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A protist ( ) or protoctist is any Eukaryote organism that is not an animal, Embryophyte, or fungus. Protists do not form a Clade, or clade, but are a Paraphyly grouping of all descendants of the last eukaryotic common ancestor excluding land plants, animals, and fungi.
Protists were historically regarded as a separate taxonomic rank 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 supergroups, such as Archaeplastida ( that includes land plants), SAR supergroup, Obazoa (which includes fungi and animals), Amoebozoa and "Excavata".
Protists represent an extremely large genetic and ecological diversity in all environments, including extreme habitats. Their Biodiversity, 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 as important components of the biogeochemical cycles and . They exist abundantly and ubiquitously in a variety of mostly unicellular forms that evolved multiple times independently, such as free-living algae, amoebae and , or as important . 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.
The names of some protists (called ambiregnal protists), because of their mixture of traits similar to both animals and land plants or fungi (e.g., slime molds and algae like euglenids), have been published under either or both of the botanical ( ICNafp) and the zoological ( ICZN) codes of nomenclature.
The evolutionary relationships of protists have been explained through molecular phylogenetics, the DNA sequencing of entire and , and electron microscopy studies of the flagellum 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 do not recognize the formal (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 branches informally named supergroups. Most photosynthetic eukaryotes fall under the Diaphoretickes clade, which contains the supergroups Archaeplastida (which includes land plants) and SAR supergroup (including, Stramenopiles, Alveolata and Rhizaria), as well as the phyla Telonemia, Cryptista and Haptista. The animals and fungi fall into the Amorphea supergroup, which contains the phylum Amoebozoa and several other protist lineages. Various groups of eukaryotes with primitive cell architecture are collectively known as the "Excavata".
Discoba includes three major groups: Jakobida, Euglenozoa and Percolozoa. Jakobida are a small group (~20 species) of free-living heterotrophic flagellates, with two cilia, that primarily eat bacteria through suspension feeding; most are aquatic aerobes, with some anaerobic species, found in marine, brackish or fresh water. (2025). 9783319281476, Springer. ISBN 9783319281476 They are best known for their bacterial-like mitochondrial genomes. Euglenozoa is a rich (>2,000 species) group of flagellates with very different lifestyles, including: the free-living heterotrophic (both osmo- and phagotrophic) and photosynthetic (e.g., the , with chloroplasts originated from green algae); the free-living and parasitic (such as Trypanosoma); the deep-sea anaerobic Symbiontida; and the elusive . Percolozoa (~150 species) are a collection of amoebae, flagellates and amoeboflagellates with complex life cycles, among which are some slime molds (). (2025). 9783319281476, Springer. ISBN 9783319281476 The two clades Euglenozoa and Percolozoa are Sister group, united under the name Discicristata, in reference to their crista shaped like discs. The species Tsukubamonas globosa is a free-living flagellate whose precise position within Discoba is not yet settled, but is probably more closely related to Discicristata than to Jakobida.
The (Metamonada) are a phylum of completely anaerobe or microaerophilic protozoa, primarily . Some are gut symbionts of animals such as , others are free-living, and others are parasitic. They include three main clades: Fornicata, Parabasalia and Preaxostyla. Fornicata (>140 species) encompasses the , with two cell nucleus (e.g., Giardia), and several smaller groups of free-living, commensal and parasitic protists (e.g., Carpediemonas, ). Parabasalia (>460 species) is a varied group of anaerobic, mostly endobiotic organisms, ranging from small parasites (like Trichomonas) to giant intestinal symbionts with numerous flagella and nuclei found in wood-eating termites and . Preaxostyla (~140 species) includes the anaerobic and endobiotic , with modified (or completely lost) mitochondria, and two genera of free-living microaerophilic bacterivorous flagellates Trimastix and Paratrimastix, with typical excavate morphology. Two genera of anaerobic flagellates of recent description and unique cell architecture, Barthelona and Skoliomonas, are closely related to the Fornicata.
The (Malawimonadida) are a small group (three species) of freshwater or marine suspension-feeding bacterivorous flagellates with typical excavate appearance, closely resembling Jakobida and some metamonads but not phylogenetically close to either in most analyses.
