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Protist

January 31, 2023

For the journal, see Protist (journal).

A protist (/ˈproʊtɪst/) is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor),[3] the exclusion of other eukaryotes means that protists do not form a natural group, or clade.[a] Therefore, some protists may be more closely related to animals, plants, or fungi than they are to other protists. However, like the groups algae, invertebrates, and protozoans, the biological category protist is used for convenience. Others classify any unicellular eukaryotic microorganism as a protist.[4] The study of protists is termed protistology.[5]

Protist
Scientific classification
Domain: Eukaryota
Groups included

Supergroups[1] and typical phyla

Many others;
classification varies

Cladistically included but traditionally excluded taxa

History

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 Ernst Haeckel also proposed a third kingdom Protista as "the kingdom of primitive forms".[6] Originally these also included prokaryotes, but with time[when?] these were removed to a fourth kingdom Monera.[b]

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.[7][8][c] In the five-kingdom system of Lynn Margulis, the term protist is reserved for microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as kelp, red algae, and slime molds.[11] 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.[12]

Description

Besides their relatively simple levels of organization, protists do not necessarily have much in common.[13] When used, the term "protists" is now considered to mean a paraphyletic assemblage of similar-appearing but diverse taxa (biological groups); these taxa do not have an exclusive common ancestor beyond being composed of eukaryotes, and have different life cycles, trophic levels, modes of locomotion, and cellular structures.[14][15]

Examples of protists include:[16]

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

In cladistic 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 eukaryotic 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 "Protozoa" 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 pathogen" may be used to denote any disease-causing organism that is not plant, animal, fungal, prokaryotic, viral, or subviral.[17]

Subdivisions

See also: Kingdom (biology) § Summary

The term Protista was first used by Ernst Haeckel in 1866. Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms such as:[6]

Protozoa
Protozoans are unicellular "animal-like" (heterotrophic, and sometimes parasitic) organisms that are further sub-divided based on characteristics such as motility, such as the (flagellated) Flagellata, the (ciliated) Ciliophora, the (phagocytic) amoeba, and the (spore-forming) Sporozoa.
Protophyta
Protophyta are "plant-like" (autotrophic) organisms that are composed mostly of unicellular algae. The dinoflagellates, diatoms and Euglena-like flagellates are photosynthetic protists.
Mold
Molds generally refer to fungi; but slime molds and water molds are "fungus-like" (saprophytic) protists, although some are pathogens. Two separate types of slime molds exist, the cellular and acellular forms.

Some protists, sometimes called ambiregnal protists, have been considered to be both protozoa and algae or fungi (e.g., slime molds and flagellated algae), and names for these have been published under either or both of the ICN and the ICZN.[18][19] Conflicts, such as these – for example the dual-classification of Euglenids and Dinobryons, which are mixotrophic – 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 phylogenetics (evolutionary relatedness among organisms). Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related phyla. For instance, the water molds are now considered to be closely related to photosynthetic organisms such as Brown algae and Diatoms, the slime molds are grouped mainly under Amoebozoa, and the Amoebozoa itself includes only a subset of the "Amoeba" group, and significant number of erstwhile "Amoeboid" genera are distributed among Rhizarians and other Phyla.

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

Classification

Main article: Taxonomy of Protista
See also: Kingdom (biology) § Summary

Historical classifications

Further information: wikispecies:Protista and wikispecies:Protoctista

Among the pioneers in the study of the protists, which were almost ignored by Linnaeus except for some genera (e.g., Vorticella, Chaos, Volvox, Corallina, Conferva, Ulva, Chara, Fucus),[20][21] were Leeuwenhoek, O. F. Müller, C. G. Ehrenberg and Félix Dujardin.[22] The first groups used to classify microscopic organism were the Animalcules and the Infusoria.[23] In 1818, the German naturalist Georg August Goldfuss introduced the word Protozoa to refer to organisms such as ciliates and corals.[24][6] After the cell theory of Schwann and Schleiden (1838–39), this group was modified in 1848 by Carl von Siebold to include only animal-like unicellular organisms, such as foraminifera and amoebae.[25] 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.[26][6] The kingdom of minerals was later removed from taxonomy in 1866 by Ernst Haeckel, leaving plants, animals, and the protists (Protista), defined as a "kingdom of primitive forms".[27][28]

