View source for Protozoa

{{Short description|Single-celled eukaryotic organisms}}
{{About|the organisms|associated infections|Protozoan infection}}
[[File:Protozoa collage 2.jpg|right|thumb|upright=1.5|Clockwise from top left: ''[[Blepharisma japonicum]]'', a [[ciliate]]; ''[[Giardia|Giardia muris]]'', a parasitic [[flagellate]]; ''[[Centropyxis aculeata]]'', a testate (shelled) [[amoeba]]; ''[[Peridinium willei]]'', a [[dinoflagellate]]; ''[[Chaos carolinense]]'', a naked [[amoebozoa]]n; ''[[Desmarella moniliformis]]'', a [[choanoflagellate]]]]

'''Protozoa''' ({{singular}}: '''protozoan''' or '''protozoon'''; alternative plural: '''protozoans''') are a [[polyphyletic]] group of [[Unicellular organism|single-celled]] [[eukaryote]]s, either free-living or [[Parasitism|parasitic]], that feed on organic matter such as other [[microorganism]]s or organic debris.<ref>{{Cite book |url=https://books.google.com/books?id=sYgKY6zz20YC&q=panno+the+cell&pg=PA130 |title=The Cell: Evolution of the First Organism |last=Panno |first=Joseph |year= 2014 |publisher=Infobase Publishing |isbn=9780816067367 |page=130}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=2zVqBgAAQBAJ&q=bertrand+microbiology&pg=PA9 |title=Environmental Microbiology: Fundamentals and Applications: Microbial Ecology |last1=Bertrand |first1=Jean-Claude |last2=Caumette |first2=Pierre |last3=Lebaron |first3=Philippe |last4=Matheron |first4=Robert |last5=Normand |first5=Philippe |last6=Sime-Ngando |first6=Télesphore |date=2015 |publisher=Springer |isbn=9789401791182 |page=9}}</ref> Historically, protozoans were regarded as "one-celled animals".

When first introduced by [[Georg August Goldfuss|Georg Goldfuss]], in 1818, the taxon Protozoa was erected as a [[Class (biology)|class]] within the Animalia,<ref name = "Goldfuß"/> 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]].<ref>{{Cite journal |last=Fenchel |first=Tom |date=1987 |title=Ecology of Protozoa |url=https://link.springer.com/book/10.1007/978-3-662-06817-5 |journal=Brock/Springer Series in Contemporary Bioscience |language=en |page=2 |doi=10.1007/978-3-662-06817-5 |isbn=978-3-662-06819-9 |s2cid=44988543 |issn=1432-0061|url-access=subscription }}</ref><ref>{{Cite book |last=Madigan |first=Michael T. |url=https://books.google.com/books?id=RawZTwEACAAJ&q=brock+biology+of+microorganisms+13th |title=Brock Biology of Microorganisms |date=2012 |publisher=Benjamin Cummings |isbn=9780321649638 |page=43}}</ref><ref>{{Cite book |last=Kudo |first=Richard R. (Richard Roksabro) |url=http://archive.org/details/protozoology1954kudo |title=Protozoology |date=1954 |publisher=Springfield, IL; C.C. Thomas |others=MBLWHOI Library |page=5}}</ref>

This classification remained widespread in the 19th and early 20th century,<ref>Copeland, HF (1956). ''The Classification of Lower Organisms''. Palo Alto, CA: Pacific Books.</ref> and even became elevated to a variety of higher ranks, including [[Phylum (biology)|phylum]], [[Subkingdom (biology)|subkingdom]], [[Kingdom (biology)|kingdom]], and then sometimes included within the paraphyletic '''[[Protist|Protoctista]]''' or '''Protista'''.<ref name=Scamardella1999>{{cite journal | last = Scamardella | first = J. M. | title = Not plants or animals: A brief history of the origin of Kingdoms Protozoa, Protista, and Protoctista | year = 1999 | journal = International Microbiology | volume = 2 | issue = 4 | pages = 207–21 | pmid = 10943416 | url = http://www.blc.arizona.edu/courses/schaffer/182h/EukaryoteOrigins/NotPlantsNotAnimals-Scamardella.pdf | access-date = 2020-07-08 | archive-date = 2021-08-25 | archive-url = https://web.archive.org/web/20210825134057/http://www.blc.arizona.edu/courses/schaffer/182h/EukaryoteOrigins/NotPlantsNotAnimals-Scamardella.pdf | url-status = dead }}</ref>

By the 1970s, it became usual to require that all taxa be [[Monophyly|monophyletic]] (derived from a common ancestor that would also be regarded as protozoan), and [[Monophyly|holophyletic]] (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, [[plant]]s, or [[fungus|fungi]]) that feed by [[heterotroph]]y.<ref name="Yaeger">{{Cite book |last=Yaeger |first=Robert G. |date=1996 |editor-last=Baron |editor-first=Samuel |title=Protozoa: Structure, Classification, Growth, and Development |url=http://www.ncbi.nlm.nih.gov/books/NBK8325/ |access-date=2020-07-07 |publisher=University of Texas Medical Branch at Galveston |pmid=21413323|isbn=9780963117212 }}</ref> Traditional textbook examples of protozoa are ''[[Amoeba (genus)|Amoeba]]'', ''[[Paramecium]]'', ''[[Euglena]]'' and ''[[Trypanosoma]]''.<ref name="Ruggiero-2015">{{Cite journal |last1=Ruggiero |first1=Michael A. |last2=Gordon |first2=Dennis P. |last3=Orrell |first3=Thomas M. |last4=Bailly |first4=Nicolas |last5=Bourgoin |first5=Thierry |last6=Brusca |first6=Richard C. |last7=Cavalier-Smith |first7=Thomas |author-link=Tom Cavalier-Smith |last8=Guiry |first8=Michael D. |last9=Kirk |first9=Paul M. |date=29 April 2015 |title=A Higher Level Classification of All Living Organisms |journal=PLOS ONE |volume=10 |issue=4 |page=e0119248 |bibcode=2015PLoSO..1019248R|doi=10.1371/journal.pone.0119248 |pmc=4418965 |pmid=25923521 |doi-access=free }}</ref>

== History of classification ==

[[File:Georg_August_Goldfuss_Protozoa_Infusoria_Monades.jpg|left|thumb|upright=1.35|Class Protozoa, order [[Infusoria]], family Monades by [[Georg August Goldfuss]], c. 1844]]

