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{{Short description|Eukaryotes other than animals, plants or fungi}}
{{For|the journal|Protist (journal){{!}}''Protist'' (journal)}}
{{pp-move-indef}}
{{Paraphyletic group
| classification_status = paraphyletic
| name = Protists
| fossil_range = {{Long fossil range|2100|0|[[Paleoproterozoic]]–[[Holocene|present]]}}
| image = Protist collage 2.jpg
| image_alt = 
| image_caption = Examples of protists. Clockwise from top left: [[red algae]], [[kelp]], [[ciliate]], [[golden alga]], [[dinoflagellate]], [[metamonad]], [[amoeba]], [[slime mold]].
| auto = yes
| parent = Eukaryota
| subdivision_ranks = Supergroups<ref name="Adl-2019"/>
|subdivision= {{Plain list|
* [[Amorphea]] <small>(including [[fungi]] and [[animal]]s)</small>
* [[Apusomonadida]]
* [[Archaeplastida]] <small>(including [[land plant]]s)</small>
* [[Breviatea]]
* [[CRuMs]]
* [[Cryptista]]
* [[Discoba]]
* [[Haptista]]
* [[Hemimastigophora]]
* [[Malawimonadida]]
* [[Metamonada]]
* [[Provora]]<ref name="Tikhonenkov-2022"/>
* [[SAR supergroup]]
* [[Telonemia]]
}}
| excludes = {{Plain list|
* [[Animalia]]
* [[Fungi]]
* [[Embryophyta]] (land plants)
}}
}}

A '''protist''' ({{IPAc-en|ˈ|p|r|oʊ|t|ᵻ|s|t}} {{respell|PROH|tist}}) or '''protoctist''' is any [[eukaryotic]] [[organism]] that is not an [[animal]], [[Embryophyte|land plant]], or [[fungus]]. Protists do not form a natural group, or [[clade]], but are a [[polyphyletic]] grouping of several independent clades that evolved from the [[last eukaryotic common ancestor]].

Protists were historically regarded as a separate [[taxonomic rank|taxonomic]] [[kingdom (biology)|kingdom]] known as '''Protista''' or '''Protoctista'''. With the advent of [[phylogenetic]] analysis and [[electron microscopy]] studies, the use of Protista as a formal [[taxon]] was gradually abandoned. In modern classifications, protists are spread across several eukaryotic clades called [[supergroup (biology)|supergroups]], such as [[Archaeplastida]] ([[photoautotroph]]s that includes land plants), [[SAR supergroup|SAR]], [[Obazoa]] (which includes fungi and animals), [[Amoebozoa]] and [[Excavata]].

Protists represent an extremely large [[genetic diversity|genetic]] and [[ecological diversity]] in all environments, including extreme habitats. Their diversity, larger than for all other eukaryotes, has only been discovered in recent decades through the study of [[environmental DNA]] and is still in the process of being fully described. They are present in all [[ecosystem]]s as important components of the [[biogeochemical cycle]]s and [[trophic web]]s. They exist abundantly and ubiquitously in a variety of forms that evolved multiple times independently, such as free-living [[algae]], [[amoebae]] and [[slime mould]]s, or as important [[parasite]]s. Together, they compose an amount of biomass that doubles that of animals. They exhibit varied types of nutrition (such as [[phototrophy]], [[phagotrophy]] or [[osmotrophy]]), sometimes combining them (in [[mixotrophy]]). They present unique adaptations not present in multicellular animals, fungi or land plants. The study of protists is termed [[protistology]].

== Definition ==
There is not a single accepted definition of what protists are. As a [[paraphyletic]] assemblage of diverse biological groups, they have historically been regarded as a [[catch-all taxon]] that includes any eukaryotic organism (i.e., living beings whose [[cell (biology)|cell]]s possess a [[nucleus (cell)|nucleus]]) that is not an animal, a [[land plant]] or a [[Dikarya|dikaryon fungus]]. Because of this definition by exclusion, protists encompass almost all of the broad spectrum of [[biological trait|biological characteristics]] expected in eukaryotes.<ref name="Fabien-2021"/>

They are generally [[unicellular]], [[microorganism|microscopic]] eukaryotes. Some species can be purely [[phototroph]]ic (generally called [[algae]]), or purely [[heterotrophic]] (traditionally called [[protozoa]]), but there is a wide range of [[mixotrophic]] protists which exhibit both [[phagotrophy]] and phototrophy together.<ref name="Fabien-2021"/> They have different [[biological life cycle|life cycles]], [[trophic level]]s, [[motility|modes of locomotion]], and [[organelle|cellular structures]].<ref name="Simonite-2005">{{cite journal | vauthors = Simonite T | title = Protists push animals aside in rule revamp | journal = Nature | volume = 438 | issue = 7064 | pages = 8–9 | date = November 2005 | pmid = 16267517 | doi = 10.1038/438008b | bibcode = 2005Natur.438....8S | doi-access = free }}</ref><ref name="Harper-2009">{{cite book | last = Harper | first = David | name-list-style = vanc | author-link = David Harper (palaeontologist)|author2= Benton, Michael |author-link2 = Michael Benton|title=Introduction to Paleobiology and the Fossil Record | url = https://archive.org/details/introductiontopa00bent_611 | url-access = limited |publisher=Wiley-Blackwell |year=2009 |page=[https://archive.org/details/introductiontopa00bent_611/page/n219 207] |isbn=978-1-4051-4157-4}}</ref> Some protists can be [[pathogen]]s.<ref name="Siddiqui-2016"/>

Examples of basic protist forms that do not represent evolutionary cohesive lineages include:<ref name="Archibald-2017b"/>
* [[Algae]], which are [[photosynthetic]] protists. Traditionally called "protophyta", they are found within most of the big evolutionary lineages or [[supergroup (biology)|supergroups]], intermingled with [[heterotrophic]] protists which are traditionally called "[[protozoa]]".<ref name="Scamardella-1999"/> There are many multicellular and colonial examples of algae, including [[kelp]], [[red algae]], some types of [[diatom]]s, and some lineages of [[green algae]].
* [[Flagellate]]s, which bear eukaryotic [[flagella]]. They are found in all lineages, reflecting that the [[last eukaryotic common ancestor|common ancestor of all living eukaryotes]] was a flagellated [[heterotroph]].
* [[Amoebae]], which usually lack flagella but move through changes in the shape and motion of their [[protoplasm]]<ref name="De Bruyn, P. P. H.-1947"/> to produce [[pseudopodia]]. They have evolved independently several times, leading to major [[radiation (evolution)|radiations]] of these lifeforms. Many lineages lack a solid shape ("naked amoebae"). Some of them have special forms, such as the "[[heliozoa]]", amoebae with [[microtubule]]-supported pseudopodia radiating from the [[cell (biology)|cell]], with at least three independent origins. Others, referred to as "[[testate amoeba]]e", grow a shell around the cell made from organic or inorganic material.
* [[Slime mold]]s, which are amoebae capable of producing stalked reproductive structures that bear spores, often through [[multicellularity|aggregative multicellularity]] (numerous amoebae aggregating together). This type of multicellularity has evolved at least seven times among protists.<ref name="Brown-2012"/>
* Fungus-like protists, which can produce [[hyphae]]-like structures and are often [[saprophytic]]. They have evolved multiple times, often very distantly from true fungi. For example, the [[oomycete]]s (water molds) or the [[myxomycete]]s.
* Parasitic protists, such as ''[[Plasmodium falciparum]]'', the cause of [[malaria]].<ref>{{Cite web|url=https://www.ecdc.europa.eu/en/malaria/facts|title=Facts about malaria|date=June 9, 2017|website=www.ecdc.europa.eu}}</ref>

The names of some protists (called [[ambiregnal]] protists), because of their mixture of traits similar to both animals and plants or fungi (e.g. [[slime molds]] and [[flagellate]]d algae like [[Euglenid#Classification|euglenid]]s), have been published under either or both of the ''[[International Code of Nomenclature for algae, fungi, and plants|ICN]]'' and the ''[[International Code of Zoological Nomenclature|ICZN]]'' codes.<ref name="Corliss-1995"/><ref name="Barnes-2001"/>

== Classification ==
[[File:Eukaryotic tree of life (Burki et al 2020).jpg|thumb|right|upright=1.5|Phylogenomic tree of eukaryotes, as regarded in 2020. [[supergroup (biology)|Supergroups]] are in color.]]
{{more|Taxonomy of Protista|Eukaryote#Phylogeny}}
The evolutionary relationships of protists have been explained through [[molecular phylogenetics]], the [[DNA sequencing|sequencing]] of entire [[genome]]s and [[transcriptome]]s, and [[electron microscopy]] studies of the [[flagellum|flagellar apparatus]] and [[cytoskeleton]]. New major lineages of protists and novel [[biodiversity]] continue to be discovered, resulting in dramatic changes to the eukaryotic tree of life. The newest classification systems of eukaryotes, revised in 2019, do not recognize the formal [[taxonomic rank]]s (kingdom, phylum, class, order...) and instead only recognize [[clades]] of related organisms, making the classification more stable in the long term and easier to update. In this new [[cladistic]] scheme, the protists are divided into various wide branches informally named [[supergroup (biology)|supergroups]]:<ref name="Archibald-2017b"/><ref name="Adl-2019"/>

