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Protozoa (also protozoan, plural protozoans) is an informal term for single-celled organisms, either free-living or parasitic, which feed on organic matter such as other or organic tissues and debris.

(2014). 9780816067367, Infobase Publishing. .
(2015). 9789401791182, Springer. .
Historically, the protozoa were regarded as "one-celled animals," because they often possess -like behaviors, such as and , and lack a cell wall, as found in plants and many .
(2018). 9780321649638, Benjamin Cummings. .
Although the traditional practice of grouping of protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by .

In some systems of biological classification, Protozoa is a high-level . When first introduced in 1818, Protozoa was erected as a taxonomic class , but in later classification schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom. The seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: , , , , , , and . Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, , , and the parasitic , all of which are classified under Kingdom . Kingdom Protozoa, as defined in this scheme, does not form a natural group or , but a group or evolutionary grade, within which the members of Fungi, Animalia and Chromista are thought to have evolved.

The use of Protozoa as a formal has been discouraged by some recent researchers, mainly because the term, which is formed from the πρῶτος prōtos "first" and ζῶον ζῶα zōa, plural of ζῶον zōon, "animal", implies kinship with animals (metazoa) and promotes an arbitrary separation of "animal-like" from "plant-like" organisms. Modern ultrastructural, biochemical, and genetic techniques have shown that protozoa, as traditionally defined, belong to widely divergent lineages, and can no longer be regarded as "primitive animals". Genetic techniques have conclusively shown that protozoa, as traditionally defined, belong to widely divergent lineages distributed across the eukaryotic tree of life, and can no longer be regarded as "primitive animals". For this reason, the terms "protists", "Protista" or "Protoctista" are sometimes preferred for the high-level classification of . In 2005, members of the Society of voted to change the name of that organization to the International Society of .

History and terminology
The word "protozoa" (singular protozoon or protozoan) was coined in 1818 by zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals" ( ‘proto-’ + ‘animal’). Goldfuss created Protozoa as a class containing what he believed to be the simplest animals. Originally, the group included not only single-celled but also some "lower" multicellular animals, such as , , , , and .

In 1848, as a result of advancements in pioneered by and Matthias Schleiden, the anatomist and zoologist C.T. von Siebold proposed that the bodies of protozoans such as and were made up 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 (animals). At the same time, he raised the group to the level of a containing two broad classes of microorganisms: (mostly ciliates and flagellated ), and Rhizopoda (). The definition of Protozoa as a phylum or sub-kingdom made up of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"—and the term came into wide use. As a phylum under Animalia, the Protozoa were firmly rooted in the old "two-kingdom" classification 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 and have for , but can also feed on organic matter and are . 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 (protophyta), which he combined together 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.

Six years later, also proposed a third kingdom of life, which he named . 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.

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, however, 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 , Robert H. Whittaker and , 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 Kingdoms Protista and Protoctista became well established in biology texts and curricula.

(1974). 9781461569466, Springer US. .

While many taxonomists have abandoned Protozoa as a high-level group, Thomas Cavalier-Smith has retained it as a kingdom in the various classifications he has proposed. As of 2015, Cavalier-Smith's Protozoa excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, and (all members of the ). In its current form, his kingdom Protozoa is a group which includes a common ancestor and most of its descendents, but excludes two important clades that branch within it: the animals and fungi.


Protozoa, as traditionally defined, range in size from as little as 1 to several , or more.
(2018). 9780471941507, Wiley. .
Among the largest are the deep-sea–dwelling , single-celled foraminifera whose shells can reach 20 cm in diameter.
Plasmodium falciparum ( parasite, trophozoite phase)1-2
(free-living amoeboid)2.3–3
(free living flagellate)5-8
Plasmodium falciparum (malaria parasite, gametocyte phase)7-14
Trypanosoma cruzi (parasitic kinetoplastid, )14-24
Entamoeba histolytica (parasitic )15–60
(parasitic ciliate)50-100
Paramecium caudatum (free-living ciliate)120-330
(free-living amoebozoan)220–760
Noctiluca scintillans (free-living )700–2000
Syringammina fragilissima ( amoeboid)up to

Free-living protozoans 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
(2018). 9780762730933, Falcon. .
and hypersaline lakes and lagoons. All protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by forming which enable them to remain dormant until conditions improve.

and protozoa live on or within other organisms, including and , as well as plants and other single-celled organisms. Some are harmless or beneficial to their host organisms; others may be significant causes of diseases, such as , and .