In 1938, Herbert Copeland resurrected Hogg's label, arguing that Haeckel's term Protista included anucleated microbes such as bacteria, which the term "Protoctista" (literally meaning "first established beings") did not. In contrast, Copeland's term included nucleated eukaryotes such as diatoms, green algae and fungi.[29] This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life.[9] The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera, leaving the protists as a group of eukaryotic microorganisms.[7] 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 monophyletic groups).[30]

Modern classifications

 
Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes
Further information: Cavalier-Smith's system of classification and Protista taxonomy

Systematists today do not treat Protista as a formal taxon, but the term "protist" is still commonly used for convenience in two ways.[31] The most popular contemporary definition is a phylogenetic one, that identifies a paraphyletic group:[32] a protist is any eukaryote that is not an animal, (land) plant, or (true) fungus; this definition[33] excludes many unicellular groups, like the Microsporidia (fungi), many Chytridiomycetes (fungi), and yeasts (fungi), and also a non-unicellular group included in Protista in the past, the Myxozoa (animal).[34] Some systematists[who?] judge paraphyletic taxa acceptable, and use Protista in this sense as a formal taxon (as found in some secondary textbooks, for pedagogical purpose).[citation needed]

The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular,[31] 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);[35] this definition excludes many brown, multicellular red and green algae, which may have tissues.

The taxonomy of protists is still changing. Newer classifications attempt to present monophyletic groups based on morphological (especially ultrastructural),[36][37][38] biochemical (chemotaxonomy)[39][40] and DNA sequence (molecular research) information.[41][42] 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, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages).[43]

Because the protists as a whole are paraphyletic, 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)[35] does not recognize formal ranks (phylum, class, etc.) and instead treats groups as clades 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.[44] Many are thought to be monophyletic, though there is still uncertainty. For instance, the Excavata are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded.[45]

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.[46]

Metabolism

Nutrition can vary according to the type of protist. Most eukaryotic algae are autotrophic, but the pigments were lost in some groups.[vague] Other protists are heterotrophic, and may present phagotrophy, osmotrophy, saprotrophy or parasitism. Some are mixotrophic. 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 Paulinella have captured a green alga (Zoochlorella) and a cyanobacterium respectively that act as replacements for chloroplast. Meanwhile, a protist, Mixotricha paradoxa that has lost its mitochondria uses endosymbiontic bacteria as mitochondria and ectosymbiontic hair-like bacteria (Treponema spirochetes) for locomotion.

Many protists are flagellate, for example, and filter feeding 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 food vacuole and digesting them internally in a process termed phagocytosis.

Nutritional types in protist metabolism
Nutritional type Source of energy Source of carbon Examples
 Photoautotrophs   Sunlight   Organic compounds or carbon fixation  Most algae 
 Chemoheterotrophs  Organic compounds   Organic compounds   Apicomplexa, Trypanosomes or Amoebae 

For most important cellular structures and functions of animal and plants, it can be found a heritage among protists.[47]

Reproduction

Some protists reproduce sexually using gametes, while others reproduce asexually by binary fission.

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.[48] 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.[49]

Eukaryotes emerged in evolution more than 1.5 billion years ago.[50] The earliest eukaryotes were likely protists. Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that sex appeared to be lacking in certain pathogenic 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, meiosis and hence sexual reproduction. For example, the common intestinal parasite Giardia lamblia 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.[51] 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.[52]

The pathogenic parasitic protists of the genus Leishmania have been shown to be capable of a sexual cycle in the invertebrate vector, likened to the meiosis undertaken in the trypanosomes.[53]

Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al.[54] 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.[54] 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.[55]

This view was further supported by a study of amoebae by Lahr et al.[56] Amoeba have generally been regarded as asexual protists. However, these authors describe evidence that most amoeboid 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.[57]

Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.[58] 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.[58]

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: felids such as domestic cats in this case).[59][60][61]

Ecology

 

Free-living protists occupy almost any environment that contains liquid water. Many protists, such as algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Protists make up a large portion of the biomass in both marine and terrestrial environments.[62]