*'''[[Archaeplastida]]'''{{efn|name=Archaeplastida|According to some classifications,<ref name="Cavalier-Smith-1998"/> all of Archaeplastida is treated as Kingdom Plantae, but others consider the algae (or non-terrestrial "plants") to be protists.<ref name="Archibald-2017b"/>}} — consists of groups that have evolved from a [[photosynthetic]] common ancestor that obtained [[chloroplast]]s directly through a single event of [[endosymbiosis]] with a [[cyanobacterium]]:
**[[Picozoa]] (1 species), non-photosynthetic predators.<ref name="Hehenberger-2019" />
**[[Glaucophyta]] (26 species), unicellular algae found in [[freshwater]] and terrestrial environments.<ref name="Leliaert-2012">{{cite book|author1=Olivier De Clerck|author2=Kenny A. Bogaert|author3=Frederik Leliaert|chapter=Chapter Two – Diversity and Evolution of Algae: Primary Endosymbiosis|editor=Gwenaël Piganeau|title=Advances in Botanical Research|publisher=Academic Press|volume=64|date=2012|pages=55–86|issn=0065-2296|isbn=9780123914996|doi=10.1016/B978-0-12-391499-6.00002-5}}</ref>
**[[Rhodophyta]] (5,000–6,000 species), mostly [[multicellular]] [[marine life|marine]] algae that lost [[chlorophyll]] and only harvest light energy through [[phycobiliprotein]]s.<ref name="Leliaert-2012"/>
**[[Rhodelphidia]] (2 species), predators with non-photosynthetic [[plastid]].<ref name="Gawryluk-2019">{{Cite journal|last1=Gawryluk|first1=Ryan M. R.|last2=Tikhonenkov|first2=Denis V.|last3=Hehenberger|first3=Elisabeth|last4=Husnik|first4=Filip|last5=Mylnikov|first5=Alexander P.|last6=Keeling|first6=Patrick J.|date=August 2019|title=Non-photosynthetic predators are sister to red algae|url=https://www.nature.com/articles/s41586-019-1398-6|journal=Nature|language=en|volume=572|issue=7768|pages=240–243|doi=10.1038/s41586-019-1398-6|pmid=31316212|s2cid=197542583|issn=1476-4687}}</ref>
**[[Viridiplantae]] or Chloroplastida, containing both [[green algae]] and [[land plant]]s which are not protists. The green algae comprise many lineages of varying diversity, such as [[Chlorophyta]] (7,000), [[Prasinodermophyta]] (10), [[Zygnematophyceae]] (4,000), [[Charophyceae]] (877), [[Klebsormidiophyceae]] (48) or [[Coleochaetophyceae]] (36).<ref name="Archibald-2017b"/>

*'''[[SAR supergroup|Sar]]''', SAR or Harosa – a clade of three highly diverse lineages exclusively containing protists.
**[[Stramenopile]]s is a wide clade of photosynthetic and [[heterotrophic]] organisms that evolved from a common ancestor with hairs in one of their two flagella. The photosynthetic stramenopiles, called [[Ochrophyta]], are a [[monophyletic]] group that acquired chloroplasts from [[secondary endosymbiosis]] with a [[red alga]]. Among these, the best known are: the unicellular or colonial [[Bacillariophyta]] (>60,000 species),<ref name="Mann-2017"/> known as diatoms; the filamentous or genuinely multicellular [[Phaeophyta]] (2,000 species),<ref name="Kawai-2017"/> known as brown algae; and the [[Chrysomonadea]] (>1,200 species). The heterotrophic stramenopiles are more diverse in forms, ranging from fungi-like organisms such as the [[Hyphochytrea]], [[Oomycota]] and [[Labyrinthulea]], to various kinds of protozoa such as the flagellates [[Opalinata]] and [[Bicosoecida]].<ref name="Archibald-2017b"/>
**[[Alveolate|Alveolata]] contains three of the most well-known groups of protists: [[Apicomplexa]], a [[parasitic]] group with species harmful to humans and animals; [[Dinoflagellata]], an ecologically important group as a main component of the [[marine life|marine]] [[microplankton]] and a main cause of [[algal bloom]]s; and [[Ciliophora]] (4,500 species),<ref name="Foissner-2009"/> the extremely diverse and well-studied group of mostly free-living heterotrophs known as ciliates.<ref name="Archibald-2017b"/>
**[[Rhizaria]] is a morphologically diverse lineage mostly comprising heterotrophic amoebae, flagellates and amoeboflagellates, and some unusual algae ([[Chlorarachniophyte|Chlorarachniophyta]]) and spore-forming parasites. The most familiar rhizarians are [[Foraminifera]] and [[Radiolaria]], groups of large and abundant marine amoebae, many of them macroscopic. Much of the rhizarian diversity lies within the phylum [[Cercozoa]], filled with free-living flagellates which usually have pseudopodia, as well as [[Phaeodaria]], a group previously considered radiolarian. Other groups comprise various amoebae like [[Vampyrellida]] or are important parasites like [[Phytomyxea]], [[Paramyxida]] or [[Haplosporida]].<ref name="Archibald-2017b"/>

*'''[[Haptista]]''' — includes the [[Haptophyta]] algae and the heterotrophic [[Centrohelida]], which are "heliozoan"-type amoebae.<ref name="Archibald-2017b"/>

*'''[[Cryptista]]''' — closely related to [[Archaeplastida]], it includes the [[Cryptophyta]] algae, with a plastid of [[red algae|red algal]] origin, and two obscure relatives with two flagella, [[katablepharid]]s and ''[[Palpitomonas]]''.<ref name="Archibald-2017b"/>

*'''[[Discoba]]''' — includes many lineages previously grouped under the paraphyletic "[[Excavata]]": the [[Jakobida]], flagellates with bacterial-like mitochondrial genomes; ''[[Tsukubamonas]]'', a free-living flagellate; and the [[Discicristata]] clade, which unites well-known phyla [[Heterolobosea]] and [[Euglenozoa]]. Heterolobosea includes amoebae, flagellates and amoeboflagellates with complex life cycles, and the unusual [[Acrasida]], a group of [[slime mold]]s. Euglenozoa encompasses a clade of algae with chloroplasts of green algal origin and many groups of anaerobic, parasitic or free-living heterotrophs.<ref name="Archibald-2017b"/>

*'''[[Metamonad]]a''' — a clade of completely [[anaerobe|anaerobic]] protozoa, primarily [[flagellate]]s. Some are [[gastrointestinal tract|gut]] symbionts of animals, others are free-living (for example, ''[[Paratrimastix pyriformis]]''), and others are well-known [[Parasitism|parasites]] (for example, ''[[Giardia lamblia]]'').<ref name="Archibald-2017b"/>

*'''[[Amorphea]]''' — unites two huge clades:
**[[Amoebozoa]] (2,400 species) is a large group of heterotrophic protists, mostly amoebae. Many lineages are [[slime mold]]s that produce spore-releasing fruiting bodies, such as [[Myxogastria]], [[Dictyostelia]] and [[Protosporangiida]], and are often studied by [[mycologist]]s. Within the non-fruiting amoebae, the [[Tubulinea]] contain many naked amoebae (such as ''[[Amoeba (genus)|Amoeba]]'' itself) and a well-studied order of testate amoebae known as [[Arcellinida]]. Other non-fruiting amoebozoans are [[Variosea]], [[Discosea]] and [[Archamoebae]].<ref name="Archibald-2017b"/>
**[[Obazoa]] includes the two kingdoms [[Metazoa]] (animals) and Fungi,{{efn|name=Fungi|Under traditional classifications, the groups [[Microsporidia]], [[Aphelida]] and [[Rozellida]] are considered to be protists, commonly grouped by the name [[Opisthosporidia]] and treated as the immediate relative of [[Eumycota|Eumycota or true fungi]].<ref name="Cavalier-Smith-2022"/> However, many researchers currently accept those three groups as part of a larger Kingdom Fungi.<ref name="Adl-2019"/><ref name="Tedersoo-2018"/><ref name="Wijayawardene-2022"/>}} and their closest protist relatives inside a clade known as [[Opisthokonta]]. The opisthokont protists are [[Nucleariida]], [[Ichthyosporea]], [[Pluriformea]], [[Filasterea]], [[Choanoflagellate|Choanoflagellata]] and the elusive ''[[Tunicaraptor]]'' (1 species).<ref name="Tikhonenkov-2020">{{cite journal|volume=30|issue=22|date=2020|pages=4500–4509|vauthors=Tikhonenkov DV, Mikhailov KV, Hehenberger E, Mylnikov AP, Aleoshin VV, Keeling PJ, etal.|doi=10.1016/j.cub.2020.08.061|journal=Current Biology|title=New Lineage of Microbial Predators Adds Complexity to Reconstructing the Evolutionary Origin of Animals|pmid=32976804 |doi-access=free}}</ref> They are flagellated or amoeboid heterotrophs of vital importance in the search for the genes that allow animal multicellularity. Sister groups to Opisthokonta are [[Apusomonadida]] (28 species)<ref name="Torruella-2022" /> and [[Breviatea]] (4 species).<ref name="Archibald-2017b"/>

Many smaller lineages do not belong to any of these supergroups, and are usually poorly known groups with limited data, often referred to as 'orphan groups'. Some, such as the '''[[CRuMs]]''' clade, '''[[Malawimonadida]]''' and '''[[Ancyromonadida]]''', appear to be related to Amorphea.<ref name="Archibald-2017b"/> Others, like '''[[Hemimastigophora]]''' (10 species)<ref name="Lax-2018"/> and '''[[Provora]]''' (7 species), appear to be related to or within [[Diaphoretickes]], a clade that unites SAR, Archaeplastida, Haptista and Cryptista.<ref name="Tikhonenkov-2022"/> A mysterious protist species, ''[[Meteora sporadica]]'', is more closely related to the latter two of these orphan groups.<ref name="Eglit-2024"/>

