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A protist () is any (that is, an organism whose cells contain a ) that is not an , , or . While it is likely that protists share a (the last eukaryotic common ancestor), the exclusion of other eukaryotes means that protists do not form a natural group, or . Therefore, some protists may be more closely related to animals, plants, or fungi than they are to other protists. However, like the groups , , and , the biological category protist is used for convenience. Others classify any unicellular eukaryotic microorganism as a protist.

(2022). 9781292235103
The study of protists is termed .

The classification of a third kingdom separate from animals and plants was first proposed by John Hogg in 1860 as the kingdom Protoctista; in 1866 also proposed a third kingdom Protista as "the kingdom of primitive forms". Originally these also included , but with time these were removed to a fourth kingdom .

In the popular five-kingdom scheme proposed by Robert Whittaker in 1969, Protista was defined as eukaryotic "organisms which are unicellular or unicellular-colonial and which form no tissues", and the fifth kingdom Fungi was established. In the five-kingdom system of , the term protist is reserved for microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as , , and .

(2009). 9780080920146, Academic Press. .
Some use the term protist interchangeably with Margulis's protoctist, to encompass both single-celled and multicellular eukaryotes, including those that form specialized tissues but do not fit into any of the other traditional kingdoms.
(2022). 9783319281476, Springer International Publishing. .

Besides their relatively simple levels of organization, protists do not necessarily have much in common. When used, the term "protists" is now considered to mean a assemblage of similar-appearing but diverse (biological groups); these taxa do not have an exclusive common ancestor beyond being composed of eukaryotes, and have different life cycles, , , and cellular structures.
(2022). 9781405141574, Wiley-Blackwell. .

Examples of protists include:

These examples are unicellular, although oomycetes can join to form filaments, and slime molds can aggregate into a tissue-like mass.

In systems (classifications based on common ancestry), there are no equivalents to the taxa Protista or Protoctista, as both terms refer to a paraphyletic group that spans the entire branch of the tree of life. In cladistic classification, the contents of Protista are mostly distributed among various supergroups: examples include the

"Protista", "Protoctista", and "" are therefore considered obsolete. However, the term "protist" continues to be used informally as a catch-all term for eukaryotic organisms that are not within other traditional kingdoms. For example, the word "protist " may be used to denote any disease-causing organism that is not plant, animal, fungal, prokaryotic, , or .

The term Protista was first used by in 1866. Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms such as:

are unicellular "animal-like" (, and sometimes ) organisms that are further sub-divided based on characteristics such as motility, such as the (flagellated) [[Flagellate#Flagellates as organisms
are "plant-like" () organisms that are composed mostly of unicellular . The dinoflagellates, diatoms and Euglena-like flagellates are photosynthetic protists.
generally refer to ; but and are "fungus-like" () protists, although some are . Two separate types of slime molds exist, the cellular and acellular forms.

Some protists, sometimes called protists, have been considered to be both protozoa and algae or fungi (e.g., and algae), and names for these have been published under either or both of the ICN and the ICZN.Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell. p. 41. . Conflicts, such as these – for example the dual-classification of Euglenids and , which are – is an example of why the kingdom Protista was adopted.

These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on ( relatedness among organisms). Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related . For instance, the are now considered to be closely related to photosynthetic organisms such as and , the are grouped mainly under , and the itself includes only a subset of the "Amoeba" group, and significant number of erstwhile "Amoeboid" are distributed among Rhizarians and other Phyla.

However, the older terms are still used as informal names to describe the morphology and of various protists. For example, the term is used to refer to species of protists that do not form filaments.


Historical classifications
Among the pioneers in the study of the protists, which were almost ignored by except for some genera (e.g., , Chaos, , , , Ulva, Chara, ),Ratcliff, Marc J. (2009). "The Emergence of the Systematics of Infusoria". In: The Quest for the Invisible: Microscopy in the Enlightenment. Aldershot: Ashgate.
(1986). 9780074519288, McGraw Hill. .
were , O. F. Müller, C. G. Ehrenberg and Félix Dujardin.Fauré-Frémiet, E. & Théodoridès, J. (1972). État des connaissances sur la structure des Protozoaires avant la formulation de la Théorie cellulaire. Revue d'histoire des sciences, 27–44. The first groups used to classify microscopic organism were the and the . The Flagellates. Unity, diversity and evolution. Ed.: Barry S. C. Leadbeater and J. C. Green Taylor and Francis, London 2000, p. 3. In 1818, the German naturalist Georg August Goldfuss introduced the word to refer to organisms such as and . From p. 1008: "Erste Klasse. Urthiere. Protozoa." (First class. Primordial animals. Protozoa.) Note: After the of and Schleiden (1838–39), this group was modified in 1848 by Carl von Siebold to include only , such as and . 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.) The formal taxonomic category Protoctista was first proposed in the early 1860s by John Hogg, who argued that the protists should include what he saw as primitive unicellular forms of both plants and animals. He defined the Protoctista as a "fourth kingdom of nature", in addition to the then-traditional kingdoms of plants, animals and minerals. From p. 223: "... I here suggest a fourth or an additional kingdom, under the title of the Primigenal kingdom, ... This Primigenal kingdom would comprise all the lower creatures, or the primary organic beings, – 'Protoctista,' – from πρώτος, first, and χτιστά, created beings; ..." The kingdom of minerals was later removed from taxonomy in 1866 by , leaving plants, animals, and the protists ( Protista), defined as a "kingdom of primitive forms". From p. 215: "VII. Character des Protistenreiches." (VII. Character of the kingdom of Protists.)