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

Parasitism: role as pathogens

See also: Antiprotozoal agent and Apicomplexa § Parasitology_and_genomics

Some protists are significant parasites of animals (e.g.; five species of the parasitic genus Plasmodium cause malaria in humans and many others cause similar diseases in other vertebrates), plants[63][64] (the oomycete Phytophthora infestans causes late blight in potatoes)[65] or even of other protists.[66][67] Protist pathogens share many metabolic pathways with their eukaryotic hosts. This makes therapeutic target development extremely difficult – a drug that harms a protist parasite is also likely to harm its animal/plant host. A more thorough understanding of protist biology may allow these diseases to be treated more efficiently. For example, the apicoplast (a nonphotosynthetic chloroplast but essential to carry out important functions other than photosynthesis) present in apicomplexans provides an attractive target for treating diseases caused by dangerous pathogens such as plasmodium.

Recent papers have proposed the use of viruses to treat infections caused by protozoa.[68][69]

Researchers from the Agricultural Research Service are taking advantage of protists as pathogens to control red imported fire ant (Solenopsis invicta) populations in Argentina. Spore-producing protists such as Kneallhazia solenopsae (recognized as a sister clade or the closest relative to the fungus kingdom now)[70] can reduce red fire ant populations by 53–100%.[71] Researchers have also been able to infect phorid fly parasitoids 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.[72]

Fossil record

Main article: Protists in the fossil record
Further information: protist shell and microfossils

Many protists have neither hard parts nor resistant spores, and their fossils are extremely rare or unknown. Examples of such groups include the apicomplexans,[73] most ciliates,[74] some green algae (the Klebsormidiales),[75] choanoflagellates,[76] oomycetes,[77] brown algae,[78] yellow-green algae,[79] Excavata (e.g., euglenids).[80] Some of these have been found preserved in amber (fossilized tree resin) or under unusual conditions (e.g., Paleoleishmania, a kinetoplastid).

Others are relatively common in the fossil record,[81] as the diatoms,[82] golden algae,[83] haptophytes (coccoliths),[84] silicoflagellates, tintinnids (ciliates), dinoflagellates,[85] green algae,[86] red algae,[87] heliozoans, radiolarians,[88] foraminiferans,[89] ebriids and testate amoebae (euglyphids, arcellaceans).[90] 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 acritarchs, spherical fossils of likely algal protists.[91] Another possible representative of early fossil eukaryotes are the Gabonionta.

See also

Footnotes

  1. ^ a b The first eukaryotes were "neither plants, animals, nor fungi", hence as defined, the category protist would include the last eukaryotic common ancestor.
  2. ^ Monera eventually became the two domains Bacteria and Archaea.[6]
  3. ^ In the original 4-kingdom model proposed in 1959, Protista included all unicellular microorganisms such as bacteria. Herbert Copeland proposed separate kingdoms, Mychota for prokaryotes and Protoctista for eukaryotes (including fungi) that were neither plants nor animals. Copeland's distinction between prokaryotic and eukaryotic cells was eventually critical in Whittaker proposing a final five-kingdom system, even though he resisted it for over a decade.[9][10]

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Bibliography

General

  • 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[permanent dead link], Charophyta, , Chrysomonadida, Coccolithophorida, Charophyta, Diatomacea & Pyrrhophyta), (Sarcodina, Chiefly "Thecamoebians" and Foraminiferida) and part D[permanent dead link] (Chiefly Radiolaria and Tintinnina). Boulder, Colorado: Geological Society of America; & Lawrence, Kansas: University of Kansas Press.

External links

 
Wikispecies has information related to Protista.
 
Wikispecies has information related to Protoctista.
 
Wikimedia Commons has media related to Protista.
  • A java applet for exploring the new higher level classification of eukaryotes
  • Holt, Jack R. and Carlos A. Iudica. (2013). Diversity of Life. http://comenius.susqu.edu/biol/202/Taxa.htm. Last modified: 11/18/13.
  • Tree of Life: Eukaryotes
  • Tsukii, Y. (1996). Protist Information Server (database of protist images). Laboratory of Biology, Hosei University.[1]. Updated: March 22, 2016.