Although the root of the tree is still unresolved, one possible topology of the eukaryotic tree of life is:<ref name="Brown-2018"/><ref name="Tikhonenkov-2022"/><ref name="Eglit-2024">{{Cite journal|title=''Meteora sporadica'', a protist with incredible cell architecture, is related to Hemimastigophora|journal=Current Biology|doi=10.1016/j.cub.2023.12.032|volume=34|pages=451–459|date=22 January 2024|first1=Yana|last1=Eglit|first2=Takashi|last2=Shiratori|first3=Jon|last3=Jerlström-Hultqvist|first4=Kelsey|last4=Williamson|first5=Andrew J.|last5=Roger|first6=Ken-Ichiro|last6=Ishida|first7=Alastair G.B.|last7=Simpson}}</ref>
{| class="collapsible collapsed" style="width:100%; border:solid 1px #aaa"
|-
! style="background:#F0F2F5" | Protist phylogeny
|-
|
{{clade|style=font-size:90%;|label1=[[Eukaryota]]|1={{clade
  |5={{clade
    |1=[[Ancyromonadida]]|state1=dashed
    |2={{clade
      |1=[[Malawimonadida]]
      |label2=[[Podiata]]|2={{clade
        |1=[[CRuMs]] [[File:12862 2018 1224 Fig1i.jpg|50px]]
        |label2=[[Amorphea]]|2={{clade
         |label2=[[Obazoa]]|2={{clade
          |1=[[Breviatea]]|2={{clade
          |1=[[Apusomonadida]] [[File:Podomonas_kaiyoae_C.jpg|50px]]
          |2=[[Opisthokonta]] [[File:Codosiga.jpg|55px]] [[File:Cétoine dorée vol.jpg|80px]] [[File:Linear arrangement of ascospores in the asci of the fungus Sordaria macrospora Cropped.jpg|65px]]
          }}}}
         |1=[[Amoebozoa]] [[File:Amoeba_proteus_with_many_pseudopodia.jpg|65px]]
         }}
        }}
      }}
    }}
  |4=[[Metamonada]] [[File:Giardia_lamblia.jpg|50px]]|state4=dashed
 |3=[[Discoba]] [[File:Euglenoid movement.jpg|50px]]|state3=dashed
 |2={{clade|1=[[Provora]]|2={{clade|1=[[Hemimastigophora]]|2=''[[Meteora sporadica]]''}}}}|state2=dashed
|1={{clade|label1=[[Diaphoretickes]]&nbsp;|1={{clade
     |1={{clade
           |2=[[Haptista]] [[File:Pavlova sp.png|40px]]
           |label1=[[TSAR]]|1={{clade
            |2=[[Telonemia]] [[File:Telonema_rivulare_(contrast_micrography).jpg|30px]]
            |label1=[[SAR supergroup|SAR]]|1={{clade
             |1={{clade|1=[[Stramenopiles]] [[File:Naviculoid diatom.jpg|65px]]
             |2=[[Alveolata]] [[File:Paramecium_bursaria.jpg|55px]]}}
             |2=[[Rhizaria]] [[File:Cribostomoides-wiesneri-quest hg.jpg|45px]]
           }}
         }}
       }}
     |2={{clade
       |1=[[Cryptista]] [[File:CSIRO_ScienceImage_6743_SEM_Cryptophyte.jpg|40px]]
       |2=[[Archaeplastida]] [[File:Спирогира.tif|55px]] [[File:200709200934 Neckarsteinach Sonnenblume.jpg|50px]] [[File:Rhod1003.jpg|50px]]
     }}
   }}
 }}}}
}}
|}

== History ==
=== Early concepts ===
[[File:Egg tree of animals Goldfuss 1820.jpg|thumb|[[Georg August Goldfuss|Goldfuss]]' system of life, introducing the Protozoa within animals.]]
From the start of the 18th century, the popular term "infusion animals" (later [[infusoria]]) referred to protists, [[bacteria]] and small [[invertebrate]] animals. In the mid-18th century, while Swedish scientist [[Carl von Linnaeus]] largely ignored the protists,{{efn|[[Carl von Linnaeus]] did not mention a single protist genus until the tenth edition of ''[[Systema Naturae]]'' of 1758, where ''[[Volvox]]'' was recorded.<ref name="Leadbeater-2000"/>}} his Danish contemporary [[Otto Friedrich Müller]] was the first to introduce protists to the [[binomial nomenclature]] system.<ref name="Leadbeater-2000"/><ref name="Ratcliff-2009"/>

In the early 19th century, German naturalist [[Georg August Goldfuss]] introduced [[Protozoa]] (meaning 'early animals') as a class within [[Kingdom Animalia]],<ref name="Goldfuß-1818"/> to refer to four very different groups: [[infusoria]] ([[ciliates]]), [[coral]]s, phytozoa (such as ''[[Cryptomonas]]'') and [[jellyfish]]. Later, in 1845, [[Carl Theodor von Siebold]] was the first to establish [[Protozoa]] as a phylum of exclusively unicellular animals consisting of two classes: Infusoria (ciliates) and [[Rhizopoda]] ([[amoebae]], [[foraminifera]]).<ref name="vonSiebold-1846"/> Other scientists did not consider all of them part of the animal kingdom, and by the middle of the century they were regarded within the groupings of ''Protozoa'' (early animals), ''Protophyta'' (early plants), ''Phytozoa'' (animal-like plants) and ''Bacteria'' (mostly considered plants). Microscopic organisms were increasingly constrained in the plant/animal dichotomy. In 1858, the palaeontolgist [[Richard Owen]] was the first to define ''Protozoa'' as a separate [[kingdom (biology)|kingdom]] of [[eukaryotic]] organisms, with "nucleated cells" and the "common organic characters" of plants and animals, although he also included [[sponges]] within protozoa.<ref name="Scamardella-1999"/>

=== Origin of the protist kingdom ===
[[File:John_Hogg_--_Primigenum_or_Protoctista.jpg|thumb|upright=1.35|[[John Hogg (biologist)|John Hogg]]'s illustration of the Four Kingdoms of Nature, showing ''"Regnum Primigenum"'' (Protoctista) as a greenish haze at the base of the Animals and Plants, 1860]]
In 1860, British [[naturalist]] [[John Hogg (biologist)|John Hogg]] proposed '''Protoctista''' (meaning 'first-created beings') as the name for a fourth kingdom of nature (the other kingdoms being [[Linnaeus]]' plant, animal and mineral) which comprised all the lower, primitive organisms, including ''protophyta'', ''protozoa'' and [[sponge]]s, at the merging bases of the plant and animal kingdoms.<ref name="Hogg-1860"/><ref name="Scamardella-1999"/>

[[File:Haeckel arbol bn.png|thumb|[[Ernst Haeckel|Haeckel]]'s 1866 tree of life, with the third kingdom Protista.]]

In 1866 the 'father of protistology', German scientist [[Ernst Haeckel]], addressed the problem of classifying all these organisms as a mixture of animal and vegetable characters, and proposed ''Protistenreich''<ref name="Haeckel-1878"/> ('''Kingdom Protista''') as the [[Kingdom_(biology)#Three_kingdoms_of_life|third kingdom of life]], comprising primitive forms that were "neither animals nor plants". He grouped both bacteria<ref name="Taylor-2003"/> and eukaryotes, both unicellular and multicellular organisms, as Protista. He retained the [[Infusoria]] in the animal kingdom, until German zoologist [[Otto Butschli]] demonstrated that they were unicellular.<ref name="Haeckel-1866"/><ref name="Rothschild-1989"/> At first, he included [[sponges]] and fungi, but in later publications he explicitly restricted Protista to predominantly unicellular organisms or colonies incapable of forming [[tissue (biology)|tissues]]. He clearly separated Protista from [[metazoa|true animals]] on the basis that the defining character of protists was the absence of [[sexual reproduction]], while the defining character of animals was the [[blastula]] stage of animal development. He also returned the terms ''protozoa'' and ''protophyta'' as subkingdoms of Protista.<ref name="Scamardella-1999"/> 

Butschli considered the kingdom to be too [[polyphyletic]] and rejected the inclusion of bacteria. He fragmented the kingdom into ''protozoa'' (only nucleated, unicellular animal-like organisms), while bacteria and the ''protophyta'' were a separate grouping. This strengthened the old dichotomy of ''protozoa''/''protophyta'' from German scientist [[Carl Theodor von Siebold]], and the German naturalists asserted this view over the worldwide scientific community by the turn of the century. However, British biologist [[C. Clifford Dobell]] in 1911 brought attention to the fact that protists functioned very differently compared to the animal and vegetable cellular organization, and gave importance to Protista as a group with a different organization that he called "acellularity", shifting away from the dogma of German cell theory. He coined the term [[protistology]] and solidified it as a branch of study independent from [[zoology]] and [[botany]].<ref name="Scamardella-1999"/> 

In 1938, American biologist [[Herbert Copeland (biologist)|Herbert Copeland]] resurrected Hogg's label, arguing that Haeckel's term ''Protista'' included anucleated microbes such as bacteria, which the term ''Protoctista'' (meaning "first established beings") did not. Under his [[Kingdom_(biology)#Four_kingdoms|four-kingdom classification]] ([[Monera]], ''Protoctista'', [[Plantae]], [[Animalia]]), the protists and bacteria were finally split apart, recognizing the difference between anucleate ([[prokaryotic]]) and nucleate ([[eukaryotic]]) organisms. To firmly separate protists from plants, he followed Haeckel's blastular definition of true animals, and proposed defining [[embryophyta|true plants]] as those with [[chlorophyll a|chlorophyll ''a'']] and [[chlorophyll b|''b'']], [[carotene]], [[xanthophyll]] and production of [[starch]]. He also was the first to recognize that the unicellular/multicellular dichotomy was invalid. Still, he kept fungi within Protoctista, together with [[red algae]], [[brown algae]] and [[protozoa]]ns.<ref name="Scamardella-1999"/><ref name="Copeland-1938">{{cite journal | vauthors = Copeland HF | year = 1938 | title = The Kingdoms of Organisms | journal = Quarterly Review of Biology | volume = 13 | issue = 4 | pages = 383–420 | doi = 10.1086/394568 | jstor=2808554| s2cid = 84634277 }}</ref> This classification was the basis for Whittaker's later definition of Fungi, [[Animalia]], [[Plantae]] and Protista as the four kingdoms of life.<ref name="Whittaker-1959">{{cite journal | vauthors = Whittaker RH | year = 1959 | title = On the Broad Classification of Organisms | journal = Quarterly Review of Biology | volume = 34 | issue = 3 | pages = 210–226 | doi = 10.1086/402733 | jstor=2816520 | pmid = 13844483| s2cid = 28836075 }}</ref>