The phylum Gyrista includes the photosynthetic Ochrophyta or Heterokontophyta (>23,000 species), which contain chloroplasts originated from a red alga. Among these are many lineages of algae that encompass a wide range of structures and morphologies. The three most diverse ochrophyte classes are: the , unicellular or colonial organisms encased in silica cell walls () that exhibit widely different shapes and ornamentations and comprise much of the marine phytoplankton; the brown algae, filamentous or 'truly' multicellular (with differentiated tissues) macroalgae that constitute the basis of many temperate and cold marine ecosystems, such as ; (2025). 9783319281476, Springer. ISBN 9783319281476 and the golden algae, unicellular or colonial flagellates that are mostly present in freshwater habitats. (2025). 9783319281476, Springer. ISBN 9783319281476 Inside Gyrista, the sister clade to Ochrophyta are the predominantly and filamentous pseudofungi (>1,200 species), which include three distinct lineages: the parasitic oomycetes or water moulds (e.g., Phytophthora), which encompass most of the pseudofungi species; the less diverse non-parasitic that maintain a fungus-like lifestyle; and the , a group of bacterivorous or eukaryovorous phagotrophs. A small group of heliozoan-like heterotrophic amoebae, Actinophryida, has an uncertain position, either within or as the sister taxon of Ochrophyta.
The little studied phylum Bigyra is an assemblage of exclusively heterotrophic organisms, most of which are free-living. It includes the labyrinthulomycetes, among which are single-celled amoeboid phagotrophs, mixotrophs, and fungus-like filamentous heterotrophs that create slime networks to move and absorb nutrients, as well as some parasites and a few testate amoebae (Amphitremida). Also included in Bigyra are the , phagotrophic flagellates that consume bacteria, and the closely related Placidozoa, which consists of several groups of heterotrophic flagellates (e.g., the deep-sea halophilic Placididea) as well as the intestinal known as Opalinata (e.g., the human parasite Blastocystis, and the highly unusual , composed of giant cells with numerous nuclei and cilia, originally misclassified as ciliates).
The remaining alveolates are grouped under the clade Myzozoa, whose common ancestor acquired chloroplasts through a secondary endosymbiosis from a red alga. One branch of Myzozoa contains the apicomplexans and their closest relatives, a small clade of flagellates known as Chrompodellida where phototrophic and heterotrophic flagellates, called and respectively, are evolutionarily intermingled. In contrast, the apicomplexans (Apicomplexa) are a large (>6,000 species) and highly specialized group of obligate parasites who have all secondarily lost their photosynthetic ability (e.g., Plasmodium). Their adult stages absorb nutrients from the host through the cell membrane, and they reproduce between hosts via sporozoites, which exhibit an organelle complex (the apicoplast) evolved from non-photosynthetic chloroplasts. (2025). 9783319281476, Springer. ISBN 9783319281476
The other branch of Myzozoa contains the dinoflagellates and their closest relatives, the perkinsids (Perkinsozoa), a small group (26 species) of aquatic intracellular parasites which have lost their photosynthetic ability similarly to apicomplexans. They reproduce through flagellated spores that infect dinoflagellates, and fish. In contrast, the dinoflagellates (Dinoflagellata) are a highly diversified (~4,500 species) (2025). 9783319281476, Springer. ISBN 9783319281476 group of aquatic algae that have mostly retained their chloroplasts, although many lineages have lost their own and instead either live as heterotrophs or reacquire new chloroplasts from other sources, including tertiary endosymbiosis and kleptoplasty. Most dinoflagellates are free-living and compose an important portion of phytoplankton, as well as a major cause of harmful algal blooms due to their toxicity; some live as symbionts of corals, allowing the creation of coral reefs. Dinoflagellates exhibit a diversity of cellular structures, such as complex eyelike ocelli, specialized vacuoles, bioluminescent organelles, and a wall surrounding the cell known as the theca.
