Protist Temporal range: Neoproterozoic - Recent Scientific classification Domain: Eukarya Kingdom: Protista* Haeckel, 1866 Typical phyla Chromalveolata Heterokontophyta Haptophyta Cryptophyta (cryptomonads) Alveolata Dinoflagellata Apicomplexa Ciliophora (ciliates) Excavata Euglenozoa Percolozoa Metamonada Rhizaria Radiolaria Foraminifera Cercozoa Archaeplastida (in part) Rhodophyta (red algae) Glaucophyta (basal archaeplastids) Unikonta (in part) Amoebozoa Choanozoa Many others; classification varies Protists (pronounced /ˈproʊtɨst/) are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy.1 Instead, it is "better regarded as a loose grouping of 30 or 40 disparate phyla with diverse combinations of trophic modes, mechanisms of motility, cell coverings and life cycles."2 The protists do not have much in common besides a relatively simple organization3—either they are unicellular, or they are multicellular without specialized tissues. This simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants. 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: the one-celled animal-like protozoa, the plant-like protophyta (mostly one-celled algae), and the fungus-like slime molds and water molds. Because these groups often overlap, they have been replaced by phylogenetic-based classifications. However, they are still useful as informal names for describing the morphology and ecology of protists.

Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protists, such as the Kinetoplastids and Apicomplexa, are responsible for a range of serious human diseases, such as malaria and sleeping sickness. Contents 1 Classification 1.1 Historical classifications 1.2 Modern classifications 2 Metabolism 3 Reproduction 4 Role as pathogens 5 See also 6 References 7 External links // Classification Historical classifications The first division of the protists from other organisms came in the 1830s, when the German biologist Georg A. Goldfuss introduced the word protozoa to refer to organisms such as ciliates and corals.4 This group was expanded in 1845 to include all "unicellular animals", such as Foraminifera and amoebae. 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.4 The kingdom of minerals was later removed from taxonomy by Ernst Haeckel, leaving plants, animals, and the protists as a “kingdom of primitive forms”.5

Herbert Copeland resurrected Hogg's label almost a century later, arguing that "Protoctista" literally meant "first established beings", Copeland complained that Haeckel's term protista included anucleated microbes such as bacteria. Copeland's use of the term protoctista did not. In contrast, Copeland's term included nucleated eukaryotes such as diatoms, green algae and fungi.6 This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life.7 The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera, leaving the protists as a group of eukaryotic microorganisms.8 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). Modern classifications Currently, the term protist is used to refer to unicellular eukaryotes that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues.9 The term protozoa is used to refer to heterotrophic species of protists that do not form filaments. These terms are not used in current taxonomy, and are retained only as convenient ways to refer to these organisms.

The taxonomy of protists is still changing. Newer classifications attempt to present monophyletic groups based on ultrastructure, biochemistry, and genetics. Because the protists as a whole are paraphyletic, such 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)9 is an example that does not bother with formal ranks (phylum, class, etc.) and instead lists organisms in hierarchical lists. 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 at right.10 Many are thought to be monophyletic, though there is still uncertainty. For instance, the excavates are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded.11 Metabolism Nutrition in some different types of protists is variable. In flagellates, for example, filter feeding may sometimes occur where the flagella find the prey. Other protists can engulf bacteria and digest them internally, by extending their cell membrane around the food material to form a food vacuole. This is then taken into the cell via endocytosis (usually phagocytosis; sometimes pinocytosis).