In the popular [[Kingdom (biology)#Five kingdoms|five-kingdom]] scheme published by American plant ecologist [[Robert Whittaker (ecologist)|Robert Whittaker]] in 1969, Protista was defined as eukaryotic "organisms which are [[Unicellular organism|unicellular]] or unicellular-colonial and which form no [[Tissue (biology)|tissues]]". Just as the prokaryotic/eukaryotic division was becoming mainstream, Whittaker, after a decade from Copeland's system,<ref name="Whittaker-1959"/> recognized the fundamental division of life between the prokaryotic Monera and the eukaryotic kingdoms: Animalia (ingestion), Plantae (photosynthesis), Fungi (absorption) and the remaining Protista.<ref name="Whittaker-1969">{{cite journal | vauthors = Whittaker RH | title = New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms | journal = Science | volume = 163 | issue = 3863 | pages = 150–160 | date = January 1969 | pmid = 5762760 | doi = 10.1126/science.163.3863.150 | bibcode = 1969Sci...163..150W | citeseerx = 10.1.1.403.5430 }}</ref><ref name="Hagen-2012"/><ref name="Scamardella-1999"/> 

In the five-kingdom system of American evolutionary biologist [[Lynn Margulis]], the term "protist" was reserved for [[microscopic organisms]], while the more inclusive kingdom Protoctista (or '''protoctists''') included certain large [[Multicellular organism|multicellular]] eukaryotes, such as [[kelp]], [[red algae]], and [[slime mold]]s.<ref>{{Cite book | url = https://books.google.com/books?id=9IWaqAOGyt4C | title = Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth | last1 = Margulis | first1 = Lynn | last2 = Chapman | first2 = Michael J. | name-list-style = vanc | date = 2009-03-19 | publisher = Academic Press | isbn = 9780080920146 }}</ref> Some use the term ''protist'' interchangeably with Margulis' ''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.<ref name="Archibald-2017a">{{Cite book|url=https://www.springer.com/gp/book/9783319281476|title=Handbook of the Protists|date=2017|publisher=Springer International Publishing|isbn=978-3-319-28147-6|editor-last=Archibald|editor-first=John M.|edition=2|pages=ix|language=en|editor-last2=Simpson|editor-first2=Alastair G. B.|editor-last3=Slamovits|editor-first3=Claudio H.}}</ref>

=== Phylogenetics and modern concepts ===
[[File:Tree of Living Organisms 2.png|thumb|Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes]]
The five-kingdom model remained the accepted classification until the development of [[molecular phylogenetic]]s in the late 20th century, when it became apparent that protists are a [[paraphyletic]] group from which animals, fungi and plants evolved, and the [[three-domain system]] (Bacteria, [[Archaea]], [[Eukarya]]) became prevalent.<ref name="Stechmann-2003"/> Today, protists are not treated as a formal [[taxon]], but the term is commonly used for convenience in two ways:<ref name="O'Malley-2012"/>
* [[Phylogenetic]] definition: protists are a [[paraphyletic]] group.<ref name="Schlegel-2007">{{cite journal | doi=10.1016/j.ode.2006.11.001 | title=Protists – A textbook example for a paraphyletic taxon☆ | journal=Organisms Diversity & Evolution| volume=7 | issue=2 | pages=166–172 | year=2007 | last1=Schlegel | first1=M.| last2=Hulsmann | first2=N.}}</ref> A protist is any [[eukaryote]] that is not an animal, [[land plant]] or [[fungus]],<ref name="microbeworld">{{cite web | url=http://www.microbeworld.org/types-of-microbes/protista | title=Protista | publisher=microbeworld.org | access-date=11 June 2016 | archive-date=13 June 2016 | archive-url=https://web.archive.org/web/20160613075238/http://www.microbeworld.org/types-of-microbes/protista | url-status=dead }}</ref> thus excluding many unicellular groups like the fungal [[Microsporidia]], [[Chytridiomycetes]] and [[yeast]]s, and the non-unicellular [[Myxozoa]]n animals included in Protista in the past.<ref>{{cite journal | vauthors = Štolc A | title = Actinomyxidies, nouveau groupe de Mesozoaires parent des Myxosporidies | year = 1899 | journal = Bull. Int. l'Acad. Sci. Bohème | volume = 12 | pages = 1–12 }}</ref>
* Functional definition: protists are essentially those eukaryotes that are never [[multicellular]],<ref name="O'Malley-2012">{{Cite journal | doi = 10.1007/s10539-012-9354-y | title = The other eukaryotes in light of evolutionary protistology | journal = Biology & Philosophy| volume = 28 | issue = 2 | pages = 299–330 | year = 2012 | vauthors = O'Malley MA, Simpson AG, Roger AJ | s2cid = 85406712 }}</ref> that either exist as independent cells, or if they occur in [[colonial organism|colonies]], do not show differentiation into tissues.<ref name="Adl-2005">{{cite journal | vauthors = 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 | title = The new higher level classification of eukaryotes with emphasis on the taxonomy of protists | journal = The Journal of Eukaryotic Microbiology | volume = 52 | issue = 5 | pages = 399–451 | year = 2005 | pmid = 16248873 | doi = 10.1111/j.1550-7408.2005.00053.x | s2cid = 8060916 | doi-access = free }}</ref> While in popular usage, this definition excludes the variety of non-colonial multicellularity types that protists exhibit, such as aggregative (e.g. [[choanoflagellates]]) or complex multicellularity (e.g. [[brown algae]]).<ref name="O'Malley-2022">{{cite book|chapter=Getting at the Basics of Multicellularity|first1=Maureen A.|last1=O'Malley|pages=9–24|title=The Evolution of Multicellularity|editor-first1=Matthew D.|editor-last1=Herron|editor-first2=Peter L.|editor-last2=Conlin|editor-first3=William C.|editor-last3=Ratcliff|publisher=CRC Press|series=Evolutionary Cell Biology|edition=1st|date=2022|doi=10.1201/9780429351907|isbn=9780429351907 |s2cid=248578172 }}</ref>

=== Kingdoms Protozoa and Chromista ===
{{more|Cavalier-Smith's system of classification}}
There is, however, one classification of protists based on traditional ranks that lasted until the 21st century. The British protozoologist [[Thomas Cavalier-Smith]], since 1998, developed a [[Kingdom_(biology)#Six_kingdoms_(1998)|six-kingdom model]]:{{efn|In 2015, Cavalier-Smith's initial six-kingdom model was revised into a [[Kingdom_(biology)#Seven_kingdoms|seven-kingdom model]] after the inclusion of [[Archaea]].<ref name="Ruggiero-2015"/>}} Bacteria, [[Animalia]], [[Plantae]], Fungi, ''[[Protozoa]]'' and ''[[Chromista]]''.<ref name="Cavalier-Smith-1998"/><ref name="Ruggiero-2015"/> In his context, paraphyletic groups take preference over clades:<ref name="Cavalier-Smith-1998"/> both protist kingdoms ''Protozoa'' and ''Chromista'' contain paraphyletic [[phylum|phyla]] such as [[Apusozoa]], [[Eolouka]] or [[Opisthosporidia]]. Additionally, [[red algae|red]] and [[green algae]] are considered true plants, while the [[fungal]] groups [[Microsporidia]], [[Rozellida]] and [[Aphelida]] are considered protozoans under the phylum [[Opisthosporidia]]. This scheme endured until 2021, the year of his last publication.<ref name="Cavalier-Smith-2022"/>

== Diversity==
=== Species diversity ===
[[File:Eukaryotic diversity morphological vs genetic.png|thumb|upright=1.2|Difference between morphological (A) and genetic (B) view of total eukaryotic diversity. Protists dominate [[DNA barcoding]] analyses, but constitute a minority of catalogued species.<ref name="Pawlowski-2012"/>]]

According to [[phylogenetics|molecular data]], protists dominate [[eukaryotic]] diversity, accounting for a vast majority of [[environmental DNA]] sequences or [[operational taxonomic unit]]s (OTUs). However, their [[species diversity]] is severely underestimated by traditional methods that differentiate species based on [[morphology (biology)|morphological]] characteristics. The number of described protistan [[species (biology)|species]] is very low (ranging from 26,000<ref name="Mora-2011"/> to 74,400<ref name="Pawlowski-2012"/> as of 2012) in comparison to the [[biodiversity|diversity]] of plants, animals and fungi, which are historically and biologically well-known and studied. The predicted number of species also varies greatly, ranging from 1.4×10{{sup|5}} to 1.6×10{{sup|6}}, and in several groups the number of predicted species is arbitrarily doubled. Most of these predictions are highly subjective.<ref name="Pawlowski-2012"/>

Molecular techniques such as [[DNA barcoding]] are being used to compensate for the lack of morphological diagnoses, but this has revealed an unknown vast diversity of protists that is difficult to accurately process because of the exceedingly large genetic divergence between the different protistan groups. Several different [[molecular marker]]s need to be used to survey the vast protistan diversity, because there is no universal marker that can be applied to all lineages.<ref name="Pawlowski-2012"/>

===Biomass===
Protists make up a large portion of the [[Biomass (ecology)|biomass]] in both [[marine life|marine]] and [[terrestrial ecosystem]]s. It has been estimated that protists account for 4 [[gigatons]] (Gt) of [[biomass (ecology)|biomass]] in the entire planet [[Earth (planet)|Earth]]. This amount is smaller than 1% of all biomass, but is still double the amount estimated for all animals (2 Gt). Together, protists, animals, [[archaea]] (7 Gt) and fungi (12 Gt) account for less than 10% of the total biomass of the planet, because plants (450 Gt) and bacteria (70 Gt) are the remaining 80% and 15% respectively.<ref name="Bar-On-2018"/>

== Ecology ==
Protists are highly abundant and diverse in all types of ecosystems, especially free-living (i.e. non-parasitic) groups. An unexpectedly enormous, taxonomically undescribed diversity of eukaryotic microbes is detected everywhere in the form of [[environmental DNA]] or [[RNA]]. The richest protist communities appear in [[soil]], followed by [[ocean]] and [[freshwater]] habitats.<ref name="Singer-2021"/>

[[Phagotroph]]ic protists (consumers) are the most diverse functional group in all ecosystems, with three main taxonomical groups of phagotrophs: [[Rhizaria]] (mainly [[Cercozoa]] in freshwater and soil habitats, and [[Radiolaria]] in oceans), ciliates (most abundant in freshwater and second most abundant in soil) and non-photosynthetic [[stramenopiles]] (third most represented overall, higher in soil than in oceans). [[Phototroph]]ic protists (producers) appear in lower proportions, probably constrained by intense predation. They exist in similar abundance in both oceans and soil. They are mostly [[dinophyte]]s in oceans, [[chrysophyte]]s in freshwater, and [[Archaeplastida]] in soil.<ref name="Singer-2021"/>

=== Marine ===
{{further|Marine protists}}
[[File:Diatoms through the microscope.jpg|thumb|Marine [[diatom]]s are important oxygen producers.]]