January 31, 2023
protist, journal, journal, protist, eukaryotic, organism, that, organism, whose, cells, contain, cell, nucleus, that, animal, plant, fungus, while, likely, that, protists, share, common, ancestor, last, eukaryotic, common, ancestor, exclusion, other, eukaryote. For the journal see Protist journal A protist ˈ p r oʊ t ɪ s t is any eukaryotic organism that is an organism whose cells contain a cell nucleus that is not an animal plant or fungus While it is likely that protists share a common ancestor the last eukaryotic common ancestor 3 the exclusion of other eukaryotes means that protists do not form a natural group or clade a Therefore some protists may be more closely related to animals plants or fungi than they are to other protists However like the groups algae invertebrates and protozoans the biological category protist is used for convenience Others classify any unicellular eukaryotic microorganism as a protist 4 The study of protists is termed protistology 5 ProtistTemporal range Paleoproterozoic a Present Pha Proterozoic Archean Had nScientific classificationDomain EukaryotaGroups includedSupergroups 1 and typical phyla Meteora sporadica 2 Amoebozoa Apusomonadida Archaeplastida in part Glaucophyta Rhodophyta red algae Breviatea Excavata Euglenozoa Metamonada Percolozoa Hacrobia Hemimastigophora Opisthokonta in part Choanoflagellatea Cristidiscoidea Filasterea Ichthyosporea Pluriformea SAR Alveolata Apicomplexa Ciliophora Dinoflagellata Rhizaria Cercozoa Foraminifera Radiolaria Stramenopiles brown algae diatoms oomycetes Many others classification variesCladistically included but traditionally excluded taxaAnimalia Metazoa Fungi Opisthosporidia Plantae Viridiplantae Contents 1 History 2 Description 3 Subdivisions 4 Classification 4 1 Historical classifications 4 2 Modern classifications 5 Metabolism 6 Reproduction 7 Ecology 7 1 Parasitism role as pathogens 8 Fossil record 9 See also 10 Footnotes 11 References 12 Bibliography 12 1 General 12 2 Physiology ecology and paleontology 13 External linksHistory EditThe classification of a third kingdom separate from animals and plants was first proposed by John Hogg in 1860 as the kingdom Protoctista in 1866 Ernst Haeckel also proposed a third kingdom Protista as the kingdom of primitive forms 6 Originally these also included prokaryotes but with time when these were removed to a fourth kingdom Monera b 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 7 8 c In the five kingdom system of Lynn Margulis the term protist is reserved for microscopic organisms while the more inclusive kingdom Protoctista or protoctists included certain large multicellular eukaryotes such as kelp red algae and slime molds 11 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 12 Description EditBesides their relatively simple levels of organization protists do not necessarily have much in common 13 When used the term protists is now considered to mean a paraphyletic assemblage of similar appearing but diverse taxa biological groups these taxa do not have an exclusive common ancestor beyond being composed of eukaryotes and have different life cycles trophic levels modes of locomotion and cellular structures 14 15 Examples of protists include 16 Amoebas including nucleariids and Foraminifera Choanoflagellates ciliates Diatoms Dinoflagellates Giardia Oomycetes including Phytophthora the cause of the Great Famine of Ireland and Plasmodium which causes malaria slime molds These examples are unicellular although oomycetes can join to form filaments and slime molds can aggregate into a tissue like mass In cladistic 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 eukaryotic branch of the tree of life In cladistic classification the contents of Protista are mostly distributed among various supergroups examples include the Archaeplastida or Plantae sensu lato Excavata which is mostly unicellular flagellates and Opisthokonta which commonly includes unicellular flagellates but also animals and fungi SAR supergroup of stramenopiles or heterokonts alveolates and Rhizaria Protista Protoctista and Protozoa 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 pathogen may be used to denote any disease causing organism that is not plant animal fungal prokaryotic viral or subviral 17 Subdivisions EditSee also Kingdom biology Summary The term Protista was first used by Ernst Haeckel in 1866 Protists were traditionally subdivided into several groups based on similarities to the higher kingdoms such as 6 Protozoa Protozoans are unicellular animal like heterotrophic and sometimes parasitic organisms that are further sub divided based on characteristics such as motility such as the flagellated Flagellata the ciliated Ciliophora the phagocytic amoeba and the spore forming Sporozoa