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For the journal, see Protist (journal). Protist_sentence_0


ProtistTemporal range: PaleoproterozoicPresent





Scientific classificationEukaryotaProtist_header_cell_0_1_0
Domain:Protist_cell_0_2_0 EukaryotaProtist_cell_0_2_1
Groups includedProtist_header_cell_0_3_0
Cladistically included but traditionally excluded taxaProtist_header_cell_0_4_0

A protist (/ˈproʊtɪst/) is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. Protist_sentence_1

While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exclusion of other eukaryotes means that protists do not form a natural group, or clade. Protist_sentence_2

So some protists may be more closely related to animals, plants, or fungi than they are to other protists; however, like algae, invertebrates, or protozoans, the grouping is used for convenience. Protist_sentence_3

The study of protists is termed protistology. Protist_sentence_4

The classification of a kingdom separate from animals and plants was first proposed by John Hogg in 1860 as the kingdom Protoctista; in 1866 Ernst Haeckel also proposed a third kingdom Protista as "the kingdom of primitive forms". Protist_sentence_5

Originally these also included prokaryotes, but with time these would be removed to a fourth kingdom Monera. Protist_sentence_6

In the popular five-kingdom scheme proposed by Robert Whittaker in 1969, Protista was defined as eukaryotic "organisms which are unicellular or unicellular-colonial and which form no tissues", and the fifth kingdom Fungi was established. Protist_sentence_7

In the five-kingdom system of Lynn Margulis, the term protist is reserved for microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as kelp, red algae and slime molds. Protist_sentence_8

Others use the term protist interchangeably with Margulis's protoctist, to encompass both single-celled and multicellular eukaryotes, including those that form specialized tissues but do not fit into any of the other traditional kingdoms. Protist_sentence_9

Besides their relatively simple levels of organization, protists do not necessarily have much in common. Protist_sentence_10

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. Protist_sentence_11

Examples of protists include: amoebas (including nucleariids and Foraminifera); choanaflagellates; ciliates; diatoms; dinoflagellates; Giardia; Plasmodium (which causes malaria); oomycetes (including Phytophthora, the cause of the Great Famine of Ireland); and slime molds. Protist_sentence_12

These examples are unicellular, although oomycetes can form filaments, and slime molds can aggregate. Protist_sentence_13

In cladistic systems (classifications based on common ancestry), there are no equivalents to the taxa Protista or Protoctista, as both terms refer to a paraphyletic group that spans the entire eukaryotic tree of life. Protist_sentence_14

In cladistic classification, the contents of Protista are mostly distributed among various supergroups: examples include the SAR supergroup (of stramenopiles or heterokonts, alveolates, and Rhizaria); Archaeplastida (or Plantae sensu lato); Excavata (which is mostly unicellular flagellates); and Opisthokonta (which commonly includes unicellular flagellates, but also animals and fungi). Protist_sentence_15

"Protista", "Protoctista", and "Protozoa" are therefore considered obsolete. Protist_sentence_16

However, the term "protist" continues to be used informally as a catch-all term for eukayotic organisms that aren't within other traditional kingdoms. Protist_sentence_17

For example, the word "protist pathogen" may be used to denote any disease-causing organism that is not plant, animal, fungal, prokaryotic, viral, or subviral. Protist_sentence_18

Subdivisions Protist_section_0

See also: Eukaryote § Five supergroups Protist_sentence_19

The term protista was first used by Ernst Haeckel in 1866. Protist_sentence_20

Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms such as: Protist_sentence_21


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. Protist_sentence_22

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. Protist_sentence_23

These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). Protist_sentence_24

Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related phyla. Protist_sentence_25

For instance, the water molds are now considered to be closely related to photosynthetic organisms such as Brown algae and Diatoms, the slime molds are grouped mainly under Amoebozoa, and the Amoebozoa itself includes only a subset of the "Amoeba" group, and significant number of erstwhile "Amoeboid" genera are distributed among Rhizarians and other Phyla. Protist_sentence_26

However, the older terms are still used as informal names to describe the morphology and ecology of various protists. Protist_sentence_27

For example, the term protozoa is used to refer to heterotrophic species of protists that do not form filaments. Protist_sentence_28

Classification Protist_section_1

See also: Kingdom (biology) § Summary Protist_sentence_29

Historical classifications Protist_section_2

Further information: and Protist_sentence_30

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) were Leeuwenhoek, O. Protist_sentence_31 F. Müller, C. Protist_sentence_32 G. Ehrenberg and Félix Dujardin. Protist_sentence_33