[[Marine life|Marine]] protists are highly diverse, have a fundamental impact on [[biogeochemical cycle]]s (particularly, the [[carbon cycle]])<ref name="Keeling-2017"/> and are at the base of the marine [[trophic network]]s as part of the [[plankton]].<ref name="Faure-2019"/>

[[Phototroph]]ic marine protists located in the [[photic zone]] as phytoplankton are vital [[primary producer]]s in the oceanic systems. They [[carbon fixation|fix as much carbon]] as all [[terrestrial plant]]s together.<ref name="Singer-2021"/> The smallest fractions, the picoplankton (<2 μm) and nanoplankton (2–20 μm), are dominated by several different algae ([[prymnesiophyte]]s, [[pelagophyte]]s, [[prasinophyte]]s); fractions larger than 5 μm are instead dominated by [[diatom]]s and [[dinoflagellate]]s. The [[heterotrophic]] fraction of marine picoplankton encompasses primarily early-branching [[stramenopiles]] (e.g. [[bicosoecid]]s and [[labyrinthulomycetes]]), as well as [[alveolate]]s, [[ciliate]]s and [[radiolarian]]s; protists of lower frequency include [[cercozoa]]ns and [[cryptophytes]].<ref name="Epstein-2007"/>

[[File:41598_2017_Article_BFsrep39892_Fig1b_HTML.png|thumb|upright=.7|''[[Prymnesium]]'', a constitutive [[mixotroph]] that participates in toxic algal blooms.]]

[[Mixotrophic]] marine protists, while not very researched, are present abundantly and ubiquitously in the global oceans, on a wide range of marine habitats. In [[DNA barcoding|metabarcoding analyses]], they constitute more than 12% of the [[environmental DNA|environmental sequences]]. They are an important and underestimated source of carbon in [[eutrophic]] and [[oligotrophic]] habitats.<ref name="Faure-2019"/> Their abundance varies [[season]]ally.<ref name="Leles-2017"/> Planktonic protists are classified into various functional groups or 'mixotypes' that present different [[biogeographies]]:

* ''Constitutive mixotrophs'', also called '[[phytoplankton]] that eat', have the innate ability to [[photosynthesize]]. They have diverse feeding behaviors: some require [[phototrophy]], others [[phagotrophy]], and others are obligate mixotrophs.<ref name="Faure-2019"/> They are responsible for harmful [[algal bloom]]s. They dominate the eukaryotic microbial biomass in the [[photic zone]], in eutrophic and oligotrophic waters across all climate zones, even in non-[[algal bloom|bloom]] conditions. They account for significant, often dominant predation of [[bacteria]].<ref name="Mitra-2016"/> 

[[File:Noctiluca_scintillans_varias.jpg|thumb|''[[Noctiluca]]'', a specialist non-constitutive mixotroph that photosynthesizes through [[endosymbiont]]s.]]

* ''Non-constitutive mixotrophs'' acquire the ability to photosynthesize by stealing [[chloroplast]]s from their prey. They can be divided into two: ''generalists'', which can use chloroplasts stolen from a variety of prey (e.g. [[oligotrich]] [[ciliate]]s), or ''specialists'', which have developed the need to only acquire chloroplasts from a few specific prey. The specialists are further divided into two: ''plastidic'', those which contain differentiated [[plastid]]s (e.g. ''[[Mesodinium]]'', ''[[Dinophysis]]''), and ''endosymbiotic'', those which contain [[endosymbiont]]s (e.g. mixotrophic [[Rhizaria]] such as [[Foraminifera]] and [[Radiolaria]], dinoflagellates like ''[[Noctiluca]]'').<ref name="Mitra-2016"/>  Both plastidic and generalist non-constitutive mixotrophs have similar biogeographies and low abundance, mostly found in eutrophic coastal waters. Generalist [[ciliate]]s can account for up to 50% of ciliate communities in the photic zone. The endosymbiotic mixotrophs are the most abundant non-constitutive type.<ref name="Faure-2019"/>

=== Freshwater ===
[[Freshwater]] planktonic protist communities are characterized by a higher "beta diversity" (i.e. highly heterogeneous between samples) than soil and marine plankton. The high diversity can be a result of the hydrological dynamic of recruiting organisms from different habitats through extreme [[flood]]s.<ref name="Metz-2022">{{cite journal|vauthors=Metz S, Huber P, Accattatis V, Lopes dos Santos A, Bigeard E, Unrein F, Chambouvet A, Not F, Lara E, Devercelli M|date=2022|title=Freshwater protists: unveiling the unexplored in a large floodplain system|journal=Environmental Microbiology|volume=24|issue=4 |pages=1731–1745|doi=10.1111/1462-2920.15838|pmid=34783136 |s2cid=244133100 }}</ref> The main [[freshwater phytoplankton|freshwater producer]]s ([[chrysophytes]], [[cryptophytes]] and [[dinophyte]]s) behave alternatively as consumers ([[mixotrophs]]). At the same time, strict consumers (non-photosynthetic) are less abundant in freshwater, implying that the consumer role is partly taken by these mixotrophs.<ref name="Singer-2021"/>

=== Soil ===
[[Soil]] protist communities are ecologically the richest. This may be due to the complex and highly dynamic distribution of water in the [[sediment]], which creates extremely heterogenous environmental conditions. The constantly changing environment promotes the activity of only one part of the community at a time, while the rest remains inactive; this phenomenon promotes high microbial diversity in [[prokaryotes]] as well as protists. Only a small fraction of the detected diversity of soil-dwelling protists has been described (8.1% as of 2017).<ref name="Singer-2021"/> Soil protists are also morphologically and functionally diverse, with four major categories:<ref name="Geisen-2018"/>

[[File:Dictyostelium_discoideum_02.jpg|thumb|[[Dictyostelid]]s are fungus-like protists present in soil.]]
[[File:Cercomonas sp.jpg|thumb|[[Cercomonad]]s ([[Rhizaria]]) are important [[phagotroph]]ic protists in soil.]]

*[[Photoautotroph]]ic soil protists, or [[algae]], are as abundant as their [[marine life|marine]] counterparts. Given the importance of marine algae, soil algae may provide a larger contribution to the global [[carbon cycle]] than previously thought, but the magnitude of their [[carbon fixation]] has yet to be quantified.<ref name="Singer-2021"/> Most soil algae belong to the [[supergroup (biology)|supergroups]] [[Stramenopiles]] ([[diatom]]s, [[Xanthophyceae]] and [[Eustigmatophyceae]]) and [[Archaeplastida]] ([[Chlorophyceae]] and [[Trebouxiophyceae]]). There is also the presence of [[environmental DNA]] from [[dinoflagellate]]s and [[haptophyte]]s in soil, but no living forms have been seen.<ref name="Geisen-2018"/>

*[[Fungus]]-like protists are present abundantly in soil. Most environmental sequences belong to the [[Oomycetes]] (Stramenopiles), an [[osmotroph]]ic and [[saprotrophic]] group that contains free-living and [[parasitic]] species of other protists, fungi, plants and animals. Another important group in soil are [[slime mold]]s (found in [[Amoebozoa]], [[Opisthokonta]], [[Rhizaria]] and [[Heterolobosea]]), which reproduce by forming fruiting bodies known as [[sporocarp (fungi)|sporocarps]] (originated from a single cell) and [[sorocarp]]s (from aggregations of cells).<ref name="Geisen-2018"/>

*[[Phagotroph]]ic protists are abundant and essential in soil ecosystems. As [[bacterial]] grazers, they have a significant role in the foodweb: they excrete [[nitrogen]] in the form of [[ammonia|NH{{sub|3}}]], making it available to plants and other microbes.<ref name="Harder-2016"/> Many soil protists are also [[mycophagous]], and facultative (i.e. non-obligate) mycophagy is a widespread evolutionary feeding mode among soil protozoa.<ref name="Geisen-2016"/> Amoeboflagellates like the [[glissomonad]]s and [[cercomonad]]s (in [[Rhizaria]]) are among the most abundant soil protists: they possess both flagella and pseudopodia, a morphological variability well suited for foraging between soil particles. [[Testate amoebae]] (e.g. [[arcellinid]]s and [[euglyphid]]s) have [[protist shell|shells]] that protect against desiccation and predation, and their contribution to the [[silica cycle]] through the [[biomineralization]] of shells is as important as that of forest trees.<ref name="Geisen-2018"/>

*[[Parasitic]] soil protists (in [[Apicomplexa]]) are diverse, ubiquitous and have an important role as parasites of soil-dwelling [[invertebrate]] animals. In [[Neotropical]] forests, [[environmental DNA]] from the apicomplexan [[gregarine]]s dominates protist diversity.<ref name="Geisen-2018"/>

=== Parasitic ===
[[File:Parasite140080-fig2 Gastrointestinal parasites in seven primates of the Taï National Park - Blastocystis.png|thumb|''[[Blastocystis]]'' ([[Stramenopiles]]) is a prevalent intestinal parasite in humans.]]
[[Parasitic]] protists represent around 15–20% of all environmental DNA in marine and soil systems, but only around 5% in freshwater systems, where [[chytrid]] fungi likely fill that [[ecological niche]]. In oceanic systems, [[parasitoid]]s (i.e. those which kill their hosts, e.g. [[Syndiniales]]) are more abundant. In soil ecosystems, true parasites (i.e. those which do not kill their hosts) are primarily animal-hosted [[Apicomplexa]] ([[Alveolata]]) and plant-hosted [[oomycetes]] ([[Stramenopiles]]) and [[plasmodiophorid]]s ([[Rhizaria]]). In freshwater ecosystems, parasitoids are mainly [[Perkinsea]] and [[Syndiniales]] (Alveolata), as well as the fungal [[Chytridiomycota]]. True parasites in freshwater are mostly [[oomycetes]], [[Apicomplexa]] and [[Ichthyosporea]].<ref name="Singer-2021"/>

