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{{short description|Unicellular organism of a diverse group of eukaryotic microorganisms}}{{for|the journal|Protist (journal)}}{{pp-move-indef}}{{Paraphyletic group| name = !Nutritional type!Source of energy!Source of carbon!Examples| Photoautotrophs | Chemoheterotrophs
2100Paleoproterozoic{{efnHolocene>Present}}| image = Protist collage 2.jpg| image_alt = | image_caption = | auto = yes| parent = Eukaryota| includes = SupergroupsJOURNAL, Adl SM, Simpson AG, Lane CE, Lukeš 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, Parfrey LW, Pawlowski J, Rueckert S, Shadwick L, Shadwick L, Schoch CL, Smirnov A, Spiegel FW, The revised classification of eukaryotes, The Journal of Eukaryotic Microbiology, 59, 5, 429–93, September 2012, 23020233, 3483872, 10.1111/j.1550-7408.2012.00644.x,weblink and typical phyla
Many others;classification varies| excludes = }}A protist ({{IPAc-en|ˈ|p|r|oʊ|t|ᵻ|s|t}}) is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant or fungus. The protists do not form a natural group, or clade, since they exclude certain eukaryotes; but, like algae or invertebrates, they are often grouped together for convenience. In some systems of biological classification, such as the popular five-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues".JOURNAL, Whittaker RH, New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms, Science, 163, 3863, 150–60, January 1969, 5762760, 10.1126/science.163.3863.150, 1969Sci...163..150W, 10.1.1.403.5430, WEB,weblink whittaker new concepts of kingdoms – Google Scholar, scholar.google.ca, 2016-02-28, {{efn-ua|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.JOURNAL, 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, 10.1525/bio.2012.62.1.11, 2012, Hagen, Joel B., vanc, }}Besides their relatively simple levels of organization, protists do not necessarily have much in common.WEB,weblink Systematics of the Eukaryota, 2009-05-31, 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.JOURNAL, Simonite T, Protists push animals aside in rule revamp, Nature, 438, 7064, 8–9, November 2005, 16267517, 10.1038/438008b, 2005Natur.438....8S, BOOK, Harper, David, vanc, David Harper (palaeontologist), Benton, Michael, Michael Benton, Introduction to Paleobiology and the Fossil Record, Wiley-Blackwell, 2009, 207, 978-1-4051-4157-4, In the classification system of Lynn Margulis, the term protist is reserved for microscopic organisms, while the more inclusive term Protoctista is applied to a biological kingdom that includes certain large multicellular eukaryotes, such as kelp, red algae and slime molds.BOOK,weblink Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth, Margulis, Lynn, Chapman, Michael J., vanc, 2009-03-19, Academic Press, 9780080920146, Others use the term protist more broadly, to encompass both microbial eukaryotes and macroscopic organisms that do not fit into the other traditional kingdoms.In cladistic systems (classifications based on common ancestry), there are no equivalents to the taxa Protista or Protoctista, both terms referring to a paraphyletic group that spans the entire eukaryotic tree of life. In cladistic classification, the contents of Protista are distributed among various supergroups (SAR, such as protozoa and some algae, Archaeplastida, such as land plants and some algae, Excavata, which are a group of unicellular organisms, and Opisthokonta, such as animals and fungi, etc.). "Protista", Protoctista and "Protozoa" are considered obsolete. However, the term "protist" continues to be used informally as a catch-all term for unicellular eukaryotic microorganisms. For example, the word "protist pathogen" may be used to denote any disease-causing microbe that is not bacteria, virus, viroid, prion, or metazoa.JOURNAL, Siddiqui R, Kulsoom H, Lalani S, Khan NA, Isolation of Balamuthia mandrillaris-specific antibody fragments from a bacteriophage antibody display library, Experimental Parasitology, 166, 94–6, July 2016, 27055361, 10.1016/j.exppara.2016.04.001,

