Protists (pronounced /ˈproʊtɨst/) are a diverse group of eukaryotic A eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the microorganisms A microorganism or microbe is an organism that is microscopic (too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design. Historically, protists were treated as the kingdom In biology, kingdom or regnum is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany. The complete sequence of ranks is life, domain, kingdom, phylum, class, order, family, genus, and species Protista 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 organization^[3]—either they are unicellular A microorganism or microbe is an organism that is microscopic (too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design, or they are multicellular Multicellular organisms are organisms that consist of more than one cell. Each cell is specialized to do a certain job for that organism. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdoms Plantae and Animalia without specialized tissues Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. This simple cellular organization distinguishes the protists from other eukaryotes, such as fungi A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/) are classified as a kingdom that is separate from plants, animals and bacteria. One major difference is that fungal cells have cell, animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also and plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004,.

The term protista was first used by Ernst Haeckel Ernst Heinrich Philipp August Haeckel , also written von Haeckel, was an eminent German biologist, naturalist, philosopher, physician, professor and artist who discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms, and coined many terms in biology, including phylum, phylogeny, ecology and the in 1866 Year 1866 was a common year starting on Monday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Saturday of the 12-day-slower Julian calendar). Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the one-celled A microorganism or microbe is an organism that is microscopic (too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design animal-like protozoa Protozoa is a subkingdom of microorganisms that are classified generally as unicellular non-fungal eukaryotes. Protozoans are a major component of the ecosystem, the plant-like protophyta The thallophytes are a polyphyletic group of non-mobile organisms traditionally described as "relatively simple plants" or "lower plants" with undifferentiated bodies . They were a defunct division of Kingdom Plantae, the Thallophyta (or Thallobionta) that included fungus and algae, and lichens occasionally bacteria and the (mostly one-celled A microorganism or microbe is an organism that is microscopic (too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design algae Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in land plants), and the fungus-like slime molds Slime mold is a broad term describing fungus-like organisms that use spores to reproduce. Slime molds were formerly classified as fungi, but are no longer considered part of this kingdom and water molds Oömycota or oömycetes (pronounced /ˌoʊ əˈmaɪ sit is/), also known as water molds (or water moulds: see spelling differences), form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms. They are filamentous microscopic, absorptive organisms that reproduce both sexually and asexually. Oomycetes occupy both saprophytic and. Because these groups often overlap, they have been replaced by phylogenetic In biology, phylogenetics is the study of evolutionary relatedness among various groups of organisms , which is discovered through molecular sequencing data and morphological data matrices. The term phylogenetics is of Greek origin from the terms phyle/phylon (φυλή/φῦλον), meaning "tribe, race," and genetikos (γενετικός-based classifications Biological classification, or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis. Modern biological classification. However, they are still useful as informal names for describing the morphology In biology "morphology" is the study of the form, structure and configuration of an organism. This includes aspects of the outward appearance [citation needed] as well as the form and structure of the internal parts like bones and organs. This is in contrast to physiology, which deals primarily with function and ecology Ecology is the scientific study of the distributions, abundance and relations of organisms and their interactions with the environment. Ecology includes the study of plant and animal populations, plant and animal communities and ecosystems. Ecosystems describe the web or network of relations among organisms at different scales of organization of protists.

Protists live in almost any environment that contains liquid water. Many protists, such as the algae Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in land plants, are photosynthetic Photosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae, and cyanobacteria, and are vital primary producers Primary producers are those organisms in an ecosystem that produce biomass from inorganic compounds . In almost all cases these are photosynthetically active organisms (plants, cyanobacteria and a number of other unicellular organisms; see article on photosynthesis). However, there are examples of archea (unicellular organisms) that produce in ecosystems, particularly in the ocean as part of the plankton Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification. They provide a crucial source of food to larger, more familiar aquatic organisms such as fish and cetacea. Other protists, such as the Kinetoplastids The kinetoplastids are a group of flagellate protozoa, including a number of parasites responsible for serious diseases in humans and other animals, as well as various forms found in soil and aquatic environments. They are included in the Euglenozoa, and are distinguished from other such forms mainly by the presence of a kinetoplast, a DNA- and Apicomplexa The Apicomplexa are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or pseudopods are absent except in certain gamete stages. This, are responsible for a range of serious human diseases, such as malaria Malaria is a mosquito-borne infectious disease caused by a eukaryotic protist of the genus Plasmodium. It is widespread in tropical and subtropical regions, including parts of the Americas , Asia, and Africa. Each year, there are approximately 350–500 million cases of malaria, killing between one and three million people, the majority of whom and sleeping sickness Human African trypanosomiasis, sleeping sickness, African lethargy, or Congo trypanosomiasis is a parasitic disease of people and animals, caused by protozoa of the species Trypanosoma brucei and transmitted by the tsetse fly. The disease is endemic in some regions of sub-Saharan Africa, covering about 36 countries and 60 million people. It is.