The word "protozoa" ''(singular ''protozoon'')'' was coined in 1818 by [[zoologist]] [[Georg August Goldfuss]] (=Goldfuß), as the Greek equivalent of the German ''{{lang|de|Urthiere}}'', meaning "primitive, or original animals" (''{{lang|de|{{linktext|ur-}}}}'' 'proto-' + ''{{lang|de|{{linktext|Thier}}}}'' 'animal').<ref>{{Cite journal |last=Rothschild |first=Lynn J. |author-link=Lynn J. Rothschild |date=1989 |title=Protozoa, Protista, Protoctista: What's in a Name? |journal=Journal of the History of Biology |volume=22 |issue=2 |pages=277–305 |doi=10.1007/BF00139515 |jstor=4331095 |pmid=11542176 |s2cid=32462158 |issn=0022-5010 |url=https://zenodo.org/record/1232387}}</ref> Goldfuss created Protozoa as a [[Class (biology)|class]] containing what he believed to be the simplest animals.<ref name = "Goldfuß">{{cite journal |last1=Goldfuß |title=Ueber die Classification der Zoophyten |language=de |trans-title=On the Classification of Zoophytes |journal=Isis, Oder, Encyclopädische Zeitung von Oken |date=1818 |volume=2 |issue=6 |pages=1008–19 |url=https://www.biodiversitylibrary.org/item/47614#page/530/mode/1up }} 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.]</ref> Originally, the group included not only single-celled [[microorganism]]s but also some "lower" [[Multicellular organism|multicellular]] animals, such as [[rotifer]]s, [[coral]]s, [[sponge]]s, [[jellyfish]], [[bryozoa]]ns and [[Polychaete|polychaete worms]].<ref>{{cite book |last1=Goldfuß |first1=Georg August |title=Handbuch der Zoologie |volume=1 |series=Handbuch der naturgeschichte ... Von dr. G. H. Schubert.3. Th. |language=de |trans-title=Handbook of Zoology. First Part. |date=1820 |publisher=Johann Leonhard Schrag |location=Nürnberg |pages=xi–xiv |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015070878148;view=1up;seq=11}}</ref> The term ''Protozoa'' is formed from the [[Ancient Greek|Greek]] words {{wikt-lang|grc|πρῶτος}} ({{grc-transl|πρῶτος}}), meaning "first", and {{wikt-lang|grc|ζῷα}} ({{grc-transl|ζῷα}}), plural of {{wikt-lang|grc|ζῷον}} ({{grc-transl|ζῷον}}), meaning "animal".<ref>{{Cite book|title=Abrégé du dictionnaire grec français|last=Bailly|first=Anatole|date=1981|publisher=Hachette|isbn=978-2010035289|location=Paris|oclc=461974285}}</ref><ref>{{Cite web|url=http://www.tabularium.be/bailly/|title=Greek-french dictionary online|last=Bailly|first=Anatole|website=www.tabularium.be|access-date=2018-10-05}}</ref>

In 1848, with better microscopes and [[Theodor Schwann]] and [[Matthias Jakob Schleiden|Matthias Schleiden]]'s [[cell theory]], the zoologist [[Karl Theodor Ernst von Siebold|C. T. von Siebold]] proposed that the bodies of protozoa such as [[ciliate]]s and [[amoeba]]e consisted of single cells, similar to those from which the [[Multicellular organism|multicellular]] tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such [[Unicellular organism|unicellular]] forms, to the exclusion of all [[Animal|Metazoa]] (animals).<ref>{{cite book |last1=Siebold (vol. 1) |last2=Stannius (vol. 2) |title=Lehrbuch der vergleichenden Anatomie |language=de |trans-title=Textbook of Comparative Anatomy |date=1848 |publisher=Veit & Co. |location=Berlin |volume= 1: ''Wirbellose Thiere'' (Invertebrate animals) |page=3 |url=https://www.biodiversitylibrary.org/item/41783#page/21/mode/1up}}  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.)</ref> At the same time, he raised the group to the level of a [[phylum]] containing two broad classes of microorganisms: [[Infusoria]] (mostly [[ciliate]]s) and [[flagellate]]s (flagellated protists and [[amoeba]]e). 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".<ref>{{Cite journal |last=Dobell |first=C. |date=April 1951 |title=In memoriam Otto Bütschli (1848–1920) 'architect of protozoology' |journal=Isis; an International Review Devoted to the History of Science and Its Cultural Influences |volume=42 |issue=127 |pages=20–22 |pmid=14831973 |doi=10.1086/349230 |s2cid=32569053}}</ref> 

[[File:John_Hogg_--_Primigenum_or_Protoctista.jpg|thumb|upright=1.35|[[John Hogg (biologist)|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.<ref name="Taylor-2003">{{Cite journal |last=Taylor |first=F.J.R. 'Max' |date=11 January 2003 |title=The collapse of the two-kingdom system, the rise of protistology and the founding of the International Society for Evolutionary Protistology (ISEP) |journal=International Journal of Systematic and Evolutionary Microbiology |volume=53 |issue=6 |pages=1707–14 |doi=10.1099/ijs.0.02587-0 |pmid=14657097 |doi-access=free }}</ref>

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 [[chloroplast]]s for [[photosynthesis]], like plants, but can also feed on organic matter and are [[Motility|motile]], like animals. In 1860, [[John Hogg (biologist)|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."<ref name="Hogg 1860"/> 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.<ref name="Hogg 1860">{{Cite journal |last=Hogg |first=John |author-link=John Hogg |date=1860 |title=On the distinctions of a plant and an animal, and on a fourth kingdom of nature |url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044089575245;view=1up;seq=232 |journal=Edinburgh New Philosophical Journal | series=2nd series | volume=12 | pages=216–25 }}</ref><ref>{{Cite journal |last=Scamardella |first=J. M. |date=December 1999 |title=Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista |journal=International Microbiology |volume=2 |issue=4 |pages=207–16 |pmid=10943416}}</ref><ref>{{Cite journal |last=Copeland |first=Herbert F. |date=September–October 1947 |title=Progress Report on Basic Classification |journal=The American Naturalist |volume=81 |issue=800 |pages=340–61 |doi=10.1086/281531 |jstor=2458229|pmid=20267535 |bibcode=1947ANat...81..340C |s2cid=36637843 }}</ref>