Protophyta Protophyta are plant like autotrophic organisms that are composed mostly of unicellular algae The dinoflagellates diatoms and Euglena like flagellates are photosynthetic protists Mold Molds generally refer to fungi but slime molds and water molds are fungus like saprophytic protists although some are pathogens Two separate types of slime molds exist the cellular and acellular forms Some protists sometimes called ambiregnal protists have been considered to be both protozoa and algae or fungi e g slime molds and flagellated algae and names for these have been published under either or both of the ICN and the ICZN 18 19 Conflicts such as these for example the dual classification of Euglenids and Dinobryons which are mixotrophic 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 phylogenetics evolutionary relatedness among organisms Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related phyla For instance the water molds are now considered to be closely related to photosynthetic organisms such as Brown algae and Diatoms the slime molds are grouped mainly under Amoebozoa and the Amoebozoa itself includes only a subset of the Amoeba group and significant number of erstwhile Amoeboid genera are distributed among Rhizarians and other Phyla However the older terms are still used as informal names to describe the morphology and ecology of various protists For example the term protozoa is used to refer to heterotrophic species of protists that do not form filaments Classification EditMain article Taxonomy of Protista See also Kingdom biology Summary Historical classifications Edit Further information wikispecies Protista and wikispecies Protoctista Among the pioneers in the study of the protists which were almost ignored by Linnaeus except for some genera e g Vorticella Chaos Volvox Corallina Conferva Ulva Chara Fucus 20 21 were Leeuwenhoek O F Muller C G Ehrenberg and Felix Dujardin 22 The first groups used to classify microscopic organism were the Animalcules and the Infusoria 23 In 1818 the German naturalist Georg August Goldfuss introduced the word Protozoa to refer to organisms such as ciliates and corals 24 6 After the cell theory of Schwann and Schleiden 1838 39 this group was modified in 1848 by Carl von Siebold to include only animal like unicellular organisms such as foraminifera and amoebae 25 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 26 6 The kingdom of minerals was later removed from taxonomy in 1866 by Ernst Haeckel leaving plants animals and the protists Protista defined as a kingdom of primitive forms 27 28 In 1938 Herbert Copeland resurrected Hogg s label arguing that Haeckel s term Protista included anucleated microbes such as bacteria which the term Protoctista literally meaning first established beings did not In contrast Copeland s term included nucleated eukaryotes such as diatoms green algae and fungi 29 This classification was the basis for Whittaker s later definition of Fungi Animalia Plantae and Protista as the four kingdoms of life 9 The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera leaving the protists as a group of eukaryotic microorganisms 7 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 monophyletic groups 30 Modern classifications Edit Phylogenetic and symbiogenetic tree of living organisms showing the origins of eukaryotes Further information Cavalier Smith s system of classification and Protista taxonomy Systematists today do not treat Protista as a formal taxon but the term protist is still commonly used for convenience in two ways 31 The most popular contemporary definition is a phylogenetic one that identifies a paraphyletic group 32 a protist is any eukaryote that is not an animal land plant or true fungus this definition 33 excludes many unicellular groups like the Microsporidia fungi many Chytridiomycetes fungi and yeasts fungi and also a non unicellular group included in Protista in the past the Myxozoa animal 34 Some systematists who judge paraphyletic taxa acceptable and use Protista in this sense as a formal taxon as found in some secondary textbooks for pedagogical purpose citation needed The other definition describes protists primarily by functional or biological criteria protists are essentially those eukaryotes that are never multicellular 31 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 35 this definition excludes many brown multicellular red and green algae which may have tissues The taxonomy of protists is still changing Newer classifications attempt to present monophyletic groups based on morphological especially ultrastructural 36 37 38 biochemical chemotaxonomy 39 40 and DNA sequence molecular research information 41 42 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 cryptic species and ii one