Subdivisions

{{see_also|Eukaryote#Five supergroups}}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:
Protozoa: These unicellular "animal-like" (heterotrophic, and sometimes parasitic) organisms are further sub-divided based on characteristics such as motility, such as the (flagellated) (Flagellate#Flagellates as organisms: the Flagellata|Flagellata), the (ciliated) Ciliophora, the (phagocytic) amoeba, and the (spore-forming) Sporozoa.
Protophyta: These "plant-like" (autotrophic) organisms are composed mostly of unicellular algae.
Molds: Slime molds and water molds are "fungus-like" (saprophytic) organisms.
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.JOURNAL, Corliss, J.O., 1995,weblink The ambiregnal protists and the codes of nomenclature: a brief review of the problem and of proposed solutions, Bulletin of Zoological Nomenclature, 52, 11–17, 10.5962/bhl.part.6717, Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell. p. 41. {{ISBN|978-0-632-04761-1}}. 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 "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

{{see_also|Kingdom (biology)#Summary}}

Historical classifications

{{further|wikispecies:Protista|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)Ratcliff, Marc J. (2009). "The Emergence of the Systematics of Infusoria". In: The Quest for the Invisible: Microscopy in the Enlightenment. Aldershot: Ashgate.Sharma, O. P. (1986). Textbook of Algae. McGraw Hill. p. 22. were Leeuwenhoek, 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 Animalcules and the Infusoria.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 Protozoa to refer to organisms such as ciliates and corals.JOURNAL, Goldfuß, Ueber die Classification der Zoophyten, Isis, Oder, Encyclopädische Zeitung von Oken, 1818, 2, 6, 1008–1019,weblink On the classification of zoophytes, German, 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.]JOURNAL, Scamardella JM, Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista, 1999, International Microbiology, 2, 207–221,weblink 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.BOOK, Siebold (vol. 1), Stannius (vol. 2), Lehrbuch der vergleichenden Anatomie [Textbook of Comparative Anatomy], 1848, Veit & Co., Berlin, (Germany), vol. 1: Wirbellose Thiere (Invertebrate animals), 3,weblink German, 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.JOURNAL, Hogg, John, 1860, On the distinctions of a plant and an animal, and on a fourth kingdom of nature,weblink 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 πρώτος, first, and χτιστά, created beings; … " 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”.JOURNAL, Rothschild LJ, Protozoa, Protista, Protoctista: what's in a name?, Journal of the History of Biology, 22, 2, 277–305, 1989, 11542176, 10.1007/BF00139515, BOOK, Haeckel, Ernst, Generelle Morphologie der Organismen, The General Morphology of Organisms, 1866, G. Reimer, Berlin, (Germany), 1, 215ff,weblink German, From p. 215: "VII. Character des Protistenreiches." (VII. Character of the kingdom of Protists.)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.JOURNAL, Copeland HF, 1938, The Kingdoms of Organisms, Quarterly Review of Biology, 13, 4, 383–420, 10.1086/394568, 2808554, This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life.JOURNAL, Whittaker RH, 1959, On the Broad Classification of Organisms, Quarterly Review of Biology, 34, 3, 210–226, 10.1086/402733, 2816520, The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera, 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 monophyletic groups).JOURNAL, Stechmann A, Cavalier-Smith T, The root of the eukaryote tree pinpointed, Current Biology, 13, 17, R665–6, September 2003, 12956967, 10.1016/S0960-9822(03)00602-X,weblink