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 [[Tissue (biology)|tissues]].<ref>(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 [[wikt:tectology|tectological]] 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.")</ref>

[[File:The foraminifera- an introduction to the study of the protozoa (1902) (14783054955).jpg|thumb|[[Frederick Chapman (palaeontologist)|Frederick Chapman]]'s The foraminifera: an introduction to the study of the protozoa (1902)]]

Despite these proposals, Protozoa emerged as the preferred taxonomic placement for [[Heterotrophic nutrition|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 (biologist)|Herbert Copeland]], [[Robert Whittaker (ecologist)|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.<ref name="Taylor-2003" /> A variety of [[Kingdom (biology)|multi-kingdom systems]] were proposed, and the kingdoms Protista and Protoctista became established in biology texts and curricula.<ref>{{Cite journal |last=Whittaker |first=R.H. |date=10 January 1969 |title=New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms |journal=Science |volume=163 |issue=3863 |pages=150–60 |bibcode=1969Sci...163..150W |doi=10.1126/science.163.3863.150 |pmid=5762760 |citeseerx=10.1.1.403.5430 }}</ref><ref>{{Cite book|last=Margulis |first=Lynn |author-link=Lynn Margulis |date=1974 |publisher=Springer |isbn=978-1-4615-6946-6 |editor-last=Dobzhansky|editor-first=Theodosius |pages=45–78 |editor-last2=Hecht |editor-first2=Max K. |editor-last3=Steere |editor-first3=William C. |doi=10.1007/978-1-4615-6944-2_2 |chapter=Five-Kingdom Classification and the Origin and Evolution of Cells |title=Evolutionary Biology}}</ref><ref>{{cite journal |last=Cavalier-Smith |first=Thomas |author-link=Tom Cavalier-Smith |date=August 1998 |title=A revised six-kingdom system of life |journal=Biological Reviews |volume=73 |issue=3 |pages=203–66 |doi=10.1111/j.1469-185X.1998.tb00030.x |pmid=9809012|s2cid=6557779 }}</ref>

By 1954, Protozoa were classified as "unicellular animals", as distinct from the "Protophyta", single-celled photosynthetic algae, which were considered primitive plants.<ref>{{Cite book |url=https://archive.org/details/protozoology1954kudo |title=Protozoology |last=Kudo |first=Richard R. (Richard Roksabro) |year=1954 |location=Springfield, IL |publisher=Charles C. Thomas |page=5}}</ref> 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.<ref>{{Cite journal |last=Honigberg |first=B. M. |author2=W. Balamuth |author3=E.C. Bovee |author4=J.O. Corliss |author5=M. Gojdics |author6=R.P. Hall |author7=R.R. Kudo |author8=N.D. Levine |author9=A.R. Lobblich |date=February 1964 |title=A Revised Classification of the Phylum Protozoa |journal=Journal of Eukaryotic Microbiology |volume=11 |issue=1 |pages=7–20 |doi=10.1111/j.1550-7408.1964.tb01715.x |pmid=14119564 |author10=J. Weiser}}</ref>

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.<ref>{{Cite journal |last=Cavalier-Smith |first=Thomas |author-link=Tom Cavalier-Smith |date=1981 |title=Eukaryote Kingdoms: Seven or Nine? |journal=Bio Systems |volume=14 |issue=3–4 |pages=461–81 |doi=10.1016/0303-2647(81)90050-2 |pmid=7337818|bibcode=1981BiSys..14..461C }}</ref><ref>{{Cite journal |last=Cavalier-Smith |first=Thomas |author-link=Tom Cavalier-Smith |date=December 1993 |title=Kingdom Protozoa and Its 18 Phyla |journal=Microbiological Reviews |volume=57 |issue=4 |pages=953–94 |pmc=372943 |pmid=8302218 |doi=10.1128/mmbr.57.4.953-994.1993}}</ref><ref>{{Cite journal |last=Cavalier-Smith |first=Thomas |author-link=Tom Cavalier-Smith |date=23 June 2010 |title=Kingdoms Protozoa and Chromista and the Eozoan Root of the Eukaryotic Tree |journal=Biology Letters |volume=6 |issue=3 |pages=342–45 |doi=10.1098/rsbl.2009.0948 |pmc=2880060 |pmid=20031978}}</ref> 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]].<ref name="Ruggiero-2015" /> This approach excludes several major groups traditionally placed among the protozoa, such as the [[ciliate]]s, [[dinoflagellate]]s, [[foraminifera]], and the parasitic [[apicomplexa]]ns, which were moved to other groups such as [[Alveolata]] and [[Stramenopiles]], under the polyphyletic [[Chromista]]. The Protozoa in this scheme were [[Paraphyly|paraphyletic]], because it excluded some descendants of Protozoa.<ref name="Ruggiero-2015" />

The continued use by some of the 'Protozoa' in its old sense<ref>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}}</ref> 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.<ref name="Ruppert">{{cite book |last=Ruppert |first=Edward E. |title=Invertebrate zoology: a functional evolutionary approach |date=2004 |location=Delhi |isbn=9788131501047 |page=12 |edition=7th}}</ref> Others class any unicellular eukaryotic microorganism as protists, and make no reference to 'Protozoa'.<ref name="Brock">{{cite book |last=Madigan |first=Michael T. |title=Brock Biology of Microorganisms |date=2019 |location=New York |isbn=9781292235103 |page=594 |edition=15th, Global}}</ref> In 2005, members of the Society of [[Protozoologist]]s voted to change its name to the International Society of [[Protistologist]]s.<ref>{{cite web |url=http://protozoa.uga.edu/artman/publish/article_37.shtml |title=New President's Address |website=protozoa.uga.edu |access-date=1 May 2015 |archive-date=29 July 2016 |archive-url=https://web.archive.org/web/20160729160637/http://protozoa.uga.edu/artman/publish/article_37.shtml |url-status=dead }}</ref>