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<ref>{{cite journal | vauthors = 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 | title = Not in your usual Top 10: protists that infect plants and algae | journal = Molecular Plant Pathology | volume = 19 | issue = 4 | pages = 1029–1044 | date = April 2018 | pmid = 29024322 | pmc = 5772912 | doi = 10.1111/mpp.12580 }}</ref><ref>{{cite journal | vauthors = 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 | title = The Top 10 oomycete pathogens in molecular plant pathology | journal = Molecular Plant Pathology | volume = 16 | issue = 4 | pages = 413–34 | date = May 2015 | pmid = 25178392 | pmc = 6638381 | doi = 10.1111/mpp.12190 }}</ref> (the [[oomycete]] ''[[Phytophthora infestans]]'' causes [[late blight]] in potatoes)<ref>Campbell, N. and Reese, J. (2008) ''Biology''. Pearson Benjamin Cummings; 8 ed. {{ISBN|0805368442}}. pp. 583, 588</ref> or even of other protists.<ref>Lauckner, G. (1980). "Diseases of protozoa". In: ''Diseases of Marine Animals''. Kinne, O. (ed.). [https://www.int-res.com/archive/doma_books/DOMA_Vol_I_(general_aspects,_protozoa_to%20gastropoda).pdf Vol. 1, p. 84], John Wiley & Sons, Chichester, UK.</ref><ref>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.</ref>

Around 100 protist species can infect humans.<ref name="Geisen-2018"/> Two papers from 2013 have proposed [[virotherapy]], the use of viruses to treat infections caused by [[protozoa]].<ref>{{cite journal | vauthors = Keen EC | title = Beyond phage therapy: virotherapy of protozoal diseases | journal = Future Microbiology | volume = 8 | issue = 7 | pages = 821–3 | date = July 2013 | pmid = 23841627 | doi = 10.2217/FMB.13.48 }}</ref><ref>{{cite journal | vauthors = Hyman P, Atterbury R, Barrow P | title = Fleas and smaller fleas: virotherapy for parasite infections | journal = Trends in Microbiology | volume = 21 | issue = 5 | pages = 215–220 | date = May 2013 | pmid = 23540830 | doi = 10.1016/j.tim.2013.02.006 }}</ref>

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)<ref name="Liu-2006">{{cite journal | vauthors = Liu YJ, Hodson MC, Hall BD | title = Loss of the flagellum happened only once in the fungal lineage: phylogenetic structure of kingdom Fungi inferred from RNA polymerase II subunit genes | journal = BMC Evolutionary Biology | volume = 6 | page = 74 | date = September 2006 | pmid = 17010206 | pmc = 1599754 | doi = 10.1186/1471-2148-6-74 | doi-access = free }}</ref> can reduce red fire ant populations by 53–100%.<ref>{{cite web
|url= http://www.ars.usda.gov/is/pr/2010/100128.htm
|title= ARS Parasite Collections Assist Research and Diagnoses
|publisher=USDA Agricultural Research Service
|date=January 28, 2010}}</ref> Researchers have also been able to infect [[Phoridae|phorid]] fly [[parasitoid]]s of the ant with the protist without harming the flies. This turns the flies into a [[Vector (epidemiology)|vector]] that can spread the pathogenic protist between red fire ant colonies.<ref>Durham, Sharon (January 28, 2010) [http://www.ars.usda.gov/is/pr/2010/100128.htm ARS Parasite Collections Assist Research and Diagnoses]. Ars.usda.gov. Retrieved 2014-03-20.</ref>

== Biology ==
=== Physiological adaptations ===
[[File:Paramecium contractile vacuoles.jpg|thumb|upright=0.8|''[[Paramecium aurelia]]'' with [[contractile vacuole]]s]]
While, in general, protists are typical [[eukaryotic cell]]s and follow the same principles of [[physiology]] and [[biochemistry]] described for those cells within the "higher" eukaryotes (animals, fungi or plants),<ref>{{cite journal | vauthors = Plattner H | year = 2018 | title = Evolutionary cell biology of proteins from protists to humans and plants | url = http://nbn-resolving.de/urn:nbn:de:bsz:352-2-1kezn7g0ruv3m7| journal = J. Eukaryot. Microbiol. | volume = 65 | issue = 2| pages = 255–289 | doi = 10.1111/jeu.12449 | pmid = 28719054 | s2cid = 206055044 }}</ref> they have evolved a variety of unique physiological adaptations that do not appear in those eukaryotes.<ref name="Levandowsky-2012"/>

* ''Osmoregulation''. [[Freshwater]] protists without [[cell wall]]s are able to [[osmoregulation|regulate their osmosis]] through [[contractile vacuole]]s, specialized [[organelle]]s that periodically excrete fluid high in [[potassium]] and [[sodium]] through a cycle of diastole and systole. The cycle stops when the cells are placed in a medium with different salinity, until the cell adapts.<ref name="Levandowsky-2012"/>

[[File:Bmc evol bio hoppenrath proterythropsis ocelloid fig1a.png|thumb|upright=0.8|[[light microscopy|Light micrograph]] of an [[ocelloid]]-containing [[dinoflagellate]]. n: [[cell nucleus|nucleus]], double arrowhead: ocelloid, scale bar: 10 μm.<ref name="Hoppenrath-2009">{{cite journal|last1=Hoppenrath|first1=M|last2=Bachvaroff|first2=TR|last3=Handy|first3=SM|last4=Delwiche|first4=CF|last5=Leander|first5=BS|title=Molecular phylogeny of ocelloid-bearing dinoflagellates (Warnowiaceae) as inferred from SSU and LSU rDNA sequences.|journal=BMC Evolutionary Biology|date=25 May 2009|volume=9|pages=116|pmid=19467154|doi=10.1186/1471-2148-9-116|pmc=2694157|doi-access=free}}</ref>|alt=An image of a single cell featuring a large nucleus and an ocelloid, which is composed of a roundish "lens" and a darkly pigmented disc-shaped retinal body.]]

* ''Energetic adaptations''. The [[last eukaryotic common ancestor]] was [[aerobic organism|aerobic]], bearing [[mitochondria]] for [[oxidative metabolism]]. Many lineages of free-living and parasitic protists have independently evolved and adapted to inhabit [[Hypoxia (environmental)|anaerobic]] or [[microaerophilic]] habitats, by modifying the early mitochondria into [[hydrogenosome]]s, organelles that generate [[Adenosine triphosphate|ATP]] anaerobically through [[fermentation]] of [[pyruvate]]. In a parallel manner, in the microaerophilic [[trypanosomatid]] protists, the fermentative [[glycosome]] evolved from the [[peroxisome]].<ref name="Levandowsky-2012"/>

* ''Sensory adaptations''. Many flagellates and probably all motile algae exhibit a positive [[phototaxis]] (i.e. they swim or glide toward a source of light). For this purpose, they exhibit three kinds of [[Photoreceptors, microbial|photoreceptor]]s or "[[Eyespot apparatus|eyespot]]s": (1) receptors with light antennae, found in many [[green algae]], [[dinoflagellates]] and [[cryptophytes]]; (2) receptors with opaque screens; and (3) complex [[ocelloid]]s with intracellular lenses, found in one group of predatory [[dinoflagellate]]s, the [[Warnowiaceae]]. Additionally, some [[ciliates]] orient themselves in relation to the Earth's [[gravitational field]] while moving ([[geotaxis]]), and others swim in relation to the concentration of dissolved [[oxygen]] in the water.<ref name="Levandowsky-2012"/>

* ''Endosymbiosis''. Protists have an accentuated tendency to include [[endosymbiont]]s in their cells, and these have produced new physiological opportunities. Some associations are more permanent, such as ''[[Paramecium bursaria]]'' and its endosymbiont ''[[Chlorella]]''; others more transient. Many protists contain captured chloroplasts, chloroplast-mitochondrial complexes, and even eyespots from algae. The [[xenosome]]s are [[bacterial]] endosymbionts found in ciliates, sometimes with a [[methanogenic]] role inside anaerobic ciliates.<ref name="Levandowsky-2012"/>

=== Sexual reproduction ===
[[File: Coleps-Konjugation.jpg|upright=0.7|thumb|Two similar-looking but sexually distinct ''[[Coleps]]'' partners connected at their front ends exchange genetic material via a plasma bridge.]]

Protists generally [[asexual reproduction|reproduce asexually]] under favorable environmental conditions, but tend to [[sexual reproduction|reproduce sexually]] under stressful conditions, such as starvation or heat shock. [[Oxidative stress]], which leads to [[DNA damage]], also appears to be an important factor in the induction of sex in protists.<ref name="Bernstein-2012">{{cite book|vauthors=Bernstein H, Bernstein C, Michod RE|date=2012|chapter=Chapter 1. DNA repair as the primary adaptive function of sex in bacteria and eukaryotes|pages=1–49|title=DNA Repair: New Research|veditors=Kimura S, Shimizu S|publisher=Nova Sci. Publ.|publication-place=Hauppauge, N.Y.|isbn=978-1-62100-808-8}}</ref>