The first groups used to classify microscopic organism were the Animalcules and the Infusoria. Protist_sentence_34

In 1818, the German naturalist Georg August Goldfuss introduced the word Protozoa to refer to organisms such as ciliates and corals. Protist_sentence_35

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. Protist_sentence_36

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. Protist_sentence_37

He defined the Protoctista as a "fourth kingdom of nature", in addition to the then-traditional kingdoms of plants, animals and minerals. Protist_sentence_38

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". Protist_sentence_39

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. Protist_sentence_40

In contrast, Copeland's term included nucleated eukaryotes such as diatoms, green algae and fungi. Protist_sentence_41

This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life. Protist_sentence_42

The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera, leaving the protists as a group of eukaryotic microorganisms. Protist_sentence_43

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). Protist_sentence_44

Modern classifications Protist_section_3

Systematists today do not treat Protista as a formal taxon, but the term "protist" is still commonly used for convenience in two ways. Protist_sentence_45

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; this definition 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). Protist_sentence_46

Some systematists judge paraphyletic taxa acceptable, and use Protista in this sense as a formal taxon (as found in some secondary textbooks, for pedagogical purpose). Protist_sentence_47

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 (but vegetative cell differentiation may occur restricted to sexual reproduction, alternate vegetative morphology, and quiescent or resistant stages, such as cysts); this definition excludes many brown, multicellular red and green algae, which may have tissues. Protist_sentence_48

The taxonomy of protists is still changing. Protist_sentence_49

Newer classifications attempt to present monophyletic groups based on morphological (especially ultrastructural), biochemical (chemotaxonomy) and DNA sequence (molecular research) information. Protist_sentence_50

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). Protist_sentence_51

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. Protist_sentence_52

The recent scheme by Adl et al. Protist_sentence_53

(2005) does not recognize formal ranks (phylum, class, etc.) and instead treats groups as clades of phylogenetically related organisms. Protist_sentence_54

This is intended to make the classification more stable in the long term and easier to update. Protist_sentence_55

Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox, upper right. Protist_sentence_56

Many are thought to be monophyletic, though there is still uncertainty. Protist_sentence_57

For instance, the Excavata are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded. Protist_sentence_58

Metabolism Protist_section_4

Nutrition can vary according to the type of protist. Protist_sentence_59

Most eukaryotic algae are autotrophic, but the pigments were lost in some groups. Protist_sentence_60

Other protists are heterotrophic, and may present phagotrophy, osmotrophy, saprotrophy or parasitism. Protist_sentence_61

Some are mixotrophic. Protist_sentence_62

Some protists that do not have / lost chloroplasts/mitochondria have entered into endosymbiontic relationship with other bacteria/algae to replace the missing functionality. Protist_sentence_63

For example, Paramecium bursaria and Paulinella have captured a green alga (Zoochlorella) and a cyanobacterium respectively that act as replacements for chloroplast. Protist_sentence_64

Meanwhile, a protist, Mixotricha paradoxa that has lost its mitochondria uses endosymbiontic bacteria as mitochondria and ectosymbiontic hair-like bacteria (Treponema spirochetes) for locomotion. Protist_sentence_65

Many protists are flagellate, for example, and filter feeding can take place where flagellates find prey. Protist_sentence_66

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. Protist_sentence_67


Nutritional types in protist metabolismProtist_table_caption_1
Nutritional typeProtist_header_cell_1_0_0 Source of energyProtist_header_cell_1_0_1 Source of carbonProtist_header_cell_1_0_2 ExamplesProtist_header_cell_1_0_3
PhotoautotrophsProtist_cell_1_1_0 SunlightProtist_cell_1_1_1 Organic compounds or carbon fixationProtist_cell_1_1_2 Most algaeProtist_cell_1_1_3
ChemoheterotrophsProtist_cell_1_2_0 Organic compoundsProtist_cell_1_2_1 Organic compoundsProtist_cell_1_2_2 Apicomplexa, Trypanosomes or AmoebaeProtist_cell_1_2_3

For most important cellular structures and functions of animal and plants, it can be found a heritage among protists. Protist_sentence_68

Reproduction Protist_section_5

Some protists reproduce sexually using gametes, while others reproduce asexually by binary fission. Protist_sentence_69