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.<ref>{{Cite journal |last1=Adl |first1=Sina M. |last2=Simpson |first2=Alastair G.B. |last3=Lane |first3=Christopher E. |last4=Lukeš |first4=Julius |last5=Bass |first5=David |last6=Bowser |first6=Samuel S. |last7=Brown |first7=Matthew W. |last8=Burki |first8=Fabien |last9=Dunthorn |first9=Micah |date=2012-09-01 |title=The Revised Classification of Eukaryotes |journal=Journal of Eukaryotic Microbiology |volume=59 |issue=5 |pages=429–514 |doi=10.1111/j.1550-7408.2012.00644.x |pmid=23020233 |pmc=3483872}}</ref>

== Phylogenetic distribution ==
{{further|Eukaryote}}

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.<ref name="Burki 2014">{{cite journal |last=Burki |first=F. |title=The eukaryotic tree of life from a global phylogenomic perspective |journal=Cold Spring Harbor Perspectives in Biology |volume=6 |issue=5 |page=a016147 |date=May 2014 |pmid=24789819 |doi=10.1101/cshperspect.a016147 |pmc=3996474 }}</ref><ref>{{cite journal |last=Burki |first=F. |title=Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista |journal=Proceedings: Biological Sciences |volume=283 |issue=1823 |page=20152802 |date=January 2016 |pmid=26817772 |pmc=4795036 |doi=10.1098/rspb.2015.2802 }}</ref> The [[Metamonada]] are hard to place, being sister possibly to [[Discoba]], possibly to [[Malawimonada]].<ref name="Burki Roger Brown Simpson 2020 pp. 43–55">{{cite journal |last1=Burki |first1=Fabien |last2=Roger |first2=Andrew J. |last3=Brown |first3=Matthew W. |last4=Simpson |first4=Alastair G.B. |name-list-style=vanc |title=The New Tree of Eukaryotes |journal=Trends in Ecology & Evolution |publisher=Elsevier BV |volume=35 |issue=1 |year=2020 |issn=0169-5347 |doi=10.1016/j.tree.2019.08.008 |pages=43–55|pmid=31606140 |s2cid=204545629 |doi-access=free |bibcode=2020TEcoE..35...43B |url=https://uu.diva-portal.org/smash/get/diva2:1387649/FULLTEXT01 }}</ref>

{{Clade|label1=[[Eukaryote]]s|style1=font-size:90%; line-height:90%
|{{clade
|1={{clade
   |1=[[Ancyromonadida]] {{font color|darkred|FLAGELLATE PROTOZOA}}
   |2={{clade
      |1=[[Malawimonad]]a {{font color|darkred|FLAGELLATE PROTOZOA}}
      |2={{clade
         |1=[[CRuMs]] {{font color|darkred|PROTOZOA, often FLAGELLATE}}
         |label2=[[Amorphea]]
         |2={{clade
            |1=[[Amoebozoa]] {{font color|darkred|AMOEBOID PROTOZOA}}
            |2={{clade
               |1=[[Breviatea]] {{font color|darkred|PARASITIC PROTOZOA}}
               |2={{clade
                  |1=[[Apusomonadida]] {{font color|darkred|FLAGELLATE PROTOZOA}}
                  |2={{clade
                     |1=[[Holomycota]] (''inc. multicellular fungi'') {{font color|magenta|FUNGAL PROTISTS}}
                     |2=[[Holozoa]] (''inc. multicellular animals'') {{font color|darkred|AMOEBOID PROTOZOA}}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
|label2=[[Diphoda]]
|sublabel2=Bikonts
|2={{clade
   |1={{clade
      |1=? [[Metamonada]] {{font color|darkred|FLAGELLATE PROTOZOA}}
      |2=[[Discoba]] {{font color|green|EUGLENOID PROTISTS}} (some photosynthetic), {{font color|darkred|FLAGELLATE/AMOEBOID PROTOZOA}}
      }}
   |label2=[[Diaphoretickes]]
   |2={{clade
      |1={{clade
         |1=[[Cryptista]] {{font color|green|PROTISTS}} (algae)
         |label2=[[Archaeplastida]]
         |2={{clade
            |1={{clade
               |1=[[Rhodophyta]] (''multicellular red algae'') {{font color|green|PROTISTS}} (red algae)
               |2=[[Picozoa]] {{font color|green|PROTISTS}} (algae)
               }}
            |2={{clade
               |1=[[Glaucophyta]] {{font color|green|PROTISTS}} (algae)
               |2=[[Viridiplantae]] (''inc. multicellular plants'') {{font color|green|PROTISTS}} (green algae)
               }} 
            }}
         }}
      |2={{clade
         |1=[[Hemimastigophora]] {{font color|darkred|FLAGELLATE PROTOZOA}}
         |2={{clade
            |1=[[Provora]] {{font color|darkred|FLAGELLATE PROTOZOA}}
            |2={{clade
               |1=[[Haptista]] {{font color|darkred|PROTOZOA}}
               |label2=[[TSAR]]
               |2={{clade
                  |1=[[Telonemia]] {{font color|darkred|FLAGELLATE PROTOZOA}}
                  |label2=[[SAR supergroup|SAR]]
                  |2={{clade
                     |1=[[Rhizaria]] {{font color|darkred|PROTOZOA, often AMOEBOID}}
                     |2={{clade
                        |1=[[Alveolata]] {{font color|darkred|PROTOZOA}}
                        |2=[[Stramenopiles]] {{font color|green|FLAGELLATE PROTISTS}} (photosynthetic)
                        }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}
}}

== Characteristics ==

=== Reproduction ===

[[Reproduction]] in Protozoa can be sexual or asexual.<ref name="Khan-2008">{{Cite book|last=Khan|first=Naveed Ahmed|url=https://books.google.com/books?id=bY9qczJ4owMC&dq=Protozoa+reproduction&pg=PA472|title=Emerging Protozoan Pathogens|date=2008|publisher=Garland Science|isbn=978-0-203-89517-7|pages=472–74|language=en}}</ref> Most Protozoa reproduce [[Asexual reproduction|asexually]] through [[binary fission]].<ref name="Rodriguez-2015">{{Cite book|last=Rodriguez|first=Margaret|url=https://books.google.com/books?id=Mc0HBgAAQBAJ&dq=Protozoa+reproduction&pg=PA135|title=Microbiology for Surgical Technologists|date=2015|publisher=Cengage Learning|isbn=978-1-133-70733-2|page=135|language=en}}</ref>