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 Protozoa is a subkingdom of microorganisms that are classified generally as unicellular non-fungal eukaryotes. Protozoans are a major component of the ecosystem to refer to organisms such as ciliates The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella. Cilia occur in all members of the group and are variously used in swimming, crawling, and corals Corals are marine organisms in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans, which secrete calcium carbonate to form a hard skeleton.^[4] This group was expanded in 1845 to include all "unicellular animals", such as Foraminifera The Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are made of calcium and amoebae "Amoeboid" and "amoeba" are often used interchangeably even by biologists, and especially refers to a creature moving by using pseudopods. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to the specific genus Amoeba. The genus Amoeba and amoeboids in general both derive. 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 Ernst Heinrich Philipp August Haeckel , also written von Haeckel, was an eminent German biologist, naturalist, philosopher, physician, professor and artist who discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms, and coined many terms in biology, including phylum, phylogeny, ecology and the, 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 The bacteria ( [bækˈtɪəriə] ; singular: bacterium)[α] are a large group of single-celled, prokaryote microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste,. Copeland's use of the term protoctista did not. In contrast, Copeland's term included nucleated In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes eukaryotes A eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the such as diatoms Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans (e.g. Meridion), zigzags (e.g. Tabellaria), or stellate colonies (e.g. Asterionella). Diatoms are producers within the food chain. A characteristic, green algae The green algae are the large group of algae from which the embryophytes (higher plants) emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic (and often just known as kingdom Plantae). The green algae include unicellular and colonial flagellates, usually but not always and fungi A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/) are classified as a kingdom that is separate from plants, animals and bacteria. One major difference is that fungal cells have cell.^[6] This classification was the basis for Whittaker's later definition of Fungi A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/) are classified as a kingdom that is separate from plants, animals and bacteria. One major difference is that fungal cells have cell, Animalia Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also, Plantae Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, and Protista as the four kingdoms of life.^[7] The kingdom Protista was later modified to separate prokaryotes The prokaryotes are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles. The word prokaryote comes from the Greek πρό- (pro-) & into the separate kingdom of Monera Monera is a now-obsolete taxonomic group in biological classification originally understood as one of five biological kingdoms. The kingdom Monera included most organisms with a prokaryotic cell organization (that is, no nucleus). For this reason, the kingdom was sometimes called Prokaryota or Prokaryotae, 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 A clade[note 1] is a group consisting of an organism and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. In).

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 A heterotroph is an organism that uses organic carbon for growth by consuming other organisms. This contrasts with autotrophs, such as plants, which can directly use sources of energy such as light to produce organic substrates from carbon dioxide species of protists that do not form filaments. These terms are not used in current taxonomy Taxonomy is the practice and science of classification. The word finds its roots in the Greek τάξις, taxis and νόμος, nomos (meaning 'law' or 'science'). Taxonomy uses taxonomic units, known as taxa (singular taxon), 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).

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 decreased from 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
• Protest

References

1. ^ Simonite T (November 2005). "Protists push animals aside in rule revamp". Nature 438 (7064): 8–9. doi:10.1038/438008b. PMID 16267517.
2. ^ Harper, David; Benton, Michael (2009). Introduction to Paleobiology and the Fossil Record. Wiley-Blackwell. pp. 207. ISBN 1-4051-4157-3.
3. ^ "Systematics of the Eukaryota". http://www.ucmp.berkeley.edu/alllife/eukaryotasy.html. Retrieved 2009-05-31.
4. ^ ^{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.
5. ^ 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.
6. ^ 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.
7. ^ 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.
8. ^ 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.
9. ^ ^{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.
10. ^ 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.
11. ^ 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.
12. ^ 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.
13. ^ 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.
14. ^ "ARS Parasite Collections Assist Research and Diagnoses". USDA Agricultural Research Service. January 28, 2010. http://www.ars.usda.gov/is/pr/2010/100128.htm.

External links

• Tree of Life: Eukaryotes
• A java applet for exploring the new higher level classification of eukaryotes

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