lineage multiple morphologies e g phenotypic plasticity multiple life cycle stages 43 Because the protists as a whole are paraphyletic 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 35 does not recognize formal ranks phylum class etc and instead treats groups as clades 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 44 Many are thought to be monophyletic though there is still uncertainty For instance the Excavata are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded 45 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 46 Metabolism EditNutrition can vary according to the type of protist Most eukaryotic algae are autotrophic but the pigments were lost in some groups vague Other protists are heterotrophic and may present phagotrophy osmotrophy saprotrophy or parasitism Some are mixotrophic 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 Paulinella have captured a green alga Zoochlorella and a cyanobacterium respectively that act as replacements for chloroplast Meanwhile a protist Mixotricha paradoxa that has lost its mitochondria uses endosymbiontic bacteria as mitochondria and ectosymbiontic hair like bacteria Treponema spirochetes for locomotion Many protists are flagellate for example and filter feeding 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 food vacuole and digesting them internally in a process termed phagocytosis Nutritional types in protist metabolism Nutritional type Source of energy Source of carbon Examples Photoautotrophs Sunlight Organic compounds or carbon fixation Most algae Chemoheterotrophs Organic compounds Organic compounds Apicomplexa Trypanosomes or Amoebae For most important cellular structures and functions of animal and plants it can be found a heritage among protists 47 Reproduction EditSome protists reproduce sexually using gametes while others reproduce asexually by binary fission 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 48 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 49 Eukaryotes emerged in evolution more than 1 5 billion years ago 50 The earliest eukaryotes were likely protists Although sexual reproduction is widespread among extant eukaryotes it seemed unlikely until recently that sex could be a primordial and fundamental characteristic of eukaryotes A principal reason for this view was that sex appeared to be lacking in certain pathogenic 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 meiosis and hence sexual reproduction For example the common intestinal parasite Giardia lamblia 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 51 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 52 The pathogenic parasitic protists of the genus Leishmania have been shown to be capable of a sexual cycle in the invertebrate vector likened to the meiosis undertaken in the trypanosomes 53 Trichomonas vaginalis a parasitic protist is not known to undergo meiosis but when Malik et al 54 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 54 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 55 This view was further supported by a study of amoebae by Lahr et al 56 Amoeba have generally been regarded as asexual protists However these authors describe evidence that most amoeboid 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 57 Protists generally reproduce asexually under favorable environmental conditions but tend to reproduce sexually under stressful conditions such as starvation or heat shock 58 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 58 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 felids such as domestic cats in this case 59 60 61 Ecology Edit Free living protists occupy almost any environment that contains liquid water Many protists such as algae are photosynthetic and are vital primary producers in ecosystems particularly in the ocean as part of the plankton Protists make up a large portion of the biomass in both marine and terrestrial environments 62 Other protists include pathogenic species such as the kinetoplastid Trypanosoma brucei which causes sleeping sickness and species of the apicomplexan Plasmodium which cause malaria Parasitism role as pathogens Edit See also Antiprotozoal agent and Apicomplexa Parasitology and genomics Some protists are significant parasites of animals e g five species of the parasitic genus Plasmodium cause malaria in humans and many others cause similar diseases in other vertebrates plants 63 64 the oomycete Phytophthora infestans causes late blight in potatoes 65 or even of other protists 66 67 Protist pathogens share many metabolic pathways with their eukaryotic hosts This makes therapeutic target development extremely difficult a drug that harms a protist parasite is also likely to harm its animal plant host A more thorough understanding of protist