Many parasitic Protozoa reproduce both asexually and [[Sexual reproduction|sexually]].<ref name="Khan-2008" /> However, sexual reproduction is rare among free-living protozoa and it usually occurs when food is scarce or the environment changes drastically.<ref name="Laybourn-Parry-2013">{{Cite book|url=https://books.google.com/books?id=ofHTBwAAQBAJ&q=protozoa+sexual+reproduction&pg=PA86|title=A Functional Biology of Free-Living Protozoa|vauthors=Laybourn-Parry J|date=2013|publisher=Springer Science & Business Media|isbn=978-1-4684-7316-2|pages=86–88|language=en}}</ref> Both [[isogamy]] and [[anisogamy]] occur in Protozoa, anisogamy being the more common form of sexual reproduction.<ref>{{Cite book|last=Khan|first=N. A.|url=https://books.google.com/books?id=R2O1DwAAQBAJ&dq=anisogamy+protozoa&pg=PA194|title=Microbial Pathogens and Human Diseases|date=2008|publisher=CRC Press|isbn=978-1-4822-8059-3|page=194|language=en}}</ref>

=== Size ===

Protozoans, as traditionally defined, range in size from as little as 1 [[micrometers|micrometre]] to several [[millimetre]]s, or more.<ref>{{Cite book |last1=Singleton |first1=Paul |last2=Sainsbury |first2=Diana |url=https://books.google.com/books?id=pddpAAAAMAAJ |title=Dictionary of microbiology and molecular biology |date=2001 |publisher=Wiley |isbn=9780471941507 }}</ref> Among the largest are the deep-sea–dwelling [[xenophyophores]], single-celled foraminifera whose shells can reach 20&nbsp;cm in diameter.<ref name="Gooday2011">{{cite journal |last1=Gooday |first1=A.J. |last2=Aranda da Silva |first2=A.P. |last3=Pawlowski |first3=J. |title=Xenophyophores (Rhizaria, Foraminifera) from the Nazaré Canyon (Portuguese margin, NE Atlantic) |journal=Deep-Sea Research Part II: Topical Studies in Oceanography |date=1 December 2011 |volume=58 |issue=24–25 |pages=2401–19 |bibcode=2011DSRII..58.2401G |doi=10.1016/j.dsr2.2011.04.005 }}</ref>

[[File:Spirostomum.jpeg|thumb|The ciliate ''Spirostomum ambiguum'' can attain 3 mm in length]]

{| class="wikitable"
!Species
!Cell type
!Size in micrometres
|-
|''[[Plasmodium falciparum]]'' || [[malaria]] [[parasite]], [[trophozoite]] phase<ref>{{Cite web |url=http://www.microbiologybook.org/parasitology/blood-malaria.htm |title=Blood and Tissue Protozoa |last=Ghaffar |first=Abdul |website=Microbiology and Immunology On-Line |access-date=2018-03-23}}</ref>
|1–2
|-
|''[[Massisteria|Massisteria voersi]]'' || free-living [[Cercozoa]] cercomonad amoebo-flagellate<ref name="Mylnikov">{{Cite journal |last1=Mylnikov |first1=Alexander P. |last2=Weber |first2=Felix |last3=Jürgens |first3=Klaus |last4=Wylezich |first4=Claudia |date=August 2015 |title=''Massisteria marina'' has a sister: ''Massisteria voersi'' sp. nov., a rare species isolated from coastal waters of the Baltic Sea|journal=European Journal of Protistology |volume=51 |issue=4 |pages=299–310 |doi=10.1016/j.ejop.2015.05.002 |pmid=26163290}}</ref>
|2.3–3
|-
|''[[Bodo saltans]]'' || free-living [[Kinetoplastida|kinetoplastid]] flagellate<ref>{{Cite journal |last1=Mitchell |first1=Gary C.|last2=Baker |first2=J.H. |last3=Sleigh |first3=M.A. |date=1 May 1988|title=Feeding of a freshwater flagellate, ''Bodo saltans'', on diverse bacteria |journal=The Journal of Protozoology |volume=35 |issue=2 |pages=219–22 |doi=10.1111/j.1550-7408.1988.tb04327.x}}</ref>
|5–8
|-
|''[[Plasmodium falciparum]]'' || [[malaria]] parasite, gametocyte phase<ref>{{Cite web |url=http://www.microbiologybook.org/parasitology/blood-malaria.htm |title=Blood and tissue Protozoa |last=Ghaffar |first=Abdul |website=Microbiology and Immunology On-Line |access-date=2018-03-23}}</ref>
|7–14
|-
|''[[Trypanosoma cruzi]]'' || parasitic kinetoplastid, [[Chagas disease]]<ref>{{Cite web |url=http://parasite.org.au/para-site/text/brucei-text.html |title=''Trypanosoma brucei'' |website=parasite.org.au |access-date=2018-03-23}}</ref>
|14–24
|-
|''[[Entamoeba histolytica]]'' || parasitic [[amoeba]]n<ref>{{Cite web |url=https://msu.edu/course/zol/316/ehisscope.htm |title=Microscopy of ''Entamoeba histolytica''|website=msu.edu|access-date=2016-08-21}}</ref>
|15–60
|-
|''[[Balantidium coli]]'' || parasitic ciliate<ref>{{Cite web |url=http://www1.udel.edu/mls/dlehman/medt372/B-coli.html |title=Diagnostic parasitology |last=Lehman |first=Don |website=University of Delaware |access-date=2018-03-23}}</ref>
|50–100
|-
|''[[Paramecium caudatum]]'' || free-living ciliate<ref>{{Cite web |url=http://eol.org/pages/484358/overview|title=''Paramecium caudatum''|last=Taylor |first=Bruce |website=Encyclopedia of Life |access-date=2018-03-23}}</ref>
|120–330
|-
|''[[Amoeba proteus]]'' || free-living amoebozoan<ref>{{Cite web |url=http://www.arcella.nl/amoeba-proteus|title=''Amoeba proteus'' {{!}} Microworld |website=www.arcella.nl |access-date=2016-08-21}}</ref>
|220–760
|-
|''[[Noctiluca scintillans]]'' || free-living [[dinoflagellate]]<ref>{{Cite web |url=http://www.imas.utas.edu.au/zooplankton/image-key/noctiluca-scintillans |title=''Noctiluca scintillans'' |date=2011-11-30 |website=University of Tasmania, Australia |access-date=2018-03-23}}</ref>
|700–2000
|-
|''[[Syringammina fragilissima]]'' || [[foraminifera]] amoeba<ref name="Gooday2011"/>
|up to {{val|200000}}
|-
|}