Nutritional types in protist metabolism Nutritional type Source of energy Source of carbon Examples  Phototrophs   Sunlight   Organic compounds or carbon fixation  Algae, Dinoflagellates or Euglena   Organotrophs  Organic compounds   Organic compounds   Apicomplexa, Trypanosomes or Amoebae  Reproduction Some protists reproduce sexually (conjugation), while others reproduce asexually (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.12 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.13 Role as pathogens Some protists are significant pathogens of both animals and plants; for example Plasmodium falciparum, which causes malaria in humans, and Phytophthora infestans, which causes potato blight. A more thorough understanding of protist biology may allow these diseases to be treated more efficiently. Researchers from the Agricultural Research Service are taking advantage of protists as pathogens in an effort to control red imported fire ant (Solenopsis invicta) populations in Argentina. With the help of spore-producing protists such as Kneallhazia solenopsae the red fire ant populations can be reduced by 53-100%.14 Researchers have also found a way to infect phorid flies with the protist without harming the flies. This is important because the flies act as a vector to infect the red fire ant population with the pathogenic protist.[1] See also Protistology References ^ Simonite T (November 2005). "Protists push animals aside in rule revamp". Nature 438 (7064): 8–9. doi:10.1038/438008b. PMID 16267517.  ^ Harper, David; Benton, Michael (2009). Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell. pp. 207. ISBN 1-4051-4157-3.  ^ "Systematics of the Eukaryota". http://www.ucmp.berkeley.edu/alllife/eukaryotasy.html. Retrieved 2009-05-31.  ^ a b Scamardella, J. M. (1999). "Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista". International Microbiology 2: 207–221. http://www.im.microbios.org/08december99/03%20Scamardella.pdf.  ^ Rothschild LJ (1989). "Protozoa, Protista, Protoctista: what's in a name?". J Hist Biol 22 (2): 277–305. doi:10.1007/BF00139515. PMID 11542176. http://www.springerlink.com/index/LW54T61737212643.pdf.  ^ Copeland, H. F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology 13 (4): 383. doi:10.1086/394568. http://links.jstor.org/sici?sici=0033-5770(193812)13%3A4%3C383%3ATKOO%3E2.0.CO%3B2-K.  ^ Whittaker, R. H. (1959). "On the Broad Classification of Organisms". Quarterly Review of Biology 34 (3): 210. doi:10.1086/402733. http://links.jstor.org/sici?sici=0033-5770(195909)34%3A3%3C210%3AOTBCOO%3E2.0.CO%3B2-J.  ^ 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 (journal) 163 (863): 150–60. doi:10.1126/science.163.3863.150. PMID 5762760.  ^ a b Adl SM, Simpson AG, Farmer MA, et al. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". J. Eukaryot. Microbiol. 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873.  ^ Cavalier-Smith T, Chao EE (October 2003). "Phylogeny and classification of phylum Cercozoa (Protozoa)". Protist 154 (3-4): 341–58. doi:10.1078/143446103322454112. PMID 14658494.  ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (2006 December). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMID 17194223.  ^ Talman AM, Domarle O, McKenzie FE, Ariey F, Robert V (July 2004). "Gametocytogenesis: the puberty of Plasmodium falciparum". Malar. J. 3: 24. doi:10.1186/1475-2875-3-24. PMID 15253774.  ^ Tibayrenc M, Kjellberg F, Arnaud J, et al. (June 1991). "Are eukaryotic microorganisms clonal or sexual? A population genetics vantage". Proc. Natl. Acad. Sci. U.S.A. 88 (12): 5129–33. doi:10.1073/pnas.88.12.5129. PMID 1675793. PMC 51825. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=1675793.  ^ "ARS Parasite Collections Assist Research and Diagnoses". USDA Agricultural Research Service. January 28, 2010. http://www.ars.usda.gov/is/pr/2010/100128.htm.  Wikimedia Commons has media related to: Protista External links Wikispecies has information related to: Protista Tree of Life: Eukaryotes A java applet for exploring the new higher level classification of eukaryotes v · d · eEukaryota Domain : Archaea · Bacteria · Eukaryota Bikonta AH/SAR AH Archaeplastida, or Plantae sensu lato Viridiplantae/Plantae sensu stricto · Rhodophyta · Glaucocystophyceae Hacrobia, or non-SAR chromalveolata Haptophyta · Cryptophyta · Centroheliozoa SAR Halvaria Heterokont ("S") Ochrophyta · Bigyra · Pseudofungi Alveolata Ciliates · Myzozoa (Apicomplexa, Dinoflagellata) Rhizaria Cercozoa · Retaria (Foraminifera, Radiolaria) Excavata Discoba (Euglenozoa, Percolozoa) · Metamonad · Malawimonas Unikonta Apusozoa Apusomonadida (Apusomonas, Amastigomonas) · Ancyromonadida (Ancyromonas) · Hemimastigida (Hemimastix, Spironema, Stereonema) Amoebozoa Lobosea · Conosa · Phalansterium · Breviata Opisthokonta Holozoa Mesomycetozoea Dermocystida · Ichthyophonida Filozoa Filasterea Capsaspora · Ministeria Choanoflagellatea Codonosigidae Metazoa or "Animalia" Eumetazoa (Bilateria, Cnidaria, Ctenophora) · Mesozoa · Parazoa (Placozoa, Porifera) Holomycota Fungi Dikarya (Ascomycota, Basidiomycota) · Glomeromycota · Zygomycota · Blastocladiomycota · Chytridiomycota/Neocallimastigomycota · Microsporidia Nucleariidae Nuclearia · Micronuclearia · Rabdiophrys · Pinaciophora · Pompholyxophrys · Fonticula v · d · eMicrobiology: Protist Protozoa locomotion Flagellate (Flagellum) · Amoeboid (Pseudopod) · Ciliate (Cilium) Other structures/organelles Chromalveolate

Ciliates: Macronucleus · Micronucleus Apicomplexans: Rhoptry · Apicoplast · Microneme Excavate Kinetoplastids: Kinetoplast · Glycosome Multiple groups Hydrogenosome M: PRO ambz, excv, chrm (strc) ambz, excv, chrm ambz, excv, chrm

Ciliates: Macronucleus · Micronucleus Apicomplexans: Rhoptry · Apicoplast · Microneme Excavate Kinetoplastids: Kinetoplast · Glycosome Multiple groups Hydrogenosome M: PRO ambz, excv, chrm (strc) ambz, excv, chrm ambz, excv, chrm

Ciliates: Macronucleus · Micronucleus Apicomplexans: Rhoptry · Apicoplast · Microneme Excavate Kinetoplastids: Kinetoplast · Glycosome Multiple groups Hydrogenosome M: PRO ambz, excv, chrm (strc) ambz, excv, chrm ambz, excv, chrm

Ciliates: Macronucleus · Micronucleus Apicomplexans: Rhoptry · Apicoplast · Microneme Excavate Kinetoplastids: Kinetoplast · Glycosome Multiple groups Hydrogenosome M: PRO ambz, excv, chrm (strc) ambz, excv, chrm ambz, excv, chrm

Ciliates: Macronucleus · Micronucleus Apicomplexans: Rhoptry · Apicoplast · Microneme Excavate Kinetoplastids: Kinetoplast · Glycosome Multiple groups Hydrogenosome M: PRO ambz, excv, chrm (strc) ambz, excv, chrm ambz, excv, chrm