Modern classifications

(File:Tree of Living Organisms 2.png|thumb|right|250px|Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes)Systematists 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 phylogenetic one, that identifies a paraphyletic group:JOURNAL, 10.1016/j.ode.2006.11.001, Protists – A textbook example for a paraphyletic taxon☆, Organisms Diversity & Evolution, 7, 2, 166–172, 2007, Schlegel, M., Hulsmann, N., a protist is any eukaryote that is not an animal, (land) plant, or (true) fungus; this definitionWEB,weblink Protista, microbeworld.org, 11 June 2016, 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).JOURNAL, Štolc A, Actinomyxidies, nouveau groupe de Mesozoaires parent des Myxosporidies, 1899, Bull. Int. l'Acad. Sci. Bohème, 12, 1–12, Some systematists{{who|date=April 2018}} 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|date=April 2018}}The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular,JOURNAL, 10.1007/s10539-012-9354-y, The other eukaryotes in light of evolutionary protistology, Biology & Philosophy, 28, 2, 299–330, 2012, O'Malley MA, Simpson AG, Roger AJ, 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);JOURNAL, Adl SM, Simpson AG, Farmer MA, Andersen RA, Anderson OR, Barta JR, Bowser SS, Brugerolle G, Fensome RA, Fredericq S, James TY, Karpov S, Kugrens P, Krug J, Lane CE, Lewis LA, Lodge J, Lynn DH, Mann DG, McCourt RM, Mendoza L, Moestrup O, Mozley-Standridge SE, Nerad TA, Shearer CA, Smirnov AV, Spiegel FW, Taylor MF, The new higher level classification of eukaryotes with emphasis on the taxonomy of protists, The Journal of Eukaryotic Microbiology, 52, 5, 399–451, 2005, 16248873, 10.1111/j.1550-7408.2005.00053.x, 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),Pitelka, D. R. (1963). Electron-Microscopic Structure of Protozoa. Pergamon Press, Oxford.Berner, T. (1993). Ultrastructure of Microalgae. Boca Raton: CRC Press. {{ISBN|0849363233}}Beckett, A., Heath, I. B., and Mclaughlin, D. J. (1974). An Atlas of Fungal Ultrastructure. Longman, Green, New York. biochemical (chemotaxonomy)Ragan M.A. & Chapman D.J. (1978). A Biochemical Phylogeny of the Protists. London, New York: Academic Press. {{ISBN|0323155618}}Lewin R. A. (1974). "Biochemical taxonomy", pp. 1–39 in Algal Physiology and Biochemistry, Stewart W. D. P. (ed.). Blackwell Scientific Publications, Oxford. {{ISBN|0520024109}} and DNA sequence (molecular research) information.Oren, A., & Papke, R. T. (2010). Molecular phylogeny of microorganisms. Norfolk, UK: Caister Academic Press. {{ISBN|1904455670}}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. {{ISBN|0203508939}} 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).JOURNAL, Lahr DJ, Laughinghouse HD, Oliverio AM, Gao F, Katz LA, How discordant morphological and molecular evolution among microorganisms can revise our notions of biodiversity on Earth, BioEssays, 36, 10, 950–9, October 2014, 25156897, 4288574, 10.1002/bies.201400056,weblink 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) 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.JOURNAL, Cavalier-Smith T, Chao EE, Phylogeny and classification of phylum Cercozoa (Protozoa), Protist, 154, 3–4, 341–58, October 2003, 14658494, 10.1078/143446103322454112, 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.JOURNAL, Parfrey LW, Barbero E, Lasser E, Dunthorn M, Bhattacharya D, Patterson DJ, Katz LA, Evaluating support for the current classification of eukaryotic diversity, PLoS Genetics, 2, 12, e220, December 2006, 17194223, 1713255, 10.1371/journal.pgen.0020220,

Metabolism

Nutrition can vary according to the type of protist. Most eukaryotic algae are autotrophic, but the pigments were lost in some groups.{{vague|reason=which pigments, in which groups?|date=April 2018}} 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.{|class="wikitable" style="margin-left: auto; margin-right: auto;"
Primary nutritional groups>Nutritional types in protist metabolism
 Sunlight  Organic compounds or carbon fixation| Most algae 
 Organic compounds  Organic compounds Apicomplexa, Trypanosomes or Amoeboid>Amoebae 
For most important cellular structures and functions of animal and plants, it can be found a heritage among protists.Plattner, H. (2018). Evolutionary cell biology of proteins from protists to humans and plants. J. Eukaryot. Microbiol. 65(2): 255-289.

