Protozoa

File:Georg_August_Goldfuss_Protozoa_Infusoria_Monades.jpg, 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').{{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}} Goldfuss created Protozoa as a class containing what he believed to be the simplest animals.{{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.] Originally, the group included not only single-celled microorganisms but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoans and polychaete worms.{{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}} The term Protozoa is formed from the 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".{{Cite book|title=Abrégé du dictionnaire grec français|last=Bailly|first=Anatole|date=1981|publisher=Hachette|isbn=978-2010035289|location=Paris|oclc=461974285}}{{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}}

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 ciliates and amoebae 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 Metazoa (animals).{{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.) At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria (mostly ciliates) and flagellates (flagellated protists and amoebae). The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology".{{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}}

File:John_Hogg_--_Primigenum_or_Protoctista.jpg'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.{{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 }}

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 chloroplasts for photosynthesis, like plants, but can also feed on organic matter and are motile, 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.{{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 }}{{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}}{{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 }}

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 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.")

File:The foraminifera- an introduction to the study of the protozoa (1902) (14783054955).jpg's The foraminifera: an introduction to the study of the protozoa (1902)]]

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.{{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 }}{{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}}{{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 }}

By 1954, Protozoa were classified as "unicellular animals", as distinct from the "Protophyta", single-celled photosynthetic algae, which were considered primitive plants.{{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}} 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.{{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}}

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.{{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 }}{{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}}{{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}} 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 ciliates, dinoflagellates, foraminifera, and the parasitic apicomplexans, which were moved to other groups such as Alveolata and Stramenopiles, under the polyphyletic Chromista. The Protozoa in this scheme were paraphyletic, 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. {{doi|10.1016/B978-0-12-814114-4.00003-5}} 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.{{cite book |last=Ruppert |first=Edward E. |title=Invertebrate zoology: a functional evolutionary approach |date=2004 |location=Delhi |isbn=9788131501047 |page=12 |edition=7th}} Others class any unicellular eukaryotic microorganism as Protists, and make no reference to 'Protozoa'.{{cite book |last=Madigan |first=Michael T. |title=Brock Biology of Microorganisms |date=2019 |location=New York |isbn=9781292235103 |page=594 |edition=15th, Global}} In 2005, members of the Society of Protozoologists voted to change its name to the International Society of Protistologists.{{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 }}

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.{{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}}

{{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.{{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 }}{{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 }} The Metamonada are hard to place, being sister possibly to Discoba, possibly to Malawimonada.{{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 }}

{{Clade|label1=Eukaryotes|style1=font-size:90%; line-height:90%

|{{clade

|1={{clade

|1=Ancyromonadida {{font color|darkred|FLAGELLATE PROTOZOA}}

|2={{clade

|1=Malawimonada {{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}}

}}

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|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)

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|2={{clade

|1=Glaucophyta {{font color|green|PROTISTS}} (algae)

|2=Viridiplantae (inc. multicellular plants) {{font color|green|PROTISTS}} (green algae)

}}

}}

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|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

|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)

}}

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Reproduction in Protozoa can be sexual or asexual.{{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}} Most Protozoa reproduce asexually through binary fission.{{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}}

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.{{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}} Both isogamy and anisogamy occur in Protozoa, anisogamy being the more common form of sexual reproduction.{{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}}

Protozoans, as traditionally defined, range in size from as little as 1 micrometre to several millimetres, or more.{{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 }} Among the largest are the deep-sea–dwelling xenophyophores, single-celled foraminifera whose shells can reach 20 cm in diameter.{{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 }}

File:Spirostomum.jpeg

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{{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}} and hypersaline lakes and lagoons.{{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}} All protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by forming resting cysts that enable them to remain dormant until conditions improve.{{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 }}

All protozoa are heterotrophic, 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 amoebae do), taking in food through a specialized mouth-like aperture called a cytostome, or using stiffened ingestion organellesFenchel, T. 1987. Ecology of protozoan: The biology of free-living phagotrophic protists. Springer-Verlag, Berlin. {{ISBN?}} {{page?|date=September 2024}}