Retaria contains the most familiar rhizarians: and , two groups of large free-living marine amoebae with pseudopodia supported by , many of which are macroscopic. The radiolarians (Radiolaria) are a diverse group (>1,000 living species) of amoebae, often bearing delicate and intricate siliceous skeletons. The forams (Foraminifera) are also diverse (>6,700 living species), and most of them are encased in multichambered tests constructed from calcium carbonate or agglutinated mineral particles. Both groups have a rich fossil record, with tens of thousands of described fossil species. (2025). 9783319281476, Springer. ISBN 9783319281476
Cercozoa (also known as Filosa) is an assemblage of free-living protists with very different morphologies. Cercozoan amoeboflagellates are important predators of other microbes in terrestrial habitats and the plant microbiota (e.g., and and , collectively known as class Sarcomonadea), and a few can generate slime molds (e.g., Helkesea). Many cercozoans are testate or scale-bearing amoebae, namely the elusive Kraken and the two classes Imbricatea (e.g., the ) and Thecofilosea. Thecofilosea also contains the Phaeodaria (~400–500 species), a group of skeleton-bearing marine amoebae previously classified as radiolarians, and both classes include some non-scaly naked flagellates (e.g., in Imbricatea and in Thecofilosea). Among the basal-branching cercozoans are the pseudopodia-lacking thecate flagellates of Metromonadea, the heliozoan-like Granofilosea and the photosynthetic chlorarachniophytes, whose chloroplasts originated from a secondary endosymbiosis with a green alga.
Endomyxa contains two major clades of parasitic protists: Ascetosporea are sporozoan-type parasites of marine invertebrates, while Phytomyxea are obligate pathogens of plants and algae, divided into the terrestrial and the marine . Also included in Endomyxa are the order of predatory amoebae Vampyrellida (48 species) and two genera of marine amoebae, the thecate Gromia and the naked Filoreta.
Besides these three phyla, Rhizaria includes numerous enigmatic and understudied lineages of uncertain evolutionary position. One such clade is the Gymnosphaerida, which includes heliozoan-type protists. (2025). 9783319281476, Springer. ISBN 9783319281476 Several clades labeled as Novel Clades (NC) are entirely composed of environmental DNA from uncultured protists, although a few have slowly been resolved over the decades with the description of new taxa (e.g., Tremulida and Aquavolonida, formerly NC11 and NC10 respectively, with a deep-branching position in Rhizaria).
The phylum Haptista includes two distinct clades with mineralized scales: and . The haptophytes (Haptophyta) are a group of over 500 living species of flagellated or coccoid algae that have acquired chloroplasts from a secondary endosymbiosis. They are mostly marine, comprise an important portion of oceanic plankton, and include the , whose calcified scales ('') contribute to the formation of sedimentary rocks and the biogeochemical cycles of carbon and calcium. Some species are capable of forming toxic blooms. (2025). 9783319281476, Springer. ISBN 9783319281476 The centrohelids (Centroplasthelida) are a small (~95 species) but widespread group of heterotrophic heliozoan-type amoebae, usually covered in scale-bearing mucous, that form an important component of benthic food webs of aquatic habitats, both marine and freshwater.
The phylum Cryptista is a clade of three distinct groups of unicellular protists: , , and the species Palpitomonas bilix. The cryptomonads (>100 species), also known as cryptophytes, are flagellated algae found in aquatic habitats of diverse salinity, characterized by extrusive organelles or called ejectisomes. Their chloroplasts, of red algal origin, contain a nucleomorph, a remnant of the eukaryotic nucleus belonging to the endosymbiotic red alga. (2025). 9783319281476, Springer. ISBN 9783319281476 The katablepharids, the closest relatives of cryptomonads, are heterotrophic flagellates with two cilia, also characterized by ejectisomes. The species Palpitomonas bilix is the most basal-branching member of Cryptista, a marine heterotrophic flagellate with two cilia, but unlike the remaining members it lacks ejectisomes.
The red algae or Rhodophyta (>7,100 species) are a group of diverse morphologies, ranging from single cells to multicellular filaments to giant thalli, all without flagella. They lack chlorophyll and only harvest light energy through . Their life cycles are varied and may include two or three generations. They are present in terrestrial, freshwater and primarily marine habitats, from the intertidal zone to deep waters; some are calcified and are vital components of marine ecosystems such as . (2025). 9783319281476, Springer. ISBN 9783319281476 Closely related to the red algae are two small lineages of non-photosynthetic predatory flagellates: the freshwater and marine Rhodelphidia (3 species), which still retain genetic evidence of relic plastids; and the marine Picozoa (1 species), which lack any remains of plastids. The evolutionary position of Picozoa may indicate that there have been two separate events of primary endosymbiosis, as opposed to one.