[[Eukaryote]]s emerged in evolution more than 1.5&nbsp;billion years ago.<ref>{{cite journal | vauthors = Javaux EJ, Knoll AH, Walter MR | s2cid = 205018792 | title = Morphological and ecological complexity in early eukaryotic ecosystems | journal = Nature | volume = 412 | issue = 6842 | pages = 66–69 | date = July 2001 | pmid = 11452306 | doi = 10.1038/35083562 | bibcode = 2001Natur.412...66J }}</ref> The earliest eukaryotes were protists. Although sexual reproduction is widespread among [[multicellular]] eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes. The main reason for this view was that sex appeared to be lacking in certain [[pathogen]]ic protists whose ancestors branched off early from the eukaryotic family tree. However, several of these "early-branching" protists that were thought to predate the emergence of meiosis and sex (such as ''[[Giardia lamblia]]'' and ''[[Trichomonas vaginalis]]'') are now known to descend from ancestors capable of [[meiosis]] and [[meiotic recombination]], because they have a set core of meiotic genes that are present in sexual eukaryotes.<ref>{{cite journal | vauthors = Ramesh MA, Malik SB, Logsdon JM | s2cid = 17013247 | title = A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis | journal = Current Biology | volume = 15 | issue = 2 | pages = 185–191 | date = January 2005 | pmid = 15668177 | doi = 10.1016/j.cub.2005.01.003 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Cooper MA, Adam RD, Worobey M, Sterling CR | s2cid = 15991722 | title = Population genetics provides evidence for recombination in Giardia | journal = Current Biology | volume = 17 | issue = 22 | pages = 1984–1988 | date = November 2007 | pmid = 17980591 | doi = 10.1016/j.cub.2007.10.020 | doi-access = free }}</ref> Most of these meiotic genes were likely present in the [[last eukaryotic common ancestor|common ancestor of all eukaryotes]],<ref name="Malik-2007">{{cite journal | vauthors = Malik SB, Pightling AW, Stefaniak LM, Schurko AM, Logsdon JM | title = An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis | journal = PLOS ONE | volume = 3 | issue = 8 | pages = e2879 | date = August 2007 | pmid = 18663385 | pmc = 2488364 | doi = 10.1371/journal.pone.0002879 | editor1-last = Hahn | bibcode = 2008PLoSO...3.2879M | editor1-first = Matthew W | doi-access = free }}</ref> which was likely capable of facultative (non-obligate) sexual reproduction.<ref>{{cite journal | vauthors = Dacks J, Roger AJ | s2cid = 9441768 | title = The first sexual lineage and the relevance of facultative sex | journal = Journal of Molecular Evolution | volume = 48 | issue = 6 | pages = 779–783 | date = June 1999 | pmid = 10229582 | doi = 10.1007/PL00013156 | bibcode = 1999JMolE..48..779D }}</ref>

This view was further supported by a 2011 study on [[amoebae]]. Amoebae have been regarded as [[asexual organism]]s, but the study describes evidence that most [[amoeboid]] lineages are ancestrally sexual, and that the majority of asexual groups likely arose recently and independently.<ref name="Lahr-2011">{{cite journal | vauthors = Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E | title = The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms | journal = Proceedings: Biological Sciences | volume = 278 | issue = 1715 | pages = 2081–2090 | date = July 2011 | pmid = 21429931 | pmc = 3107637 | doi = 10.1098/rspb.2011.0289 }}</ref> Even in the early 20th century, some researchers interpreted phenomena related to chromidia ([[chromatin]] granules free in the [[cytoplasm]]) in amoebae as sexual reproduction.<ref name="Dobell-1909">{{cite journal |author=Dobell, C. |year=1909 |title=Chromidia and the binuclearity hypotheses: A review and a criticism |journal=Quarterly Journal of Microscopical Science |volume=53 |pages=279–326 |url=http://jcs.biologists.org/content/s2-53/210/279.full.pdf}}</ref>

==== Sex in pathogenic 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 (biology)|host]] – but can undergo sexual reproduction only in the primary or definitive [[Host (biology)|host]] (for example: [[felid]]s such as [[domestic cat]]s in this case).<ref>{{cite web |title=CDC – Toxoplasmosis – Biology |url=https://www.cdc.gov/parasites/toxoplasmosis/biology.html |date=17 March 2015 |access-date=14 June 2015}}</ref><ref>{{cite web |title=Cat parasite linked to mental illness, schizophrenia |date=5 June 2015 |url=http://www.cbsnews.com/news/cat-parasite-toxoplasma-gondii-linked-to-mental-illness-schizophrenia/ |publisher=CBS |access-date=23 September 2015}}</ref><ref>{{cite web |title=CDC – About Parasites |url=https://www.cdc.gov/parasites/about.html |access-date=12 March 2013}}</ref>

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.<ref>{{cite journal | vauthors = Talman AM, Domarle O, McKenzie FE, Ariey F, Robert V | title = Gametocytogenesis: the puberty of Plasmodium falciparum | journal = Malaria Journal | volume = 3 | pages = 24 | date = July 2004 | pmid = 15253774 | pmc = 497046 | doi = 10.1186/1475-2875-3-24 | doi-access = free }}</ref> 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.<ref>{{cite journal | vauthors = Tibayrenc M, Kjellberg F, Arnaud J, Oury B, Brenière SF, Dardé ML, Ayala FJ | title = Are eukaryotic microorganisms clonal or sexual? A population genetics vantage | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 88 | issue = 12 | pages = 5129–33 | date = June 1991 | pmid = 1675793 | pmc = 51825 | doi = 10.1073/pnas.88.12.5129 | bibcode = 1991PNAS...88.5129T | display-authors = 1 | doi-access = free }}</ref>

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.<ref>{{cite journal | vauthors = Akopyants NS, Kimblin N, Secundino N, Patrick R, Peters N, Lawyer P, Dobson DE, Beverley SM, Sacks DL | title = Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector | journal = Science | volume = 324 | issue = 5924 | pages = 265–268 | date = April 2009 | pmid = 19359589 | pmc = 2729066 | doi = 10.1126/science.1169464 | bibcode = 2009Sci...324..265A | display-authors = 1 }}</ref>

==Fossil record==
{{main|Protists in the fossil record}}
{{further|protist shell|microfossils}}
===Mesoproterozoic===
{{further|Protosterol biota}}
By definition, all [[eukaryotes]] before the existence of [[plants]], [[animals]] and [[fungi]] are considered protists. For that reason, this section contains information about the deep ancestry of all eukaryotes.

All living [[eukaryotes]], including protists, evolved from the [[last eukaryotic common ancestor]] (LECA). Descendants of this ancestor are known as "[[crown-group]]" or "modern" eukaryotes. [[Molecular clock]]s suggest that LECA originated between 1200 and more than 1800&nbsp;million years ago (Ma). Based on all molecular predictions, modern eukaryotes reached [[morphology (biology)|morphological]] and [[ecological diversity]] before 1000 Ma in the form of [[multicellular]] [[algae]] capable of [[sexual reproduction]], and unicellular protists capable of [[phagocytosis]] and [[cell locomotion|locomotion]]. However, the fossil record of modern eukaryotes is very scarce around this period, which contradicts the predicted diversity.<ref name="Brocks-2023"/>

Instead, the fossil record of this period contains "[[stem-group]] eukaryotes". These fossils cannot be assigned to any known crown group, so they probably belong to extinct lineages that originated before LECA. They appear continuously throughout the [[Mesoproterozoic]] fossil record (1650–1000 Ma). They present defining eukaryote characteristics such as complex [[cell wall]] ornamentation and [[cell membrane]] protrusions, which require a flexible [[endomembrane system]]. However, they had a major distinction from crown eukaryores: the composition of their cell membrane. Unlike crown eukaryotes, which produce "crown [[sterol]]s" for their cell membranes (e.g. [[cholesterol]] and [[ergosterol]]), stem eukaryotes produced "[[Protosterol biota|protosterols]]", which appear earlier in the [[biosynthetic pathway]].<ref name="Brocks-2023"/>

Crown sterols, while metabolically more expensive, may have granted several evolutionary advantages for LECA's descendants. Specific unsaturation patterns in crown sterols protect against [[osmotic shock]] during desiccation and rehydration cycles. Crown sterols can also receive [[Ethyl group|ethyl]] groups, thus enhancing cohesion between [[lipid]]s and adapting cells against extreme cold and heat. Moreover, the additional steps in the biosynthetic pathway allow cells to regulate the proportion of different sterols in their membranes, in turn allowing for a wider habitable temperature range and unique mechanisms such as [[asymmetric cell division]] or membrane repair under exposure to [[UV light]]. A more speculative role of these sterols is their protection against the Proterozoic [[Great Oxidation Event|changing oxygen levels]]. It is theorized that all of these sterol-based mechanisms allowed LECA's descendants to live as [[extremophile]]s of their time, [[Evolutionary radiation|diversifying]] into [[ecological niche]]s that experienced cycles of desiccation and rehydration, daily extremes of high and low temperatures, and elevated UV radiation (such as [[mudflat]]s, rivers, agitated shorelines and [[subaerial]] soil).<ref name="Brocks-2023"/>

In contrast, the named mechanisms were absent in stem-group eukaryotes, as they were only capable of producing protosterols. Instead, these protosterol-based life forms occupied open marine waters. They were facultative [[anaerobe]]s that thrived in [[Mesoproterozoic]] waters, which at the time were low on oxygen. Eventually, during the [[Tonian]] period ([[Neoproterozoic]] era), oxygen levels increased and the crown eukaryotes were able to expand to open marine environments thanks to their preference for more oxygenated habitats. Stem eukaryotes may have been driven to extinction as a result of this competition. Additionally, their protosterol membranes may have posed a disadvantage during the cold of the [[Cryogenian]] "[[Snowball Earth]]" [[glaciation]]s and the extreme global heat that came afterwards.<ref name="Brocks-2023"/>