In 1938, resurrected Hogg's label, arguing that Haeckel's term Protista included anucleated microbes such as , which the term "Protoctista" (literally meaning "first established beings") did not. In contrast, Copeland's term included such as , and . This classification was the basis for Whittaker's later definition of , , and Protista as the four kingdoms of life. The kingdom Protista was later modified to separate into the separate kingdom of , leaving the protists as a group of eukaryotic microorganisms. These five kingdoms remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that neither protists nor monera were single groups of related organisms (they were not ).

Modern classifications
today do not treat Protista as a formal taxon, but the term "protist" is still commonly used for convenience in two ways. The most popular contemporary definition is a one, that identifies a group: a protist is any that is not an , (land) , or (true) ; this definition excludes many unicellular groups, like the (fungi), many (fungi), and (fungi), and also a non-unicellular group included in Protista in the past, the (animal). Some systematists judge paraphyletic taxa acceptable, and use Protista in this sense as a formal taxon (as found in some secondary textbooks, for pedagogical purpose).

The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular, that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues (but vegetative cell differentiation may occur restricted to sexual reproduction, alternate vegetative morphology, and quiescent or resistant stages, such as cysts); this definition excludes many , multicellular and , which may have tissues.

The taxonomy of protists is still changing. Newer classifications attempt to present groups based on morphological (especially ),Pitelka, D. R. (1963). Electron-Microscopic Structure of Protozoa. Pergamon Press, Oxford.Berner, T. (1993). Ultrastructure of Microalgae. Boca Raton: CRC Press. Beckett, A., Heath, I. B., and Mclaughlin, D. J. (1974). An Atlas of Fungal Ultrastructure. Longman, Green, New York. ()Ragan M.A. & Chapman D.J. (1978). A Biochemical Phylogeny of the Protists. London, New York: Academic Press. Lewin R. A. (1974). "Biochemical taxonomy", pp. 1–39 in Algal Physiology and Biochemistry, Stewart W. D. P. (ed.). Blackwell Scientific Publications, Oxford. and (molecular research) information.Oren, A., & Papke, R. T. (2010). Molecular phylogeny of microorganisms. Norfolk, UK: Caister Academic Press. Horner, D. S., & Hirt, R. P. (2004). "An overview on eukaryote origins and evolution: the beauty of the cell and the fabulous gene phylogenies", pp. 1–26 in Hirt, R.P. & D.S. Horner. Organelles, Genomes and Eukaryote Phylogeny, An Evolutionary Synthesis in the Age of Genomics. New York: CRC Press. However, there are sometimes discordances between molecular and morphological investigations; these can be categorized as two types: (i) one morphology, multiple lineages (e.g. morphological convergence, ) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages).

Because the protists as a whole are , new systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al. (2005) does not recognize formal ranks (phylum, class, etc.) and instead treats groups as of phylogenetically related organisms. This is intended to make the classification more stable in the long term and easier to update. Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox, upper right. Many are thought to be monophyletic, though there is still uncertainty. For instance, the are probably not monophyletic and the are probably only monophyletic if the and are excluded.

In 2015 a Higher Level Classification of all Living Organisms was arrived at by consensus with many authors including Cavalier-Smith. This classification proposes two superkingdoms and seven kingdoms. The superkingdoms are those of Prokaryotes and Eukaryotes. The Prokaryotes include two kingdoms of Bacteria and Archaea; the Eukaryotes include five kingdoms of Protozoa, Chromista, Fungi, Plantae, and Animalia. The scheme retains fourteen taxonomic ranks. Eukaryotic unicellular organisms are referred to as protists.