The green algae, unlike the Monophyly glaucophytes and rhodophytes, are a Paraphyly group from which land plants evolved. Together they compose the Chloroplastida or Viridiplantae clade. The earliest branching member is the phylum Prasinodermophyta (ten species), whose members are exclusively marine coccoid cells or small macroscopic thalli. The remaining green algae are distributed in two major clades. One clade is the phylum Chlorophyta (>7,900 species), which includes numerous lineages of scaly unicellular flagellate algae known collectively as along with the Prasinodermophyta, but also includes a variety of morphologies such as coccoids, palmelloids, colonies, and macroscopic filamentous, foliose or tubular thalli, present in aquatic and terrestrial habitats. The opposed clade is Streptophyta, which contains the land plants and a paraphyletic group of green algae collectively known as phylum Charophyta, composed of several classes: Zygnematophyceae (>4,300 species), containing unicellular, colonial and filamentous flagella-lacking organisms found almost exclusively in freshwater habitats; (2025). 9783319281476, Springer. ISBN 9783319281476 Charophyceae (450 living species), also known as stoneworts, consisting of complex multicellular thalli only found in freshwater habitats; (2025). 9783319281476, Springer. ISBN 9783319281476 Klebsormidiophyceae (52 species), with unbranched filamentous thalli; Coleochaetophyceae (36 species), containing branched filamentous thalli; Mesostigmatophyceae, composed of a single species of scaly flagellates; and Chlorokybophyceae (five species), with sarcinoid forms. (2025). 9783319281476, Springer. ISBN 9783319281476
The phylum Amoebozoa (2,400 species) is a large group of morphologically diverse phagotrophic protists, mostly amoebae. A considerable portion of amoebozoans are lobosa, meaning they produce round, blunt-ended pseudopods. It includes the 'archetypal' amoebae, known as the naked lobose amoebae or 'gymnamoebae' (such as Amoeba itself), among which is a genus of sorocarp-forming slime moulds, Copromyxa. Some gymnamoebae are important pathogens to animals (e.g., Acanthamoeba). Other relevant lobose amoebae are the Arcellinida, a diverse order of testate amoebae and one of the most conspicuous protist groups overall. The remaining, non-lobose amoebozoans include the Eumycetozoa or 'true slime moulds', comprising the sorocarp-producing bacterivorous and the sporocarp-producing omnivorous and Protosporangiida. Due to the fungus-like appearance of their fruiting bodies, eumycetozoans are often studied by mycologists. Closely related to the eumycetozoans are two lineages: the Variosea, a heterogeneous assortment of amoeboid, reticulate or flagellated organisms (including some sorocarp-producing organisms); and the anaerobic Archamoebae, some of which live as intestinal symbionts of some animals (e.g., Entamoeba).
Opisthokonta includes the animal and fungal kingdoms, as well as their closest protist relatives. The branch leading to the fungi is known as Nucletmycea or Holomycota, while the branch leading to the animals is called Holozoa. The Holomycota includes the closest relatives of fungi, the , a small group (~50 species) of free-living naked or scale-bearing phagotrophic amoebae with filose pseudopodia, some of which can aggregate into slime moulds. Within the wider definition of fungi, three groups are studied as protists by some authors: Aphelida (15 species), Rozellida (27 species) and Microsporidia (~1,300 species), (2025). 9783319281476, Springer. ISBN 9783319281476 collectively known as Opisthosporidia, as opposed to the 'true' or osmotrophic fungi. Both aphelids and rozellids are single-celled phagotrophic flagellates that feed in an endobiotic manner, penetrating the cells of their respective hosts. Microsporidians are obligate intracellular parasites that feed through osmotrophy, much like true fungi. Aphelids and true fungi are closest relatives, and generally feed on cellulose-walled organisms (many algae and plants). Conversely, rozellids and microsporidians form a separate clade, and generally feed on chitin-walled organisms (fungi and animals).