=== Habitat ===

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<ref>{{Cite book|url=https://books.google.com/books?id=mJAxvgAACAAJ&q=yellowstone+microorganisms|title=Seen and Unseen: Discovering the Microbes of Yellowstone|last=Sheehan|first=Kathy B.|date=2005 |publisher=Falcon |isbn=9780762730933}}</ref> and hypersaline lakes and lagoons.<ref>{{Cite journal|last1=Post|first1=F.J.|last2=Borowitzka|first2=L.J.|last3=Borowitzka|first3=M.A. |last4=Mackay |first4=B. |last5=Moulton|first5=T.|date=1983-09-01|title=The protozoa of a Western Australian hypersaline lagoon|journal=Hydrobiologia |volume=105 |issue=1 |pages=95–113|doi=10.1007/BF00025180 |bibcode=1983HyBio.105...95P |s2cid=40995213 |issn=0018-8158}}</ref> All protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by forming [[Microbial cyst|resting cysts]] that enable them to remain dormant until conditions improve.<ref>{{cite journal|title=Resting cysts: A survival strategy in Protozoa Ciliophora|year=2011 |doi=10.1080/11250003.2011.560579 |url=https://www.tandfonline.com/doi/full/10.1080/11250003.2011.560579|accessdate=8 September 2022|last1=Verni |first1=F. |last2=Rosati |first2=G. |journal=Italian Journal of Zoology |volume=78 |issue=2 |pages=134–45 |s2cid=84550678 |url-access=subscription }}</ref>

=== Feeding ===

All protozoa are [[heterotroph]]ic, deriving nutrients from other organisms, either by ingesting them whole by [[phagocytosis]] or taking up dissolved organic matter or micro-particles ([[osmotrophy]]). [[Phagocytosis]] may involve engulfing organic particles with [[pseudopodia]] (as [[amoeba]]e do), taking in food through a specialized mouth-like aperture called a [[cytostome]], or using stiffened ingestion organelles<ref name="Fenchel, T 1987">Fenchel, T. 1987. ''Ecology of protozoan: The biology of free-living phagotrophic protists''. Springer-Verlag, Berlin. {{ISBN?}} {{page?|date=September 2024}}</ref>

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.<ref>{{Cite web |url=http://www.tulane.edu/~wiser/malaria/fv.html |title=Biochemistry of Plasmodium |last=Wiser |first=Mark F. |website=The Wiser Page |access-date=2018-03-22 |archive-date=2017-07-04 |archive-url=https://web.archive.org/web/20170704155236/http://www.tulane.edu/~wiser/malaria/fv.html |url-status=dead }}</ref>

[[File:Paramecium bursaria 001.jpg|thumb|upright|''[[Paramecium bursaria]]'', is one example of a variety of freshwater ciliates that host [[endosymbiont]] chlorophyte algae from the genus ''[[Chlorella]]'']]

Protozoa may also live as [[mixotroph]]s, combining a heterotrophic diet with some form of [[autotroph]]y. 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 [[chloroplast]]s 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 [[plastid]]s 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.<ref>{{Cite journal |last1=Nishitani |first1=Goh |last2=Nagai |first2=Satoshi |last3=Baba|first3=Katsuhisa |last4=Kiyokawa |first4=Susumu |last5=Kosaka|first5=Yuki |last6=Miyamura |first6=Kazuyoshi |last7=Nishikawa |first7=Tetsuya |last8=Sakurada |first8=Kiyonari |last9=Shinada |first9=Akiyoshi |display-authors=3 |date=May 2010 |title=High-Level Congruence of Myrionecta rubra Prey and Dinophysis Species Plastid Identities as Revealed by Genetic Analyses of Isolates from Japanese Coastal Waters |journal=Applied and Environmental Microbiology |volume=76|issue=9 |pages=2791–98 |doi=10.1128/AEM.02566-09 |pmc=2863437 |pmid=20305031 |bibcode=2010ApEnM..76.2791N }}</ref>

=== Motility ===

Organisms traditionally classified as protozoa are abundant in [[aqueous]] environments and [[soil]], occupying a range of [[trophic level]]s. The group includes [[flagellate]]s (which move with the help of undulating and beating [[flagella]]). [[Ciliate]]s (which move by using hair-like structures called [[cilia]]) and [[amoeba]]e (which move by the use of temporary extensions of cytoplasm called [[pseudopodia]]). 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 ([[Sessility (zoology)|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.{{cn|date=August 2022}}

=== Walls, pellicles, scales, and skeletons ===

Unlike plants, fungi and most types of algae, most protozoa do not have a rigid external [[cell wall]] but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoa, such as the ciliates and [[euglenozoa]]ns, the outer membrane of the cell is supported by a cytoskeletal infrastructure, which may be referred to as a "pellicle". The pellicle gives shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. In [[ciliate]]s and [[Apicomplexa]], the pellicle includes a layer of closely packed vesicles called alveoli. In [[euglenid]]s, the pellicle is formed from [[protein]] strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the [[euglenoids]] and the ciliate ''[[Paramecium]]''. In some protozoa, the pellicle hosts [[Epibiont|epibiotic]] bacteria that adhere to the surface by their [[Pilus#Fimbriae|fimbriae]] (attachment pili).

