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Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled , either free-living or Parasitism, that feed on organic matter such as other or organic debris. Historically, protozoans were regarded as "one-celled animals".
When first introduced by Georg Goldfuss, in 1818, the taxon Protozoa was erected as a class within the Animalia, with the word 'protozoa' meaning "first animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.
This classification remained widespread in the 19th and early 20th century,Copeland, HF (1956). The Classification of Lower Organisms. Palo Alto, CA: Pacific Books. and even became elevated to a variety of higher ranks, including phylum, subkingdom, kingdom, and then sometimes included within the paraphyletic Protist or Protista.
By the 1970s, it became usual to require that all taxa be Monophyly (derived from a common ancestor that would also be regarded as protozoan), and Monophyly (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, , or fungus) that feed by .Robert G. Yaeger (1996). 9780963117212, University of Texas Medical Branch at Galveston. . ISBN 9780963117212 Traditional textbook examples of protozoa are Amoeba, Paramecium, Euglena and Trypanosoma.
In 1848, with better microscopes and Theodor Schwann and Matthias Schleiden's cell theory, the zoologist C. T. von Siebold proposed that the bodies of protozoa such as and consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all Animal (animals). 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.) At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria (mostly ) and (flagellated protists and ). The definition of Protozoa as a phylum or subkingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology".
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.
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 algae Euglena and Dinobryon have for photosynthesis, like plants, but can also feed on organic matter and are Motility, 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." 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.
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 of tissues.(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 essential 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.")
Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic 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 as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi. A variety of multi-kingdom systems were proposed, and the kingdoms Protista and Protoctista became established in biology texts and curricula.
By 1954, Protozoa were classified as "unicellular animals", as distinct from the "Protophyta", single-celled photosynthetic algae, which were considered primitive plants. 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.
Despite awareness that the traditional Protozoa was not a clade, 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 by Thomas Cavalier-Smith and collaborators since 1981, the taxon Protozoa was applied to certain groups of eukaryotes, and ranked as a kingdom. A scheme presented by Ruggiero et al. in 2015, placed eight not closely related phyla within kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. This approach excludes several major groups traditionally placed among the protozoa, such as the , , foraminifera, and the parasitic , which were moved to other groups such as Alveolata and Stramenopiles, under the polyphyletic Chromista. The Protozoa in this scheme were Paraphyly, because it excluded some descendants of Protozoa.
The continued use by some of the 'Protozoa' in its old senseEl-Bawab, F. 2020. Invertebrate Embryology and Reproduction, Chapter 3 – Phylum Protozoa. Academic Press, pp 68–102. 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. Others class any unicellular eukaryotic microorganism as protists, and make no reference to 'Protozoa'. In 2005, members of the Society of voted to change its name to the International Society of .
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.
Many parasitic Protozoa reproduce both asexually and sexually. However, sexual reproduction is rare among free-living protozoa and it usually occurs when food is scarce or the environment changes drastically. Both isogamy and anisogamy occur in Protozoa, anisogamy being the more common form of sexual reproduction.
| malaria parasite, trophozoite phase | 1–2 |
| free-living Cercozoa cercomonad amoebo-flagellate | 2.3–3 |
| free-living Kinetoplastida flagellate | 5–8 |
| malaria parasite, gametocyte phase | 7–14 |
| parasitic kinetoplastid, Chagas disease | 14–24 |
| parasitic | 15–60 |
| parasitic ciliate | 50–100 |
| free-living ciliate | 120–330 |
| free-living amoebozoan | 220–760 |
| free-living dinoflagellate | 700–2000 |
| foraminifera amoeba | up to |
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 parasite Plasmodium feeds by pinocytosis during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cell.
Protozoa may also live as , combining a heterotrophic diet with some form of . 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 practice kleptoplasty, stealing from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliate Mesodinium rubrum retains functioning 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 genus Dinophysis, which prey on Mesodinium rubrum but keep the enslaved plastids for themselves. Within Dinophysis, these plastids can continue to function for months.
Some protozoa live within loricasloose fitting but not fully intact enclosures. For example, many collar flagellates () have an organic lorica or a lorica made from silicous sectretions. Loricas are also common among some green euglenids, various ciliates (such as the , various testate amoebae and foraminifera. The surfaces of a variety of protozoa are covered with a layer of scales and or spicules. Examples include the amoeba Cochliopodium, many centrohelid heliozoa, . 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 in Acantharea, Pylocystinea, the 'Radiolaria', and Ebriida).
Additional experiments by Smith-Sonneborn, Holmes and Holmes, and Gilley and Blackburn showed that, during clonal aging, DNA damage increases dramatically. (1991). 9780120928606, Academic Press. ISBN 9780120928606 Thus, DNA damage in the macronucleus appears to be the principal cause of clonal aging in P. tetraurelia. In this single-celled protozoan, aging appears to proceed in a manner similar to that of multicellular (see DNA damage theory of aging).
Most species of free-living protozoa live in similar habitats in all parts of the world.Lee, W.J. & Patterson, D.J. 1999. "Are communities of heterotrophic flagellates determined by their geography?" In Ponder, W. and Lunney, D. The other 99%. The conservation and biodiversity of Invertebrates. Trans. R. Soc. New South Wales, Mosman, Sydney, pp 232–35Lee, W.J. & Patterson, D.J. 1998. "Diversity and geographic distribution of free-living heterotrophic flagellates=analysis by PRIMER. Protist, 149: 229–43
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