, George E.; Holmes, Norreen R. (July 1986). "Accumulation of DNA damages in agingParamecium tetraurelia".Molecular and General Genetics.204 (1):108–14.doi:10.1007/bf00330196.PMID3091993.S2CID11992591.
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