Some protozoa live within loricas{{snd}}loose fitting but not fully intact enclosures.  For example, many collar flagellates ([[Choanoflagellate]]s) have an organic lorica or a lorica made from silicous sectretions. Loricas are also common among some green euglenids, various ciliates (such as the [[folliculinid]]s, 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]], [[synurophyte]]s.  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]], [[Phaeodarea]]{{snd}}collectively the '[[Radiolaria]]', and [[Ebriida]]).
<!--<ref>[https://archive.org/stream/protozoainbiolog00calk#page/1008/mode/2up "Organisms Living On and In Protozoa" from ''Protozoa in Biological Research'', by Gary N. Calkins and Francis M. Summers, Columbia University Press, 1941]</ref> not sure what this is doing here but it's very old and not obviously usable for anything but history-->

=== Life cycle ===

Protozoa mostly reproduce asexually by [[binary fission]] or multiple fission. Many protozoa also exchange genetic material by sexual means (typically, through [[Isogamy#Conjugation|conjugation]]), but this is generally decoupled from reproduction.<ref>{{cite web |url=http://www.cambridge.org/ca/academic/subjects/life-sciences/evolutionary-biology/sex-and-death-protozoa-history-obsession |title=Sex and Death in Protozoa |website=Cambridge University Press |access-date=2015-06-09}}</ref> [[Origin and function of meiosis|Meiotic sex]] is widespread among [[eukaryote]]s, and must have originated early in their evolution, as it has been found in many protozoan lineages that diverged early in eukaryotic evolution.<ref>Bernstein, H.; Bernstein, C. (2013). ''Evolutionary Origin and Adaptive Function of Meiosis''. Meiosis. InTech. {{ISBN|978-953-51-1197-9}} {{pn|date=May 2023}}</ref>

===Aging===

In the well-studied protozoan species ''Paramecium 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 by [[Transplantation chimera|transplantation]] experiments of Aufderheide in 1986.<ref>{{cite journal|last=Aufderheide|first=Karl J.|year=1986|title=Clonal aging in ''Paramecium tetraurelia''. II. Evidence of functional changes in the macronucleus with age|journal=Mechanisms of Ageing and Development|volume=37|issue=3|pages=265–79|doi=10.1016/0047-6374(86)90044-8|pmid=3553762|s2cid=28320562}}</ref>  These experiments demonstrated that the macronucleus, and not the cytoplasm, is responsible for clonal aging. 

Additional experiments by Smith-Sonneborn,<ref>{{cite journal|last=Smith-Sonneborn|first=J.|year=1979|title=DNA repair and longevity assurance in ''Paramecium tetraurelia''|journal=[[Science (journal)|Science]]|volume=203|issue=4385|pages=1115–17|doi=10.1126/science.424739|pmid=424739|bibcode=1979Sci...203.1115S}}</ref> Holmes and Holmes,<ref>{{cite journal|last1=Holmes|first1=George E.|last2=Holmes|first2=Norreen R.|title=Accumulation of DNA damages in aging ''Paramecium tetraurelia''|journal=Molecular and General Genetics|date=July 1986|volume=204|issue=1|pages=108–14|doi=10.1007/bf00330196|pmid=3091993|s2cid=11992591}}</ref> and Gilley and Blackburn<ref>{{cite journal|last1=Gilley|first1=David|last2=Blackburn|first2=Elizabeth H.|year=1994|title=Lack of telomere shortening during senescence in ''Paramecium''|url=http://www.pnas.org/content/pnas/91/5/1955.full.pdf|journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|volume=91|issue=5|pages=1955–58|doi=10.1073/pnas.91.5.1955|pmc=43283|pmid=8127914|bibcode=1994PNAS...91.1955G|doi-access=free}}</ref> showed that, during clonal aging, [[DNA damage]] increases dramatically.<ref>{{cite book| last1=Bernstein |first1= H |last2= Bernstein |first2= C |year= 1991| title= Aging, Sex, and DNA Repair| publisher= Academic Press| place= San Diego| isbn=  978-0120928606| pages= 153–56}}</ref>  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 [[eukaryote]]s (see [[DNA damage theory of aging]]).

== Ecology ==

=== Free-living ===

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 the [[microbial food web]] they include the most important bacterivores.<ref name="Fenchel, T 1987"/> In part, they facilitate the transfer of bacterial and algal production to successive [[Trophic pyramid|trophic levels]], but also they solubilize the nutrients within microbial biomass, allowing stimulation of microbial growth. As consumers, protozoa prey upon [[unicellular]] or [[filamentous algae]], [[bacteria]], [[microfungi]], and micro-carrion. In the context of older ecological models of the [[Fauna (animals)#Microfauna|micro-]] and [[Fauna (animals)#Meiofauna|meiofauna]], protozoa may be a food source for [[microinvertebrate]]s.

Most species of free-living protozoa live in similar habitats in all parts of the world.<!-- as was first observed by [[Vladimir Shevyakov|W. Shewiakoff]] ?1926?.--><ref>{{cite journal |last1=Fenchel |first1=T. |last2=Finlay |first2=B.J. |year=2004 |title=The ubiquity of small species: Patterns of local and global diversity |journal=BioScience |volume=54 |issue=8 |pages=777–84|doi=10.1641/0006-3568(2004)054[0777:TUOSSP]2.0.CO;2 |s2cid=85884588 |doi-access=free }}</ref><ref>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 232–35</ref><ref>Lee, W.J. & Patterson, D.J. 1998. "Diversity and geographic distribution of free-living heterotrophic flagellates={{snd}}analysis by PRIMER. Protist, 149: 229–43</ref><!-- not sure what we are supposed to conclude from a highly specific 1ry research paper on ciliates living in Bromeliads, really...<ref name="Dunthorn Stoeck Wolf Breiner 2012 pp. 195–205">{{cite journal | last=Dunthorn | first=Micah | last2=Stoeck | first2=Thorsten | last3=Wolf | first3=Klaus | last4=Breiner | first4=Hans-Werner | last5=Foissner | first5=Wilhelm | title=Diversity and endemism of ciliates inhabiting Neotropical phytotelmata | journal=Systematics and Biodiversity | volume=10 | issue=2 | year=2012 | issn=1477-2000 | doi=10.1080/14772000.2012.685195 | pages=195–205}}</ref>-->

=== Parasitism ===
{{main|Protozoan infection}}
{{further|List of parasites of humans}}

Many protozoan [[pathogen]]s are [[human parasite]]s, causing serious diseases such as [[malaria]], [[giardiasis]], [[toxoplasmosis]], and [[African trypanosomiasis|sleeping sickness]]. Some of these protozoa have two-phase life cycles, alternating between proliferative stages (e.g., [[trophozoite]]s) and resting [[Microbial cyst|cysts]], enabling them to survive harsh conditions.<ref name="Yaeger 1996">{{cite book |last1=Yaeger |first1=Robert G. |editor1-last=Baron |editor1-first=S |title=Medical Microbiology |date=1996 |publisher=University of Texas Medical Branch at Galveston |location=Galveston |edition=4th |url=https://www.ncbi.nlm.nih.gov/books/NBK8325/ |chapter=Ch. 77: Protozoa: Structure, Classification, Growth, and Development|pmid=21413323 |isbn=9780963117212 }}</ref>