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.JOURNAL, Talman AM, Domarle O, McKenzie FE, Ariey F, Robert V, Gametocytogenesis: the puberty of Plasmodium falciparum, Malaria Journal, 3, 24, July 2004, 15253774, 497046, 10.1186/1475-2875-3-24, 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.JOURNAL, Tibayrenc M, Kjellberg F, Arnaud J, Oury B, Brenière SF, Dardé ML, Ayala FJ, Are eukaryotic microorganisms clonal or sexual? A population genetics vantage, Proceedings of the National Academy of Sciences of the United States of America, 88, 12, 5129–33, June 1991, 1675793, 51825, 10.1073/pnas.88.12.5129, 1991PNAS...88.5129T, 1, Eukaryotes emerged in evolution more than 1.5 billion years ago.JOURNAL, Javaux EJ, Knoll AH, Walter MR, Morphological and ecological complexity in early eukaryotic ecosystems, Nature, 412, 6842, 66–9, July 2001, 11452306, 10.1038/35083562, 2001Natur.412...66J, 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.JOURNAL, Ramesh MA, Malik SB, Logsdon JM, A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis, Current Biology, 15, 2, 185–91, January 2005, 15668177, 10.1016/j.cub.2005.01.003, 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.JOURNAL, Cooper MA, Adam RD, Worobey M, Sterling CR, Population genetics provides evidence for recombination in Giardia, Current Biology, 17, 22, 1984–8, November 2007, 17980591, 10.1016/j.cub.2007.10.020, 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.JOURNAL, Akopyants NS, Kimblin N, Secundino N, Patrick R, Peters N, Lawyer P, Dobson DE, Beverley SM, Sacks DL, Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector, Science, 324, 5924, 265–8, April 2009, 19359589, 2729066, 10.1126/science.1169464, 2009Sci...324..265A, 1, Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al.JOURNAL, Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM, An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis, PLOS ONE, 3, 8, e2879, August 2007, 18663385, 2488364, 10.1371/journal.pone.0002879, Hahn, 2008PLoSO...3.2879M, Matthew W, 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.JOURNAL, Dacks J, Roger AJ, The first sexual lineage and the relevance of facultative sex, Journal of Molecular Evolution, 48, 6, 779–83, June 1999, 10229582, 10.1007/PL00013156, 1999JMolE..48..779D, This view was further supported by a study of amoebae by Lahr et al.JOURNAL, Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E, The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms, Proceedings: Biological Sciences, 278, 1715, 2081–90, July 2011, 21429931, 3107637, 10.1098/rspb.2011.0289, 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.JOURNAL, Dobell, C., 1909, Chromidia and the binuclearity hypotheses: a review and a criticism, Quarterly Journal of Microscopical Science, 53, 279–326,weblink 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. {{ISBN|978-1-62100-808-8}} 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: felids such as domestic cats in this case).WEB, CDC – Toxoplasmosis – Biology,weblink 17 March 2015, 14 June 2015, WEB, Cat parasite linked to mental illness, schizophrenia,weblink CBS, 23 September 2015, WEB, CDC – About Parasites,weblink 12 March 2013,