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. Protist_sentence_70

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. Protist_sentence_71

Eukaryotes emerged in evolution more than 1.5 billion years ago. Protist_sentence_72

The earliest eukaryotes were likely protists. Protist_sentence_73

Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes. Protist_sentence_74

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. Protist_sentence_75

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. Protist_sentence_76

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. Protist_sentence_77

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. Protist_sentence_78

These results suggested that G. lamblia is capable of meiosis and thus sexual reproduction. Protist_sentence_79

Furthermore, direct evidence for meiotic recombination, indicative of sex, was also found in G. lamblia. Protist_sentence_80

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. Protist_sentence_81

Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al. Protist_sentence_82

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. Protist_sentence_83

These findings suggest that T. vaginalis may be capable of meiosis. Protist_sentence_84

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. Protist_sentence_85

These two species are descendants of protist lineages that are highly divergent among eukaryotes, leading Malik et al. Protist_sentence_86

to suggest that these meiotic genes were likely present in a common ancestor of all eukaryotes. Protist_sentence_87

Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes. Protist_sentence_88

This view was further supported by a study of amoebae by Lahr et al. Protist_sentence_89

Amoeba have generally been regarded as asexual protists. Protist_sentence_90

However, these authors describe evidence that most amoeboid lineages are anciently sexual, and that the majority of asexual groups likely arose recently and independently. Protist_sentence_91

Early researchers (e.g., Calkins) have interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoeboid organisms as sexual reproduction. Protist_sentence_92

Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock. Protist_sentence_93

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. Protist_sentence_94

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). Protist_sentence_95

Ecology Protist_section_6

Free-living Protists occupy almost any environment that contains liquid water. Protist_sentence_96

Many protists, such as algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Protist_sentence_97

Protists make up a large portion of the biomass in both marine and terrestrial environments. Protist_sentence_98

Other protists include pathogenic species, such as the kinetoplastid Trypanosoma brucei, which causes sleeping sickness, and species of the apicomplexan Plasmodium, which cause malaria. Protist_sentence_99

Parasitism: role as pathogens Protist_section_7

See also: Antiprotozoal agent and Apicomplexa § Parasitology_and_genomics Protist_sentence_100

Some protists are significant parasites of animals (e.g.; five species of the parasitic genus Plasmodium cause malaria in humans and many others cause similar diseases in other vertebrates), plants (the oomycete Phytophthora infestans causes late blight in potatoes) or even of other protists. Protist_sentence_101

Protist pathogens share many metabolic pathways with their eukaryotic hosts. Protist_sentence_102

This makes therapeutic target development extremely difficult – a drug that harms a protist parasite is also likely to harm its animal/plant host. Protist_sentence_103

A more thorough understanding of protist biology may allow these diseases to be treated more efficiently. Protist_sentence_104

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. Protist_sentence_105

Recent papers have proposed the use of viruses to treat infections caused by protozoa. Protist_sentence_106

Researchers from the Agricultural Research Service are taking advantage of protists as pathogens to control red imported fire ant (Solenopsis invicta) populations in Argentina. Protist_sentence_107

Spore-producing protists such as Kneallhazia solenopsae (recognized as a sister clade or the closest relative to the fungus kingdom now) can reduce red fire ant populations by 53–100%. Protist_sentence_108

Researchers have also been able to infect phorid fly parasitoids of the ant with the protist without harming the flies. Protist_sentence_109

This turns the flies into a vector that can spread the pathogenic protist between red fire ant colonies. Protist_sentence_110

Fossil record Protist_section_8

Many protists have neither hard parts nor resistant spores, and their fossils are extremely rare or unknown. Protist_sentence_111

Examples of such groups include the apicomplexans, most ciliates, some green algae (the Klebsormidiales), choanoflagellates, oomycetes, brown algae, yellow-green algae, Excavata (e.g., euglenids). Protist_sentence_112

Some of these have been found preserved in amber (fossilized tree resin) or under unusual conditions (e.g., Paleoleishmania, a kinetoplastid). Protist_sentence_113

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). Protist_sentence_114

Some are even used as paleoecological indicators to reconstruct ancient environments. Protist_sentence_115

More probable eukaryote fossils begin to appear at about 1.8 billion years ago, the acritarchs, spherical fossils of likely algal protists. Protist_sentence_116

Another possible representative of early fossil eukaryotes are the Gabonionta. Protist_sentence_117

See also Protist_section_9


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