=== Commensalism ===

A wide range of protozoa live [[Commensalism|commensally]] in the rumens of [[ruminant]] animals, such as cattle and sheep. These include flagellates, such as ''[[Trichomonas]]'', and ciliated protozoa, such as ''[[Isotricha]]'' and ''[[Entodinium]]''.<ref>{{Cite book |title=The Rumen Microbial Ecosystem |last1=Williams |first1=A.G. |last2=Coleman |first2=G.S. |chapter=The rumen protozoa |date=1997 |publisher=Springer, Dordrecht |isbn=9789401071499 |pages=73–139 |doi=10.1007/978-94-009-1453-7_3}}</ref>  The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms.<ref>{{Cite book|url=https://books.google.com/books?id=hY0WAQAAIAAJ|title=An illustrated guide to the protozoa: organisms traditionally referred to as protozoa, or newly discovered groups |last1=Lee |first1=John J. |last2=Leedale |first2=Gordon F. |last3=Bradbury |first3=Phyllis Clarke |date=2000 |publisher=Society of Protozoologists |isbn=9781891276231 |page=634}}</ref>

=== Mutualism ===

Association between protozoan symbionts and their host organisms can be [[Mutualism (biology)|mutually beneficial]]. Flagellated protozoa such as ''[[Trichonympha]]'' and ''[[Pyrsonympha]]'' inhabit the guts of [[termite]]s, where they enable their insect host to digest wood by helping to break down complex [[sugar]]s into smaller, more easily digested molecules.<ref>{{Cite web|url=http://www.kennethnoll.uconn.edu/nsf-termite-project/termite-gut-microbes.html |title=Termite gut microbes {{!}} NOLL LAB|website=www.kennethnoll.uconn.edu|access-date=2018-03-21|archive-url=https://web.archive.org/web/20180321060255/http://www.kennethnoll.uconn.edu/nsf-termite-project/termite-gut-microbes.html |archive-date=2018-03-21 |url-status=dead}}</ref>

<gallery class=center mode=nolines widths=200 heights=200>
File:Trophozoites_of_Entamoeba_histolytica_with_ingested_erythrocytes.JPG|Trophozoites of ''[[Entamoeba histolytica]]'', a disease-causing [[Parasitism|parasite]] with engulfed [[red blood cell]]s (dark circles)
File:Isotricha intestinalis.jpg|''[[Isotricha intestinalis]]'', a [[Commensalism|commensal]] ciliate in the [[rumen]] of sheep
File:Trichonympha campanula.png|''Trichonympha campanula'', a [[Mutualism (biology)|mutualist]] partner of [[termite]]s
</gallery>

== References ==
{{Reflist|28em}}

== Bibliography ==
; General
* Dogiel, V. A., revised by J.I. Poljanskij and E. M. Chejsin. ''General Protozoology'', 2nd ed., Oxford University Press, 1965. {{ISBN?}}
* Hausmann, K., N. Hulsmann. ''Protozoology''. Thieme Verlag; New York, 1996. {{ISBN?}}
* Kudo, R.R. ''[[iarchive:protozoology1954kudo|Protozoology]]''. Springfield, IL: C.C. Thomas, 1954; 4th ed. {{ISBN?}}
* Manwell, R.D. ''Introduction to Protozoology'', 2nd rev. ed., Dover Publications Inc.: New York, 1968. {{ISBN?}}
* Roger Anderson, O. ''Comparative protozoology: ecology, physiology, life history''. Berlin [etc.]: Springer-Verlag, 1988.
* Sleigh, M. ''The Biology of Protozoa''. E. Arnold: London, 1981. {{ISBN?}}
; Identification
* 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. {{ISBN?}}
* Lee, J.J., Leedale, G.F. & Bradbury, P. ''An Illustrated Guide to the Protozoa''. Lawrence, KS: Society of Protozoologists, 2000; 2nd ed. {{ISBN?}}
* Patterson, D.J. ''Free-Living Freshwater Protozoa. A Colour Guide''. Manson Publishing: London, 1996. {{ISBN?}}
* Patterson, D.J., M.A. Burford. ''A Guide to the Protozoa of Marine Aquaculture Ponds''. CSIRO Publishing, 2001.{{ISBN?}}
; Morphology
* 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. [https://www.biodiversitylibrary.org/bibliography/7050#/summary ''Electron-Microscopic Structure of Protozoa'']. Pergamon Press, Oxford. {{ISBN?}}
; Physiology and biochemistry
* Nisbet, B. 1984. ''Nutrition and feeding strategies in Protozoa.'' Croom Helm Publ.: London, 280 pp. {{ISBN?}}
* Coombs, G.H. & North, M. 1991. ''Biochemical protozoology''. Taylor & Francis, London, Washington. {{ISBN?}}
* 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''. Vols. 1, 2, and 3. Academic Press: New York; 2nd ed.
* Sukhareva-Buell, N.N. 2003. ''Biologically active substances of protozoa''. Dordrecht: Kluwer. {{ISBN?}}
; Ecology
* 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.
; Parasitology
* Kreier, J.P. (ed.). 1991–1995. ''Parasitic Protozoa'', 2nd ed. 10 vols (1–3 coedited by Baker, J.R.). Academic Press: San Diego, CA, [https://books.google.com/books/about/Parasitic_Protozoa.html?id=6OFTrgEACAAJ].
; Methods
* Lee, J.J., & Soldo, A.T. (1992). ''Protocols in protozoology''. Lawrence, KS: Society of Protozoologists, [http://protozoa.uga.edu/pub/Protocols_in_protozoology.pdf] {{Webarchive|url=https://web.archive.org/web/20160729032313/http://protozoa.uga.edu/pub/Protocols_in_protozoology.pdf |date=2016-07-29 }}.

== External links ==
{{Wikispecies|Protozoa}}{{Commonscat}}
{{Eukaryota classification}}
{{Protozoa protist}}
{{Infectious disease}}
{{Taxonbar|from=Q101274}}
{{Authority control}}

[[Category:1670s in science]]
[[Category:Paraphyletic groups]]