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.{{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 }}

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

Protozoa may also live as mixotrophs, combining a heterotrophic diet with some form of autotrophy. 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 chloroplasts 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 plastids 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.{{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 }}

Organisms traditionally classified as protozoa are abundant in aqueous environments and soil, occupying a range of trophic levels. The group includes flagellates (which move with the help of undulating and beating flagella). Ciliates (which move by using hair-like structures called cilia) and amoebae (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 (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}}

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 euglenozoans, 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 ciliates and Apicomplexa, the pellicle includes a layer of closely packed vesicles called alveoli. In euglenids, 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 epibiotic bacteria that adhere to the surface by their fimbriae (attachment pili).

Some protozoa live within loricas{{snd}}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 the folliculinids, 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, 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 in Acantharea, Pylocystinea, Phaeodarea{{snd}}collectively the 'Radiolaria', and Ebriida).

Protozoa mostly reproduce asexually by binary fission or multiple fission. Many protozoa also exchange genetic material by sexual means (typically, through conjugation), but this is generally decoupled from reproduction.{{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}} Meiotic sex is widespread among eukaryotes, and must have originated early in their evolution, as it has been found in many protozoan lineages that diverged early in eukaryotic evolution.Bernstein, H.; Bernstein, C. (2013). Evolutionary Origin and Adaptive Function of Meiosis. Meiosis. InTech. {{ISBN|978-953-51-1197-9}} {{pn|date=May 2023}}

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 experiments of Aufderheide in 1986.{{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}} These experiments demonstrated that the macronucleus, and not the cytoplasm, is responsible for clonal aging.

Additional experiments by Smith-Sonneborn,{{cite journal|last=Smith-Sonneborn|first=J.|year=1979|title=DNA repair and longevity assurance in Paramecium tetraurelia|journal=Science|volume=203|issue=4385|pages=1115–17|doi=10.1126/science.424739|pmid=424739|bibcode=1979Sci...203.1115S}} Holmes and Holmes,{{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}} and Gilley and Blackburn{{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}} showed that, during clonal aging, DNA damage increases dramatically.{{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}} 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 eukaryotes (see DNA 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 the microbial food web they include the most important bacterivores. In part, they facilitate the transfer of bacterial and algal production to successive 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 micro- and meiofauna, protozoa may be a food source for microinvertebrates.

Most species of free-living protozoa live in similar habitats in all parts of the world.{{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 }}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={{snd}}analysis by PRIMER. Protist, 149: 229–43

{{main|Protozoan infection}}

{{further|List of parasites of humans}}

Many protozoan pathogens are human parasites, causing serious diseases such as malaria, giardiasis, toxoplasmosis, and sleeping sickness. Some of these protozoa have two-phase life cycles, alternating between proliferative stages (e.g., trophozoites) and resting cysts, enabling them to survive harsh conditions.{{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 }}

A wide range of protozoa live 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.{{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}} The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms.{{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}}

Association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoa such as Trichonympha and Pyrsonympha inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex sugars into smaller, more easily digested molecules.{{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}}

File:Trophozoites_of_Entamoeba_histolytica_with_ingested_erythrocytes.JPG|Trophozoites of Entamoeba histolytica, a disease-causing parasite with engulfed red blood cells (dark circles)

File:Isotricha intestinalis.jpg|Isotricha intestinalis, a commensal ciliate in the rumen of sheep

File:Trichonympha campanula.png|Trichonympha campanula, a mutualist partner of termites

{{Reflist|28em}}

; 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. 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 }}.

{{Wikispecies|Protozoa}}{{Commonscat}}

{{Eukaryota classification}}

{{Protozoa protist}}

{{Infectious disease}}

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{{Authority control}}

Category:1670s in science

Category:Paraphyletic groups