Temporal range: Neoproterozoic – Recent
In some biological taxonomy schemes, Protists // are a large and diverse group of eukaryotic microorganisms, which belong to the kingdom Protista. There have been attempts to remove the kingdom from modern taxonomy but it is still very much in use. The use of Protoctista is also preferred by various organisations and institutions. Molecular information has been used to redefine this group in modern taxonomy as diverse and often distantly related phyla. The group of protists is now considered to mean diverse phyla that are not closely related through evolution and have different life cycles, trophic levels, modes of locomotion and cellular structures. Besides their relatively simple levels of organization, the protists do not have much in common. They are unicellular, or they are multicellular without specialized tissues, and 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 unicellular "animal-like" protozoa, the "plant-like" protophyta (mostly unicellular algae), and the "fungus-like" slime molds and water molds. These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the morphology and ecology of various 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 include pathogenic species such as the kinetoplastid Trypanosoma brucei, which causes sleeping sickness and species of the apicomplexan Plasmodium which cause malaria.
The first groups used to include microscopic organism were the Animalcules, and after the Infusoria. In the 1817, when the German biologist Georg August Goldfuss introduced the word Protozoa to refer to organisms such as ciliates and corals. 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. The kingdom of minerals was later removed from taxonomy by Ernst Haeckel, leaving plants, animals, and the protists as a “kingdom of primitive forms”.
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. This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life. 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).
Some protists, sometimes called ambiregnal protists, have been considered to be both protozoa and algae or fungi (e.g., slime molds and mixotrophic algae), and names for these have been published under either or both of the ICN and the ICZN.
Although systematists today do not treat protists as a formal taxon, the term protist is currently used in two ways. The most popular contemporary definition is a phylogenetic one that identifies a paraphyletic group: a protist is any eukaryote that is not an animal, (land) plant, or (true) fungus, excluding unicellular groups like the Myxosporida, Microsporidia, many Chytridiomycetes and yeasts. The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular, that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues, excluding so the brown algae and many red and green algae. 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) 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, upper right. 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.
Nutrition can vary according to the type of protist. Many protists are flagellate, for example, and filter feeding can take place where the flagella find prey. Other protists can engulf bacteria and other food particles, by extending their cell membrane around them to form a food vacuole and digest them internally, in a process termed phagocytosis.
|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|
Some species, for example Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually. However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protists may be clonal lines that rarely exchange genes with other members of their species.
Eukaryotes emerged in evolution more than 1.5 billion years ago. The earliest eukaryotes were likely protists. Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that 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. 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.
Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al. tested for 29 genes that function in meiosis, they found 27 to be present, including 8 of 9 genes specific to meiosis in model eukaryotes. These findings suggest that T. vaginalis may be capable of meiosis. Since 21 of the 29 meiotic genes were also present in G. lamblia, it appears that most of these meiotic genes were likely present in a common ancestor of T. vaginalis and G. lamblia. These two species are descendants of protist lineages that are highly divergent among eukaryotes, leading Malik et al. to suggest that these meiotic genes were likely present in a common ancestor of all eukaryotes.
Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes.
This view was further supported by a study of amoebae by Lahr et al. Amoeba have generally been regarded as asexual protists. However these authors describe evidence that most amoeboid lineages are anciently sexual, and that the majority of asexual groups likely arose recently and independently. It should be noted that early researchers (e.g., Calkins) have interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoeboid organims as sexual reproduction.
Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock. 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.
Role as pathogens
There are some protists that are significant pathogens of animals and others that are pathogens of plants; for example there are five species of the parasitic genus Plasmodium, which cause malaria in humans; and the oomycete Phytophthora infestans, which causes late blight in potatoes. 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 (this is more widely recognized as belonging to the fungus kingdom now) the red fire ant populations can be reduced by 53–100%. 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.