The Holozoa includes various lineages with complex life cycles involving different cell types and associated with the origin of animal multicellularity. The closest relatives to animals are the (~360 species), free-living flagellates that feed through a collar of microvilli surrounding a larger cilium and often form colonies. (2025). 9783319281476, Springer. ISBN 9783319281476 The Ichthyosporea (>40 species), otherwise known as mesomycetozoans, are a group of fungus-like pathogenic holozoans specialized in infecting fish and other animals. The Filasterea (six species) are a heterogeneous group of free-living, endosymbiotic, or parasitic amoebae or flagellates. Lastly, the Pluriformea are two species of free-living holozoans with life cycles that include multicellular aggregates. An elusive flagellate species Tunicaraptor unikontum has an uncertain evolutionary position among these holozoan groups.
There are also many genera of uncertain affiliation among eukaryotes because their DNA has not been DNA sequencing, and consequently their phylogenetic affinities are unknown. One enigmatic heliozoan species is so large that it does not match the description of any known genus, and was consequently transferred to a separate genus Berkeleyaesol with an unclear position, although it probably belongs to Diaphoretickes along with all other heliozoa. The organism Parakaryon is harder to place, as it shares traits from both prokaryotes and eukaryotes.
Heterotrophic protists obtain organic molecules synthesized by other organisms, and can be further divided according to the size of their nutrients. Those that feed on soluble molecules or macromolecules under 0.5 μm in size are called , and they absorb them by diffusion, ciliary pits, transport proteins of the cell membrane, and a type of endocytosis (i.e., invagination of the cell membrane into , called ) known as pinocytosis or fluid-phase endocytosis. Those that feed on organic particles over 0.5 μm in size or entire cells are called , and they ingest them through a type of endocytosis known as phagocytosis. Endocytosis is considered one of the most important adaptations in the origin of eukaryotes because it increased the potential food supply, and phagocytosis allowed the endosymbiosis and development of mitochondria and . In both osmotrophs and phagotrophs, endocytosis is often restricted to a specific region of the cell membrane, known as the cytostome, which may be followed by a cytopharynx, a specialized tract supported by .
Probably all eukaryotes are capable of osmotrophy, but some have no alternative of acquiring nutrients. Obligate osmotrophs and saprotrophs include some , some green algae, the human parasite Blastocystis, some , the parasitic , and the fungus-like oomycetes and .
Phagotrophic protists can be further classified according to how they approach the nutrients. The filter feeders acquire small, suspended food particles or prokaryotic cells and accumulate them by filtration into the cytostome (e.g., , some , most ciliates); filter-feeding flagellates accumulate particles by propelling them with a flagellum through a collar of rigid tentacles or pseudopodia that act as a filter, while filter-feeding ciliates generate water currents through cilia and membranelle zones surrounding the cytostome. The raptorial feeders (e.g., , chrysomonads, , some euglenids, many and ciliates), instead of retaining all particles in bulk, capture each particle individually. Among raptorial protists, the grazers search and ingest prey from surfaces covered with potential food items such as , while the Predation actively pursue scarce prey. Predators that feed on filamentous algae or fungal hyphae either swallow the filaments entirely or penetrate the cell wall and ingest the cytoplasm (e.g., Viridiraptoridae). Predators may have adaptations to hunt prey, such as 'toxicysts' that immobilize prey cells. Certain ciliates have developed a specialized kind of raptorial feeding called histophagy, where they attack damaged but live animals (e.g., annelids and small crustaceans), enter the wounds, and ingest animal tissue. Large raptorial amoebae enclose their prey in a "food cup" of pseudopodia, prior to the formation of the food vacuole. Lastly, diffusion feeders (e.g., heliozoa, foraminifera and many other amoebae, ciliates) engulf prey that happen to collide with their pseudopods or, in the case of ciliates, tentacles that carry toxicysts or extrusomes to immobilize the prey.
Consumers of prokaryotes are popularly called (e.g., most amoebae), while consumers (including osmotrophic parasites) of eukaryotes are known as eukaryovores. In particular, eukaryovores that feed on unicellular protists are cytotrophs (e.g., , , many amoebae, some ciliates); those that feed on fungi are mycophages or mycotrophs (e.g., the ciliate family Grossglockneriidae of obligate mycophages); those that prey on are nematophages; and those that feed on algae are phycotrophs (e.g., ).