Nutrition can vary according to the type of protist. Most eukaryotic are , but the pigments were lost in some groups. Other protists are , and may present , , saprotrophy or . Some are . Some protists that do not have / lost chloroplasts/mitochondria have entered into endosymbiontic relationship with other bacteria/algae to replace the missing functionality. For example, Paramecium bursaria and have a ( ) and a respectively that act as replacements for . Meanwhile, a protist, Mixotricha paradoxa that has lost its uses bacteria as mitochondria and hair-like bacteria ( ) for locomotion.

Many protists are , for example, and can take place where flagellates find prey. Other protists can engulf bacteria and other food particles, by extending their cell membrane around them to form a and digesting them internally in a process termed .

+ Nutritional types in protist metabolism
   Sunlight  Organic compounds or carbon fixation Most  
  Organic compounds  Organic compounds  , or  

For most important cellular structures and functions of animal and plants, it can be found a heritage among protists.

Some protists reproduce sexually using , while others reproduce asexually by .

Some species, for example Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually. However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protists may be clonal lines that rarely exchange genes with other members of their species.

emerged in evolution more than 1.5 billion years ago. The earliest eukaryotes were likely protists. Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that sex appeared to be lacking in certain protists whose ancestors branched off early from the eukaryotic family tree. However, several of these protists are now known to be capable of, or to recently have had the capability for, and hence sexual reproduction. For example, the common intestinal parasite was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, G. lamblia was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes. These results suggested that G. lamblia is capable of meiosis and thus sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of sex, was also found in G. lamblia.

The parasitic protists of the genus have been shown to be capable of a sexual cycle in the invertebrate vector, likened to the meiosis undertaken in the trypanosomes.

Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al. tested for 29 genes that function in meiosis, they found 27 to be present, including 8 of 9 genes specific to meiosis in model eukaryotes. These findings suggest that T. vaginalis may be capable of meiosis. Since 21 of the 29 meiotic genes were also present in G. lamblia, it appears that most of these meiotic genes were likely present in a common ancestor of T. vaginalis and G. lamblia. These two species are descendants of protist lineages that are highly divergent among eukaryotes, leading Malik et al. to suggest that these meiotic genes were likely present in a common ancestor of all eukaryotes.

Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes.

This view was further supported by a study of amoebae by Lahr et al. Amoeba have generally been regarded as asexual protists. However, these authors describe evidence that most lineages are anciently sexual, and that the majority of asexual groups likely arose recently and independently. Early researchers (e.g., Calkins) have interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoeboid organisms as sexual reproduction.

Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.Bernstein H, Bernstein C, Michod RE (2012). "DNA repair as the primary adaptive function of sex in bacteria and eukaryotes". Chapter 1: pp. 1–49 in DNA Repair: New Research, Sakura Kimura and Sora Shimizu (eds.). Nova Sci. Publ., Hauppauge, N.Y. Oxidative stress, which is associated with the production of reactive oxygen species leading to DNA damage, also appears to be an important factor in the induction of sex in protists.

Some commonly found protist pathogens such as Toxoplasma gondii are capable of infecting and undergoing asexual reproduction in a wide variety of animals – which act as secondary or intermediate host – but can undergo sexual reproduction only in the primary or definitive host (for example: such as in this case).

Free-living protists occupy almost any environment that contains liquid water. Many protists, such as , are and are vital in ecosystems, particularly in the ocean as part of the . Protists make up a large portion of the biomass in both marine and terrestrial environments.

Other protists include pathogenic species, such as the Trypanosoma brucei, which causes sleeping sickness, and species of the , which cause .

Parasitism: role as pathogens
Some protists are significant parasites of (e.g.; five species of the parasitic genus cause in humans and many others cause similar diseases in other vertebrates), (the Phytophthora infestans causes in potatoes)Campbell, N. and Reese, J. (2008) Biology. Pearson Benjamin Cummings; 8 ed. . pp. 583, 588 or even of other protists.Lauckner, G. (1980). "Diseases of protozoa". In: Diseases of Marine Animals. Kinne, O. (ed.). Vol. 1, p. 84, John Wiley & Sons, Chichester, UK.Cox, F.E.G. (1991). "Systematics of parasitic protozoa". In: Kreier, J.P. & J. R. Baker (ed.). Parasitic Protozoa, 2nd ed., vol. 1. San Diego: Academic Press. Protist pathogens share many metabolic pathways with their hosts. This makes therapeutic target development extremely difficult – a drug that harms a protist is also likely to harm its / host. A more thorough understanding of protist biology may allow these diseases to be treated more efficiently. For example, the (a nonphotosynthetic but essential to carry out important functions other than photosynthesis) present in provides an attractive target for treating diseases caused by dangerous pathogens such as .