biology may allow these diseases to be treated more efficiently For example the apicoplast a nonphotosynthetic chloroplast but essential to carry out important functions other than photosynthesis present in apicomplexans provides an attractive target for treating diseases caused by dangerous pathogens such as plasmodium Recent papers have proposed the use of viruses to treat infections caused by protozoa 68 69 Researchers from the Agricultural Research Service are taking advantage of protists as pathogens to control red imported fire ant Solenopsis invicta populations in Argentina Spore producing protists such as Kneallhazia solenopsae recognized as a sister clade or the closest relative to the fungus kingdom now 70 can reduce red fire ant populations by 53 100 71 Researchers have also been able to infect phorid fly parasitoids 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 72 Fossil record EditMain article Protists in the fossil record Further information protist shell and microfossils Many protists have neither hard parts nor resistant spores and their fossils are extremely rare or unknown Examples of such groups include the apicomplexans 73 most ciliates 74 some green algae the Klebsormidiales 75 choanoflagellates 76 oomycetes 77 brown algae 78 yellow green algae 79 Excavata e g euglenids 80 Some of these have been found preserved in amber fossilized tree resin or under unusual conditions e g Paleoleishmania a kinetoplastid Others are relatively common in the fossil record 81 as the diatoms 82 golden algae 83 haptophytes coccoliths 84 silicoflagellates tintinnids ciliates dinoflagellates 85 green algae 86 red algae 87 heliozoans radiolarians 88 foraminiferans 89 ebriids and testate amoebae euglyphids arcellaceans 90 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 acritarchs spherical fossils of likely algal protists 91 Another possible representative of early fossil eukaryotes are the Gabonionta See also EditEvolution of sexual reproduction Marine protists Protist locomotion ProtistologyFootnotes Edit a b The first eukaryotes were neither plants animals nor fungi hence as defined the category protist would include the last eukaryotic common ancestor Monera eventually became the two domains Bacteria and Archaea 6 In the original 4 kingdom model proposed in 1959 Protista included all unicellular microorganisms such as bacteria Herbert Copeland proposed separate kingdoms Mychota for prokaryotes and Protoctista for eukaryotes including fungi that were neither plants nor animals Copeland s distinction between prokaryotic and eukaryotic cells was eventually critical in Whittaker proposing a final five kingdom system even though he resisted it for over a decade 9 10 References Edit Adl SM Simpson AG Lane CE Lukes J Bass D Bowser SS Brown MW Burki F Dunthorn M Hampl V Heiss A Hoppenrath M Lara E Le Gall L Lynn DH McManus H Mitchell EA Mozley Stanridge SE Earnest H Aurther T Parfrey LW Pawlowski J Rueckert S Shadwick L Shadwick L Schoch CL Smirnov A Spiegel FW September 2012 The revised classification of eukaryotes PDF The Journal of Eukaryotic Microbiology 59 5 429 493 doi 10 1111 j 1550 7408 2012 00644 x PMC 3483872 PMID 23020233 Archived from the original PDF on 2016 06 16 Retrieved 2015 09 29 Galindo Luis Javier Lopez Garcia Purificacion Moreira David 2022 First Molecular Characterization of the Elusive Marine Protist Meteora sporadica Protist 173 4 125896 doi 10 1016 j protis 2022 125896 ISSN 1434 4610 PMID 35841658 S2CID 250059723 O Malley Maureen A Leger Michelle M Wideman Jeremy G Ruiz Trillo Inaki 2019 02 18 Concepts of the last eukaryotic common ancestor Nature Ecology amp Evolution Springer Science and Business Media LLC 3 3 338 344 doi 10 1038 s41559 019 0796 3 hdl 10261 201794 ISSN 2397 334X PMID 30778187 S2CID 67790751 Madigan Michael T 2019 Brock biology of microorganisms Fifteenth Global ed NY NY p 594 ISBN 9781292235103 Taylor F J R M 2003 11 01 The collapse of the two kingdom system the rise of protistology and the founding of the International Society for Evolutionary Protistology ISEP International Journal of Systematic and Evolutionary Microbiology Microbiology Society 53 6 1707 1714 doi 10 1099 ijs 0 02587 0 ISSN 1466 5026 PMID 14657097 a b c d e Scamardella JM 1999 Not plants or animals A brief history of the origin of Kingdoms Protozoa Protista and Protoctista PDF International Microbiology 2 4 207 221 PMID 10943416 a b Whittaker RH January 1969 New concepts of kingdoms or organisms Evolutionary relations are better represented by new classifications than by the traditional two kingdoms Science 163 3863 150 160 Bibcode 1969Sci 163 150W CiteSeerX 10 1 1 403 5430 doi 10 1126 science 163 3863 150 PMID 5762760 whittaker new concepts of kingdoms Google Scholar scholar google ca