Among exclusively heterotrophic protists, variation of nutritional modes is also observed. The , which inhabit deep waters where photosynthesis is absent, can flexibly switch between osmotrophy and bacterivory depending on the environmental conditions.
The contractile vacuoles are surrounded by the spongiome, an array of cytoplasmic vesicles or tubes that slowly collect fluid from the cytoplasm into the vacuole. The vacuoles then contract and discharge the fluid outside of the cell through a pore. The contractile mechanism varies depending on the protist: in ciliates, the spongiome is composed of irregular tubules and actin filaments wind around the pore and over the vacuole surface, together with microtubules; in most flagellates and amoebae, the spongiome is composed of both vesicles and tubules; in dinoflagellates, a flagellar rootlet branches to form a contractile sheath around the vacuole (known as pusule). The location and amount also varies: unicellular flagellated algae (cryptomonads, euglenids, prasinophytes, golden algae, haptophytes, etc.) typically have a single contractile vacuole in a fixed position; naked amoebae have numerous small vesicles that fuse into one vacuole and then split again after excretion. Marine or parasitic protists (e.g., metamonads), as well as those with rigid cell walls, lack these vacuoles.
Unicellular protists often multiply via binary fission, similarly to bacteria. They can also divide through budding, similarly to , or through multiple fissions, a process known as schizogony. In multicellular protists, vegetative reproduction can take the form of fragmentation of body parts, or specialized propagules composed of numerous cells (e.g., in red algae).
Free-living protists tend to 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.
Some protist pathogens undergo asexual reproduction in a wide variety of organisms – which act as secondary or intermediate hosts – but can undergo sexual reproduction only in the primary or definitive host (e.g., Toxoplasma gondii in such as ). (2025). 9780123964816, Academic Press. ISBN 9780123964816 Others, such as Leishmania, are capable of performing syngamy in the secondary vector. In apicomplexans, sexual reproduction is obligatory for parasite transmission.
Despite undergoing sexual reproduction, it is unclear how frequently there is genetic exchange between different strains of pathogenic protists, as most populations may be clonal lines that rarely exchange genes with other members of their species.
Macroalgae (namely red algae, green algae and brown algae), unlike phytoplankton, generally require a fixation point, which limits their marine distribution to coastal waters, and particularly to rocky substrates. They support numerous herbivorous animals, especially benthic ones, as both food and refuge from predators. Some communities of exist adrift on the ocean surface, serving as a refuge and means of dispersal for associated organisms. (2012). 9783642284519, Springer. ISBN 9783642284519
Phototrophic protists are as abundant in soils as their aquatic counterparts. Given the importance of aquatic 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. Most soil algae are stramenopiles (, and ) and (green algae). There is also presence of environmental DNA from and in soil, but no living forms have been seen.
Contrary to the common division between phytoplankton and zooplankton, much of the marine plankton is composed of mixotrophic protists, which pose a largely underestimated importance and abundance (around 12% of all marine environmental DNA sequences). Mixotrophs have varied presence due to abundance and depending on their specific type of mixotrophy. Constitutive mixotrophs are present in almost the entire range of oceanic conditions, from eutrophic shallow habitats to oligotrophic subtropical waters but mostly dominating the photic zone, and they account for most of the predation of bacteria. They are also responsible for harmful algal blooms. Plastidic and generalist non-constitutive mixotrophs have similar biogeographies and low abundance, mostly found in eutrophic coastal waters, with generalist dominating up to half of ciliate communities in the photic zone. Lastly, endosymbiotic mixotrophs are by far the most widespread and abundant non-constitutive type, representing over 90% of all mixotroph sequences (mostly ).
In the of soils, protists are the main consumers of both bacteria and fungi, the two main pathways of nutrient flow towards higher trophic levels. Amoeboflagellates like the and 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 are also acclimated to the soil environment, as their shells protect against desiccation. As bacterial grazers, they have a significant role in the foodweb: they excrete nitrogen in the form of ammonia, making it available to plants and other microbes. Traditionally, protists were considered primarily bacterivorous due to biases in cultivation techniques, but many (e.g., , cercomonads, gymnamoebae, testate amoebae, small flagellates) are omnivores that feed on a wide range of soil eukaryotes, including fungi and even some animals such as . Bacterivorous and mycophagous protists amount to similar biomasses.