Many protists have no hard parts or don't produce resistant spores, and their fossils are extremely rare or unknown, as the apicomplexans, most ciliates, some green algae (the Klebsormidiales), choanoflagellates, oomycetes, brown algae, yellow-green algae, excavates (e.g., euglenids). Some of these have been found preserved in amber (fossilized tree resin) or under unusual conditions.
Others are relatively common in the fossil record, as the diatoms, golden algae, haptophytes (coccoliths), silicoflagellates, tintinnids (ciliates), dinoflagellates, green algae, red algae, heliozoans, radiolarians, foraminiferans, ebriids and testate amoebae (euglyphids, arcellaceans). 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. Another possible representant of early fossil eukaryotes are the Gabonionta.
- Gooday, A. J. (2003). "Benthic foraminifera (Protista) as tools in deep-water palaeoceanography: Environmental influences on faunal characteristics". Advances in marine biology 46: 1–90. doi:10.1016/s0065-2881(03)46002-1. PMID 14601411.
- Kirk, N. L.; Ritson-Williams, R.; Coffroth, M. A.; Miller, M. W.; Fogarty, N. D.; Santos, S. R. (2013). "Tracking Transmission of Apicomplexan Symbionts in Diverse Caribbean Corals". PLoS ONE 8 (11): e80618. doi:10.1371/journal.pone.0080618. PMC 3833926. PMID 24260438.
- Zhao, Y.; Gentekaki, E.; Yi, Z.; Lin, X. (2013). "Genetic Differentiation of the Mitochondrial Cytochrome Oxidase c Subunit I Gene in Genus Paramecium (Protista, Ciliophora)". PLoS ONE 8 (10): e77044. doi:10.1371/journal.pone.0077044. PMC 3812207. PMID 24204730.
- Miloslavich, P.; Díaz, J. M.; Klein, E.; Alvarado, J. J.; Díaz, C.; Gobin, J.; Escobar-Briones, E.; Cruz-Motta, J. J.; Weil, E.; Cortés, J.; Bastidas, A. C.; Robertson, R.; Zapata, F.; Martín, A.; Castillo, J.; Kazandjian, A.; Ortiz, M. (2010). "Marine Biodiversity in the Caribbean: Regional Estimates and Distribution Patterns". PLoS ONE 5 (8): e11916. doi:10.1371/journal.pone.0011916. PMC 2914069. PMID 20689856.
- Martínez-Girón, R; Ribas-Barceló, A (2007). "Pitfall in sputum cytology: Protoctista resembling adenocarcinoma cells". Diagnostic Cytopathology 35 (1): 32–3. doi:10.1002/dc.20584. PMID 17173305.
- Osaka, T; Beika, A; Hattori, A; Kohno, Y; Kato, K. H.; Mizutani, T (2003). "The protozoa dinoflagellate Oxyrrhis marina contains selenoproteins and the relevant translation apparatus". Biochemical and Biophysical Research Communications 300 (1): 236–40. doi:10.1016/S0006-291X(02)02806-1. PMID 12480549.
- 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. p. 207. ISBN 1-4051-4157-3.
- "Systematics of the Eukaryota". Retrieved 2009-05-31.
- The Flagellates. Unity, diversity and evolution. Ed.: Barry S. C. Leadbeater and J. C. Green Taylor and Francis, London 2000, p. 3.
- 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.
- Rothschild LJ (1989). "Protozoa, Protista, Protoctista: what's in a name?". J Hist Biol 22 (2): 277–305. doi:10.1007/BF00139515. PMID 11542176.
- Copeland, H. F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology 13 (4): 383. doi:10.1086/394568. JSTOR 2808554.
- Whittaker, R. H. (1959). "On the Broad Classification of Organisms". Quarterly Review of Biology 34 (3): 210. doi:10.1086/402733. JSTOR 2816520.
- 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): 150–60. doi:10.1126/science.163.3863.150. PMID 5762760.
- Stechmann, Alexandra; Thomas Cavalier-Smith (2003). "The root of the eukaryote tree pinpointed". Current Biology 13 (17): R665–R666. doi:10.1016/S0960-9822(03)00602-X. PMID 12956967. Retrieved 15 May 2011.