Ecology

(File:Biomass by life form.jpg|thumb|442x442px)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.JOURNAL, Bar-On, Yinon M., Phillips, Rob, Milo, Ron, 2018-05-17, The biomass distribution on Earth, Proceedings of the National Academy of Sciences, 115, 25, en, 6506–6511, 10.1073/pnas.1711842115, 0027-8424, 29784790, 6016768, 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|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), plantsJOURNAL, Schwelm A, Badstöber J, Bulman S, Desoignies N, Etemadi M, Falloon RE, Gachon CM, Legreve A, Lukeš J, Merz U, Nenarokova A, Strittmatter M, Sullivan BK, Neuhauser S, Not in your usual Top 10: protists that infect plants and algae, Molecular Plant Pathology, 19, 4, 1029–1044, April 2018, 29024322, 5772912, 10.1111/mpp.12580, JOURNAL, Kamoun S, Furzer O, Jones JD, Judelson HS, Ali GS, Dalio RJ, Roy SG, Schena L, Zambounis A, Panabières F, Cahill D, Ruocco M, Figueiredo A, Chen XR, Hulvey J, Stam R, Lamour K, Gijzen M, Tyler BM, Grünwald NJ, Mukhtar MS, Tomé DF, Tör M, Van Den Ackerveken G, McDowell J, Daayf F, Fry WE, Lindqvist-Kreuze H, Meijer HJ, Petre B, Ristaino J, Yoshida K, Birch PR, Govers F, The Top 10 oomycete pathogens in molecular plant pathology, Molecular Plant Pathology, 16, 4, 413–34, May 2015, 25178392, 10.1111/mpp.12190, (the oomycete Phytophthora infestans causes late blight in potatoes)Campbell, N. and Reese, J. (2008) Biology. Pearson Benjamin Cummings; 8 ed. {{ISBN|0805368442}}. 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 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.JOURNAL, Keen EC, Beyond phage therapy: virotherapy of protozoal diseases, Future Microbiology, 8, 7, 821–3, July 2013, 23841627, 10.2217/FMB.13.48, JOURNAL, Hyman P, Atterbury R, Barrow P, Fleas and smaller fleas: virotherapy for parasite infections, Trends in Microbiology, 21, 5, 215–20, May 2013, 23540830, 10.1016/j.tim.2013.02.006, 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)JOURNAL, Liu YJ, Hodson MC, Hall BD, Loss of the flagellum happened only once in the fungal lineage: phylogenetic structure of kingdom Fungi inferred from RNA polymerase II subunit genes, BMC Evolutionary Biology, 6, 74, September 2006, 17010206, 1599754, 10.1186/1471-2148-6-74, can reduce red fire ant populations by 53–100%.WEB,weblink ARS Parasite Collections Assist Research and Diagnoses, USDA Agricultural Research ServicePhoridae>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.Durham, Sharon (January 28, 2010) ARS Parasite Collections Assist Research and Diagnoses. Ars.usda.gov. Retrieved 2014-03-20.

Fossil record

Many protists have neither hard parts nor resistant spores, and their fossils are extremely rare or unknown. Examples of such groups include the apicomplexans,Introduction to the Apicomplexa. Ucmp.berkeley.edu. Retrieved 2014-03-20. most ciliates,Fossil Record of the Ciliata. Ucmp.berkeley.edu. Retrieved 2014-03-20. some green algae (the Klebsormidiales),Klebsormidiales. Ucmp.berkeley.edu. Retrieved 2014-03-20. choanoflagellates,Introduction to the Choanoflagellata. Ucmp.berkeley.edu. Retrieved 2014-03-20. oomycetes,Introduction to the Oomycota. Ucmp.berkeley.edu. Retrieved 2014-03-20. brown algae,Introduction to the Phaeophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. yellow-green algae,Introduction to the Xanthophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. Excavata (e.g., euglenids).Introduction to the Basal Eukaryotes. Ucmp.berkeley.edu. Retrieved 2014-03-20. 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,Why Is The Museum On The Web?. Ucmp.berkeley.edu. Retrieved 2014-03-20. as the diatoms,Fossil Record of Diatoms. Ucmp.berkeley.edu. Retrieved 2014-03-20. golden algae,Introduction to the Chrysophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. haptophytes (coccoliths),Introduction to the Prymnesiophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. silicoflagellates, tintinnids (ciliates), dinoflagellates,Fossil Record of the Dinoflagellata. Ucmp.berkeley.edu. Retrieved 2014-03-20. green algae,Systematics of the "Green Algae", Part 1. Ucmp.berkeley.edu. Retrieved 2014-03-20. red algae,Fossil Record of the Rhodophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. heliozoans, radiolarians,Fossil Record of the Radiolaria. Ucmp.berkeley.edu. Retrieved 2014-03-20. foraminiferans,Fossil Record of Foraminifera. Ucmp.berkeley.edu. Retrieved 2014-03-20. ebriids and testate amoebae (euglyphids, arcellaceans).Introduction to the Testaceafilosea. Ucmp.berkeley.edu. 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 acritarchs, spherical fossils of likely algal protists.Fossil Record of the Eukaryota. Ucmp.berkeley.edu. Retrieved 2014-03-20. Another possible representative of early fossil eukaryotes are the Gabonionta.

See also

Notes

{{reflist|group=upper-alpha}}{{notes| notes ={{efn| name= origin|Since the first eukaryotes were unicellular, they were neither plants, animals, nor fungi.}}}

References

{{Reflist|35em}}

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, 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.

External links

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