Recent papers have proposed the use of viruses to treat infections caused by .

Researchers from the Agricultural Research Service are taking advantage of protists as pathogens to control red imported fire ant ( Solenopsis invicta) populations in . Spore-producing protists such as Kneallhazia solenopsae (recognized as a or the closest relative to the now) can reduce red fire ant populations by 53–100%. Researchers have also been able to infect fly of the ant with the protist without harming the flies. This turns the flies into a vector that can spread the pathogenic protist between red fire ant colonies.Durham, Sharon (January 28, 2010) ARS Parasite Collections Assist Research and Diagnoses. Retrieved 2014-03-20.

Fossil record
Many protists have neither hard parts nor resistant spores, and their are extremely rare or unknown. Examples of such groups include the , Introduction to the Apicomplexa. Retrieved 2014-03-20. most , Fossil Record of the Ciliata. Retrieved 2014-03-20. some green algae (the ), Klebsormidiales. Retrieved 2014-03-20. , Introduction to the Choanoflagellata. Retrieved 2014-03-20. , Introduction to the Oomycota. Retrieved 2014-03-20. , Introduction to the Phaeophyta . Retrieved 2014-03-20. yellow-green algae, Introduction to the Xanthophyta. Retrieved 2014-03-20. (e.g., ). Introduction to the Basal Eukaryotes. Retrieved 2014-03-20. Some of these have been found preserved in (fossilized tree resin) or under unusual conditions (e.g., , a ).

Others are relatively common in the fossil record, Why Is The Museum On The Web?. Retrieved 2014-03-20. as the , Fossil Record of Diatoms. Retrieved 2014-03-20. , Introduction to the Chrysophyta. Retrieved 2014-03-20. (coccoliths), Introduction to the Prymnesiophyta. Retrieved 2014-03-20. , (ciliates), , Fossil Record of the Dinoflagellata. Retrieved 2014-03-20. , Systematics of the "Green Algae", Part 1. Retrieved 2014-03-20. , Fossil Record of the Rhodophyta. Retrieved 2014-03-20. , , Fossil Record of the Radiolaria. Retrieved 2014-03-20. , Fossil Record of Foraminifera. Retrieved 2014-03-20. and (, ). Introduction to the Testaceafilosea. Retrieved 2014-03-20. Some are even used as paleoecological indicators to reconstruct ancient environments.

More probable eukaryote fossils begin to appear at about 1.8 billion years ago, the , spherical fossils of likely algal protists. Fossil Record of the Eukaryota. Retrieved 2014-03-20. Another possible representative of early fossil eukaryotes are the .

See also



  • Haeckel, E. Das Protistenreich. Leipzig, 1878.
  • Hausmann, K., N. Hulsmann, R. Radek. Protistology. Schweizerbart'sche Verlagsbuchshandlung, Stuttgart, 2003.
  • Margulis, L., J.O. Corliss, M. Melkonian, D.J. Chapman. Handbook of Protoctista. Jones and Bartlett Publishers, Boston, 1990.
  • Margulis, L., K.V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3rd ed. New York: W.H. Freeman, 1998.
  • Margulis, L., L. Olendzenski, H.I. McKhann. Illustrated Glossary of the Protoctista, 1993.
  • Margulis, L., M.J. Chapman. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. Amsterdam: Academic Press/Elsevier, 2009.
  • Schaechter, M. Eukaryotic microbes. Amsterdam, Academic Press, 2012.

Physiology, ecology and paleontology
  • Foissner, W.; D.L. Hawksworth. Protist Diversity and Geographical Distribution. Dordrecht: Springer, 2009
  • Fontaneto, D. Biogeography of Microscopic Organisms. Is Everything Small Everywhere? Cambridge University Press, Cambridge, 2011.
  • Levandowsky, M. Physiological Adaptations of Protists. In: Cell physiology sourcebook : essentials of membrane biophysics. Amsterdam; Boston: Elsevier/AP, 2012.
  • Moore, R. C., and other editors. Treatise on Invertebrate Paleontology. Protista, part B ( vol. 1, Charophyta, vol. 2, Chrysomonadida, Coccolithophorida, Charophyta, Diatomacea & Pyrrhophyta), part C (Sarcodina, Chiefly "Thecamoebians" and Foraminiferida) and part D (Chiefly Radiolaria and Tintinnina). Boulder, Colorado: Geological Society of America; & Lawrence, Kansas: University of Kansas Press.

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