- Corliss, J.O. (1995). "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.
- Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell. p. 41. ISBN 978-0-632-04761-1.
- O'Malley MA, Simpson AGB, and Roger AJ (2013). The other eukaryotes in light of evolutionary protistology. Biology and Philosophy 28(2): 299–330.
- 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 (December 2006). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMC 1713255. 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. PMC 497046. 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. PMC 51825. PMID 1675793.
- Javaux EJ, Knoll AH, Walter MR (July 2001). "Morphological and ecological complexity in early eukaryotic ecosystems". Nature 412 (6842): 66–9. doi:10.1038/35083562. PMID 11452306.
- Ramesh MA, Malik SB, Logsdon JM (January 2005). "A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis". Curr. Biol. 15 (2): 185–91. doi:10.1016/j.cub.2005.01.003. PMID 15668177.
- Cooper MA, Adam RD, Worobey M, Sterling CR (November 2007). "Population genetics provides evidence for recombination in Giardia". Curr. Biol. 17 (22): 1984–8. doi:10.1016/j.cub.2007.10.020. PMID 17980591.
- Akopyants NS, Kimblin N, Secundino N, et al. (April 2009). "Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector". Science 324 (5924): 265–8. doi:10.1126/science.1169464. PMC 2729066. PMID 19359589.
- Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM (2008). "An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis". In Hahn, Matthew W. PLoS ONE 3 (8): e2879. doi:10.1371/journal.pone.0002879. PMC 2488364. PMID 18663385.
- Dacks J, Roger AJ (1999). "The first sexual lineage and the relevance of facultative sex". J. Mol. Evol. 48 (6): 779–83. doi:10.1007/PL00013156. PMID 10229582.
- Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E (July 2011). "The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms". Proc. Biol. Sci. 278 (1715): 2081–90. doi:10.1098/rspb.2011.0289. PMC 3107637. PMID 21429931.
- Dobell, C. (1909) Chromidia and the binuclearity hypotheses: a review and a criticism. Quarterly Journal of Microscopical Science, 53 279-326, .
- 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
- Campbell, N. and Reese, J. (2008) Biology. Pearson Benjamin Cummings; 8 ed. ISBN 0805368442. pp. 583, 588
- Keen, E. C. (2013). "Beyond phage therapy: Virotherapy of protozoal diseases". Future Microbiology 8 (7): 821–823. doi:10.2217/FMB.13.48.
- Hyman, P.; Atterbury, R.; Barrow, P. (2013). "Fleas and smaller fleas: Virotherapy for parasite infections". Trends in Microbiology 21 (5): 215–220. doi:10.1016/j.tim.2013.02.006. PMID 23540830.
- "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 on 2014-03-20.
- Introduction to the Apicomplexa. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Fossil Record of the Ciliata. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Klebsormidiales. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Choanoflagellata. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Oomycota. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Phaeophyta. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Xanthophyta. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Basal Eukaryotes. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Why Is The Museum On The Web?. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Fossil Record of Diatoms. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Chrysophyta. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Prymnesiophyta. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Fossil Record of the Dinoflagellata. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Systematics of the "Green Algae", Part 1. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Fossil Record of the Rhodophyta. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Fossil Record of the Radiolaria. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Fossil Record of Foraminifera. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Introduction to the Testaceafilosea. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
- Fossil Record of the Eukaryota. Ucmp.berkeley.edu. Retrieved on 2014-03-20.
|Wikimedia Commons has media related to Protista.|
- 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.
|Wikispecies has information related to: Protista|
|Wikispecies has information related to: Protoctista|
- Tree of Life: Eukaryotes
- A java applet for exploring the new higher level classification of eukaryotes
- Plankton Chronicles – Protists – Cells in the Sea – video
- Database of protist images
- Holt, Jack R. and Carlos A. Iudica. 2013. Diversity of Life. http://comenius.susqu.edu/biol/202/Taxa.htm. Last modified: 11/18/13.