Association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoans such as and inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex into smaller, more easily-digested molecules. A wide range of protozoans live in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as , and ciliated protozoa, such as Isotricha and .
(1997). 9789401071499, Springer, Dordrecht. .
The ciliate subclass Astomatia is made up entirely of mouthless symbionts adapted for life in the guts of annelid worms.

By definition, all protozoans are , deriving nutrients from other organisms, either by ingesting them whole or consuming their organic remains and waste-products. Some protozoans take in food by , engulfing organic particles with (as do), or taking in food through a specialized mouth-like aperture called a . Some protozoans take in food by , absorbing dissolved nutrients through their .

Parasitic protozoans 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 feeds by during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding (cytostome) as it matures within a host's red blood cell.

Protozoa may also live as , supplementing a heterotrophic diet with some form of . Some protozoa form close associations with symbiotic photosynthetic algae, which live and grow within the membranes of the larger cell and provide nutrients to the host. Others practice , 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. These, in turn, may be passed along to dinoflagellates of the genus , which prey on Mesodinium rubrum but keep the enslaved plastids for themselves. Within Dinophysis, these plastids can continue to function for months.

Organisms traditionally classified as protozoa are abundant in environments and , occupying a range of . The group includes (which move with the help of whip-like structures called ), (which move by using hair-like structures called ) and (which move by the use of foot-like structures called ). Some protozoa are sessile, and do not move at all.

Unlike plants, fungi and most types of algae, protozoans do not typically have a rigid , but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoans, such as the ciliates and , the cell is supported by a composite membranous envelope called the "pellicle." The pellicle gives some shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. In and , the pellicle is supported by closely packed vesicles called alveoli. In , it is formed from strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the and the ciliate . In some protozoa, the pellicle hosts bacteria that adhere to the surface by their fimbriae (attachment pili). Protozoa in biological research

Life cycle
Some protozoa have two-phase life cycles, alternating between proliferative stages (e.g., ) and dormant . As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for periods of time. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of (Greek tropho = to nourish), they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation.

All protozoans reproduce asexually by or multiple fission. Many protozoan species exchange genetic material by sexual means (typically, through conjugation); however, sexuality is generally decoupled from the process of reproduction, and does not immediately result in increased population.

Although meiotic sex is widespread among present day , it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Due to recent advances in detection and other techniques, evidence has been found for some form of meiotic sex in an increasing number of protozoans of ancient lineage that diverged early in eukaryotic evolution.Bernstein H, Bernstein C (2013). Bernstein C, Bernstein H eds. Evolutionary Origin and Adaptive Function of Meiosis'. Meiosis. InTech. (See eukaryote reproduction.) Thus, such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles , , , Plasmodium falciparum biology, , Toxoplasma gondii, Trichomonas vaginalis and Trypanosoma brucei.

Historically, the Protozoa were classified as "unicellular animals", as distinct from the Protophyta, single-celled photosynthetic organisms (algae) which were considered primitive plants. Both groups were commonly given the rank of phylum, under the kingdom Protista. In older systems of classification, the phylum Protozoa was commonly divided into several sub-groups, reflecting the means of locomotion: Classification schemes differed, but throughout much of the 20th century the major groups of Protozoa included:

With the emergence of molecular phylogenetics and tools enabling researchers to directly compare the DNA of different organisms, it became evident that, of the main sub-groups of Protozoa, only the ciliates (Ciliophora) formed a natural group, or (that is, a distinct lineage of organisms sharing common ancestry). The other classes or subphyla of Protozoa were all groups made up of organisms that, despite similarities of appearance or way of life, were not necessarily closely related to one another. In the system of classification currently endorsed by the International Society of Protistologists, members of the old phylum Protozoa have been distributed among a variety of supergroups.