===Neoproterozoic===
Modern eukaryotes began to appear abundantly in the [[Tonian]] period (1000–720 Ma), fueled by the proliferation of [[red algae]]. The oldest fossils assigned to modern eukaryotes belong to two [[photosynthetic]] protists: the multicellular [[red alga]] ''[[Bangiomorpha]]'' (from 1050 Ma), and the [[chlorophyte]] [[green alga]] ''[[Proterocladus]]'' (from 1000 Ma).<ref name="Brocks-2023">{{cite journal|first1=Jochen J.|last1=Brocks|first2=Benjamin J.|last2=Nettersheim|first3=Pierre|last3=Adam|first4=Philippe|last4=Schaeffer|first5=Amber J. M.|last5=Jarrett|first6=Nur|last6=Güneli|first7=Tharika|last7=Liyanage|first8=Lennart M.|last8=van Maldegem|first9=Christian|last9=Hallmann|first10=Janet M.|last10=Hope |title= Lost world of complex life and the late rise of the eukaryotic crown |journal=Nature|volume=618|pages=767–773|date=2023|issue=7966 |doi=10.1038/s41586-023-06170-w|pmid=37286610 |s2cid=259111647 |url=https://hal.science/hal-04273175/file/Brocks%20et%20al.%20Text%202023%20%2B%20Supplementary.pdf }}</ref> Abundant fossils of [[heterotrophic]] protists appear later, around 900 Ma, with the emergence of [[fungi]].<ref name="Brocks-2023"/> For example, the oldest fossils of [[Amoebozoa]] are vase-shaped microfossils resembling modern [[testate amoebae]], found in 800 million-year-old rocks.<ref name="Porter-2003"/><ref name="Parfrey-2011"/> [[Radiolaria]]n shells are found abundantly in the fossil record after the [[Cambrian]] period (~500 Ma), but more recent paleontological studies are beginning to interpret some [[Precambrian]] fossils as the earliest evidence of radiolarians.<ref name="Chang-2018">{{cite journal |last1=Chang |first1=Shan |last2=Feng |first2=Qinglai |last3=Zhang |first3=Lei |title=New Siliceous Microfossils from the Terreneuvian Yanjiahe Formation, South China: The Possible Earliest Radiolarian Fossil Record |journal=Journal of Earth Science |date=14 August 2018 |volume=29 |issue=4 |pages=912–919 |doi=10.1007/s12583-017-0960-0|s2cid=134890245 }}</ref><ref name="Zhang-2019">{{cite journal |last1=Zhang |first1=Ke |last2=Feng |first2=Qing-Lai |title=Early Cambrian radiolarians and sponge spicules from the Niujiaohe Formation in South China |journal=Palaeoworld |date=September 2019 |volume=28 |issue=3 |pages=234–242 |doi=10.1016/j.palwor.2019.04.001|s2cid=146452469 }}</ref><ref name="Maletz-2017">{{cite journal |last1=Maletz |first1=Jörg |title=The identification of putative Lower Cambrian Radiolaria |journal=Revue de Micropaléontologie |date=June 2017 |volume=60 |issue=2 |pages=233–240 |doi=10.1016/j.revmic.2017.04.001}}</ref>

== See also ==
* [[Evolution of sexual reproduction]]
* [[Protist locomotion]]

== Footnotes ==
{{Notelist}}

== References ==
{{Reflist|25em|refs=

<ref name="Bar-On-2018">{{Cite journal |last1=Bar-On |first1=Yinon M. |last2=Phillips |first2=Rob |last3=Milo |first3=Ron |date=2018-05-17 |df=dmy-all |title=The biomass distribution on Earth |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=25 |language=en |pages=6506–6511 |doi=10.1073/pnas.1711842115 |issn=0027-8424 |pmid=29784790 |pmc=6016768|bibcode=2018PNAS..115.6506B |doi-access=free }}</ref>

<ref name="Adl-2019">{{cite journal|vauthors=Adl SM, Bass D, Lane CE, Lukeš J, Schoch CL, Smirnov A, Agatha S, Berney C, Brown MW, Burki F, Cárdenas P, Čepička I, Chistyakova L, del Campo J, Dunthorn M, Edvardsen B, Eglit Y, Guillou L, Hampl V, Heiss AA, Hoppenrath M, James TY, Karnkowska A, Karpov S, Kim E, Kolisko M, Kudryavtsev A, ((Lahr DJG)), Lara E, Le Gall L, Lynn DH, Mann DG, Massana R, ((Mitchell EAD)), Morrow C, Park JS, Pawlowski JW, Powell MJ, Richter DJ, Rueckert S, Shadwick L, Shimano S, Spiegel FW, Torruella G, Youssef N, Zlatogursky V, Zhang Q|year=2019|title=Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes|journal=Journal of Eukaryotic Microbiology|volume=66|issue=1 |pages=4–119|doi=10.1111/jeu.12691|pmid=30257078 |pmc=6492006 }}</ref>

<ref name="Leadbeater-2000">{{cite book|title=The Flagellates. Unity, diversity and evolution|editor1=Barry S. C. Leadbeater|editor2=J. C. Green|chapter=Chapter 1. The flagellates: historical perspectives|author1=Barry S. C. Leadbeater|author2=Sharon M. M. McReady|publisher=Taylor & Francis|publication-place=London|date=2000|pages=1–26|doi=10.1201/9781482268225|isbn=9780429182136 }}</ref>

<ref name="Ratcliff-2009">{{cite book|author=Marc J. Ratcliff|publisher=Ashgate|date=2009|url=https://books.google.com/books?id=GTZt1c5IYioC|chapter=The Emergence of the Systematics of Infusoria|title=The Quest for the Invisible: Microscopy in the Enlightenment|pages=177–216|isbn=9781409480266 }}</ref>

<ref name="Goldfuß-1818">{{cite journal |last1=Goldfuß |title=Ueber die Classification der Zoophyten |journal=Isis, Oder, Encyclopädische Zeitung von Oken |date=1818 |volume=2 |issue=6 |pages=1008–1019 |url=https://www.biodiversitylibrary.org/item/47614#page/530/mode/1up |trans-title=On the classification of zoophytes |language=de}} 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.]</ref>

<ref name="Scamardella-1999">{{cite journal | vauthors = Scamardella JM |title = Not plants or animals: A brief history of the origin of Kingdoms Protozoa, Protista, and Protoctista | year = 1999 | journal = International Microbiology | volume = 2 |issue = 4 | pages = 207–221 |pmid = 10943416 | url = http://revistes.iec.cat/index.php/IM/article/viewFile/9219/9217}}</ref> 

<ref name="vonSiebold-1846">{{cite book|author1=Carl Theodor Ernst von Siebold|author2=Hermann Stannius|title=Lehrbuch der vergleichenden Anatomie Vol. 1: Wirbellose Thiere|trans-title=Textbook of Comparative Anatomy Vol. 1: Invertebrate Animals|date=1846–1848|volume=1 |publisher=Veit|publication-place=Berlin, Germany|page=3|url=https://www.biodiversitylibrary.org/item/41783#page/21/mode/1up|language=de|quote-page=3|quote=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.|trans-quote=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.}}</ref>

<ref name="Hogg-1860">{{cite journal|author=John Hogg|date=1860|title=On the distinctions of a Plant and an Animal, and on a Fourth Kingdom of Nature|url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044089575245;view=1up;seq=232 |journal=Edinburgh New Philosophical Journal|series=2nd series|volume=12|pages=216–225|quote-page=223|quote=... 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''; ...}}</ref>

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<ref name="Haeckel-1878">{{cite book|title=Das protistenreich. Eine populäre uebersicht über das formengebiet der niedersten lebewesen|first1=Ernst|last1=Haeckel|author-link1=Ernst Haeckel|date=1878|publication-place=Leipzig|publisher=E. Günther|trans-title=The protistan kingdom. A popular survey of the forms of the lowest living beings|language=German|doi=10.5962/bhl.title.58542|url=https://www.biodiversitylibrary.org/bibliography/58542 }}</ref>

<ref name="Faure-2019">{{cite journal|last1=Faure|first1=Emile|last2=Not|first2=Fabrice|last3=Benoiston|first3=Anne-Sophie|last4=Labadie|first4=Karine|last5=Bittner|first5=Lucie|last6=Ayata|first6=Sakina-Dorothée|date=April 2019|title=Mixotrophic protists display contrasted biogeographies in the global ocean|journal=The ISME Journal|volume=13|issue=4|pages=1072–1083|pmc=6461780|pmid=30643201|doi=10.1038/s41396-018-0340-5}}</ref>

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<ref name="Geisen-2018">{{cite journal|first1=Stefan |last1=Geisen |first2=Edward A. D. |last2=Mitchell |first3=Sina |last3=Adl |first4=Michael |last4=Bonkowski |first5=Micah |last5=Dunthorn |first6=Flemming |last6=Ekelund |first7=Leonardo D. |last7=Fernández |first8=Alexandre |last8=Jousset |first9=Valentyna |last9=Krashevska |first10=David |last10=Singer |first11=Frederick W. |last11=Spiegel |first12=Julia |last12=Walochnik |first13=Enrique |last13=Lara|title=Soil protists: a fertile frontier in soil biology research|journal=FEMS Microbiology Reviews|volume=42|issue=3|date=May 2018|pages=293–323|doi=10.1093/femsre/fuy006|pmid=29447350 |doi-access=free }}</ref>

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}}

==Bibliography==

===General===
* 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===
* Fontaneto, D. ''Biogeography of Microscopic Organisms. Is Everything Small Everywhere?'' Cambridge University Press, Cambridge, 2011.
* Moore, R. C., and other editors. ''[[Treatise on Invertebrate Paleontology]]''. Protista, part B (vol. 1{{Dead link|date=May 2020 |bot=InternetArchiveBot |fix-attempted=yes }}, Charophyta, vol. 2, Chrysomonadida, Coccolithophorida, Charophyta, Diatomacea & Pyrrhophyta), part C (Sarcodina, Chiefly "Thecamoebians" and Foraminiferida) and part D{{Dead link|date=May 2020 |bot=InternetArchiveBot |fix-attempted=yes }} (Chiefly Radiolaria and Tintinnina). Boulder, Colorado: Geological Society of America; & Lawrence, Kansas: University of Kansas Press.
== External links ==
{{Wikispecies|Protista}}
{{Wikispecies|Protoctista}}
{{Commons category|Protista}}
* [https://eukmap.unieuk.net/ UniEuk Taxonomy App]
* [http://tolweb.org/Eukaryotes/3 Tree of Life: Eukaryotes]
* Tsukii, Y. (1996). ''Protist Information Server'' (database of protist images). Laboratory of Biology, Hosei University. [http://protist.i.hosei.ac.jp/ Protist Information Server]. Updated: March 22, 2016.

{{Eukaryota}}
{{Protozoa protist}}
{{Life on Earth}}
{{Organisms et al.}}
{{Taxonbar|from=Q10892}}
{{Authority control}}

[[Category:Protista| ]]
[[Category:Obsolete eukaryote taxa]]
[[Category:Paraphyletic groups]]

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