Retrieved 2016 02 28 a b Whittaker RH 1959 On the Broad Classification of Organisms Quarterly Review of Biology 34 3 210 226 doi 10 1086 402733 JSTOR 2816520 PMID 13844483 S2CID 28836075 Hagen JB 2012 depiction of Whittaker s early four kingdom system based on three modes of nutrition and the distinction between unicellular and multicellular body plans BioScience 62 67 74 doi 10 1525 bio 2012 62 1 11 Margulis L Chapman MJ 2009 03 19 Kingdoms and Domains An Illustrated Guide to the Phyla of Life on Earth Academic Press ISBN 9780080920146 Archibald John M Simpson Alastair G B Slamovits Claudio H eds 2017 Handbook of the Protists 2 ed Springer International Publishing pp ix ISBN 978 3 319 28147 6 Systematics of the Eukaryota Retrieved 2009 05 31 Simonite T November 2005 Protists push animals aside in rule revamp Nature 438 7064 8 9 Bibcode 2005Natur 438 8S doi 10 1038 438008b PMID 16267517 Harper D Benton Michael 2009 Introduction to Paleobiology and the Fossil Record Wiley Blackwell p 207 ISBN 978 1 4051 4157 4 Protists basicbiology net basicbiology net 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Hogg John 1860 On the distinctions of a plant and an animal and on a fourth kingdom of nature Edinburgh New Philosophical Journal 2nd series 12 216 225 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 prwtos first and xtista created beings Rothschild Lynn J 1989 Protozoa Protista Protoctista what s in a name Journal of the History of Biology 22 2 277 305 doi 10 1007 BF00139515 PMID 11542176 S2CID 32462158 Haeckel Ernst 1866 Generelle Morphologie der Organismen The General Morphology of Organisms in German Vol 1 Berlin Germany G Reimer pp 215ff From p 215 VII Character des Protistenreiches VII Character of the kingdom of Protists Copeland HF 1938 The Kingdoms of Organisms Quarterly Review of Biology 13 4 383 420 doi 10 1086 394568 JSTOR 2808554 S2CID 84634277 Stechmann A Cavalier Smith T September 2003 The root of the 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PMID 17010206 ARS Parasite Collections Assist Research and Diagnoses USDA Agricultural Research Service January 28 2010 Durham Sharon January 28 2010 ARS Parasite Collections Assist Research and Diagnoses Ars usda gov Retrieved 2014 03 20 Introduction to the Apicomplexa Ucmp berkeley edu Retrieved 2014 03 20 Fossil Record of the Ciliata Ucmp berkeley edu Retrieved 2014 03 20 Klebsormidiales Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Choanoflagellata Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Oomycota Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Phaeophyta Archived 2008 12 21 at the Wayback Machine Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Xanthophyta Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Basal Eukaryotes Ucmp berkeley edu Retrieved 2014 03 20 Why Is The Museum On The Web Ucmp berkeley edu Retrieved 2014 03 20 Fossil Record of Diatoms Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Chrysophyta Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Prymnesiophyta Ucmp berkeley edu Retrieved 2014 03 20 Fossil Record of the Dinoflagellata Ucmp berkeley edu Retrieved 2014 03 20 Systematics of the Green Algae Part 1 Ucmp berkeley edu Retrieved 2014 03 20 Fossil Record of the Rhodophyta Ucmp berkeley edu Retrieved 2014 03 20 Fossil Record of the Radiolaria Ucmp berkeley edu Retrieved 2014 03 20 Fossil Record of Foraminifera Ucmp berkeley edu Retrieved 2014 03 20 Introduction to the Testaceafilosea Ucmp berkeley edu Retrieved 2014 03 20 Fossil Record of the Eukaryota Ucmp berkeley edu Retrieved 2014 03 20 Bibliography EditGeneral Edit 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 Edit 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 permanent dead link Charophyta vol 2 Chrysomonadida Coccolithophorida Charophyta Diatomacea amp Pyrrhophyta part C Sarcodina Chiefly Thecamoebians and Foraminiferida and part D permanent dead link Chiefly Radiolaria and Tintinnina Boulder Colorado Geological Society of America amp Lawrence Kansas University of Kansas Press External links Edit Wikispecies has information related to Protista Wikispecies has information related to Protoctista Wikimedia Commons has media related to Protista A java applet for exploring the new higher level classification of eukaryotes Holt Jack R and Carlos A Iudica 2013 Diversity of Life http comenius susqu edu biol 202 Taxa htm Last modified 11 18 13 Plankton Chronicles Protists Cells in the Sea video Tree of Life Eukaryotes Tsukii Y 1996 Protist Information Server database of protist images Laboratory of Biology Hosei University 1 Updated March 22 2016 Retrieved from https en wikipedia org w index php title Protist amp oldid 1133800650, wikipedia, wiki, book, books, library,

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