Ecological role
As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive is important. As predators, they prey upon or filamentous algae, , and . Protozoan species include both and in the decomposer link of the . They also control bacteria populations and to some extent.


In humans
A number of protozoan are , causing diseases such as (by ), , , , cryptosporidiosis, , , , African trypanosomiasis (sleeping sickness), amoebic dysentery, acanthamoeba keratitis, and primary amoebic meningoencephalitis (naegleriasis).

In other animals
The protozoan Ophryocystis elektroscirrha is a parasite of , passed from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to , and have shortened lifespans, but parasite levels vary in populations. Infection creates a effect, whereby infected migrating animals are less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration. This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.

  • Dogiel, V. A., revised by J.I. Poljanskij and E. M. Chejsin. General Protozoology, 2nd ed., Oxford University Press, 1965.
  • Hausmann, K., N. Hulsmann. Protozoology. Thieme Verlag; New York, 1996.
  • Kudo, R.R. . Springfield, Illinois: C.C. Thomas, 1954; 4th ed.
  • Manwell, R.D. Introduction to Protozoology, second revised edition, Dover Publications Inc., New York, 1968.
  • Roger Anderson, O. Comparative protozoology: ecology, physiology, life history. Berlin etc.: Springer-Verlag, 1988.
  • Sleigh, M. The Biology of Protozoa. E. Arnold: London, 1981.
  • Jahn,T.L.- Bovee, E.C. & Jahn, F.F. How to Know the Protozoa. Wm. C. Brown Publishers, Div. of McGraw Hill, Dubuque, Iowa, 1979; 2nd ed.
  • Lee, J.J., Leedale, G.F. & Bradbury, P. An Illustrated Guide to the Protozoa. Lawrence, Kansas, U.S.A: Society of Protozoologists, 2000; 2nd ed.
  • Patterson, D.J. Free-Living Freshwater Protozoa. A Colour Guide. Manson Publishing; London, 1996.
  • Patterson, D.J., M.A. Burford. A Guide to the Protozoa of Marine Aquaculture Ponds. CSIRO Publishing, 2001.
  • Harrison, F.W., Corliss, J.O. (ed.). 1991. Microscopic Anatomy of Invertebrates, vol. 1, Protozoa. New York: Wiley-Liss, 512 pp.
  • Pitelka, D. R. 1963. Electron-Microscopic Structure of Protozoa. Pergamon Press, Oxford.
Physiology and biochemistry
  • Nisbet, B. 1984. Nutrition and feeding strategies in Protozoa. Croom Helm Publ., London, 280 pp.
  • Coombs, G.H. & North, M. 1991. Biochemical protozoology. Taylor & Francis, London, Washington.
  • Laybourn-Parry J. 1984. A Functional Biology of Free-Living Protozoa. Berkeley, California: University of California Press.
  • Levandowski, M., S.H. Hutner (eds). 1979. Biochemistry and physiology of protozoa. Volumes 1, 2, and 3. Academic Press: New York, NY; 2nd ed.
  • Sukhareva-Buell, N.N. 2003. Biologically active substances of protozoa. Dordrecht: Kluwer.
  • Capriulo, G.M. (ed.). 1990. Ecology of Marine Protozoa. Oxford Univ. Press, New York.
  • Darbyshire, J.F. (ed.). 1994. Soil Protozoa. CAB International: Wallingford, U.K. 2009 pp.
  • Laybourn-Parry, J. 1992. Protozoan plankton ecology. Chapman & Hall, New York. 213 pp.
  • Fenchel, T. 1987. Ecology of protozoan: The biology of free-living phagotrophic protists. Springer-Verlag, Berlin. 197 pp.
  • Kreier, J.P. (ed.). 1991-1995. Parasitic Protozoa, 2nd ed. 10 vols (1-3 coedited by Baker, J.R.). Academic Press, San Diego, California, [3].
Lee, J. J., & Soldo, A. T. (1992). Protocols in protozoology. Kansas, USA: Society of Protozoologists, Lawrence, [4].

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