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), land plants (the oomycete Phytophthora infestans causes late blight in potatoes)Campbell, N. and Reese, J. (2008) Biology. Pearson Benjamin Cummings; 8 ed. . pp. 583, 588 or even of other protists.Lauckner, G. (1980). "Diseases of protozoa". In: Diseases of Marine Animals. Kinne, O. (ed.). Vol. 1, p. 84, John Wiley & Sons, Chichester, UK.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. Around 100 protist species can infect humans.
In 1820, German naturalist Georg August Goldfuss coined the term "Protozoa" (meaning 'early animals') as a class within Kingdom Animalia that consisted of four groups: Infusoria (ciliates), Lithozoa (), Phytozoa, and Medusinae (jellyfish). Later, in 1845, Carl Theodor von Siebold used the term "Protozoa" as a phylum of exclusively unicellular animals consisting of two classes: Infusoria (ciliates) and Rhizopoda (amoebae, foraminifera). Other scientists did not consider all protozoans part of the animal kingdom, and by the middle of the century most biologists grouped microorganisms into Protozoa, Protophyta (primitive plants), Phytozoa (animal-like plants), and Bacteria (mostly considered plants). In 1860, palaeontolgist Richard Owen was the first to define Protozoa as its own kingdom of eukaryotes, although he also included sponges within his group.
In 1860, British naturalist John Hogg proposed "Protoctista" as the name for a fourth kingdom, (the other kingdoms being plant, animal and mineral) which he described as containing "all the lower creatures, or the primary organic beings", which included Protophyta, Protozoa and .
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 (Kingdom Protista) as the third kingdom of life, comprising primitive forms that were "neither animals nor plants". He grouped both bacteria and eukaryotes, both unicellular and multicellular organisms, as Protista. He retained the Infusoria in the animal kingdom, until German zoologist Otto Bütschli demonstrated that they were unicellular. At first, he included sponges and fungi, but in later publications he explicitly restricted Protista to predominantly unicellular organisms or colonies incapable of forming tissues. He clearly separated Protista from metazoa 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.
In 1938, American 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 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 as those with chlorophyll a and chlorophyll 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 . This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life.
In the popular five-kingdom scheme published by American plant ecologist Robert Whittaker in 1969, Protista was defined as eukaryotic "organisms which are unicellular or unicellular-colonial and which form no tissues". Just as the prokaryotic/eukaryotic division was becoming mainstream, Whittaker, after a decade from Copeland's system, recognized the fundamental division of life between the prokaryotic Monera and the eukaryotic kingdoms: Animalia (ingestion), Plantae (photosynthesis), Fungi (absorption) and the remaining Protista.
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 eukaryotes, such as kelp, red algae, and . 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.
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 six-kingdom model: Bacteria, Animalia, Plantae, Fungi, Protozoa and Chromista. In his context, paraphyletic groups take preference over clades: both protist kingdoms Protozoa and Chromista contain paraphyletic phylum such as Apusozoa, Eolouka or Opisthosporidia. Additionally, red algae 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.
Crown-group eukaryotes achieved significant morphological and ecological diversity before 1000 Ma, with multicellular algae capable of sexual reproduction and unicellular protists exhibiting modern phagocytosis and locomotion. Their advanced but metabolically expensive sterols likely provided numerous Adaptation due to the increased membrane flexibility, including resilience to osmotic shock during desiccation and rehydration cycles, extreme temperatures, UV light exposure, and protection against changing oxygen levels. These adaptations allowed crown-group eukaryotes to colonize diverse and harsh environments (e.g., , rivers, agitated shorelines and land). In contrast, stem-group eukaryotes occupied the low-oxygen marine waters as . The oldest definitive crown-group eukaryotic fossils include Rafatazmia and Ramathallus, both putative red algae, dated at 1600 Ma.
Abundant fossils of heterotrophic protists appear significantly later, parallel to the emergence of fungi. Vase-shaped microfossils (VSMs), widespread rocks dated at 780–720 Ma (Tonian to Cryogenian), have been described as a variety of organisms across the decades (e.g., algae, , ), but current scientific consensus relates most VSMs to marine testate amoebae. As such, VSMs comprise the oldest known fossils of both filose (Cercozoa) and lobose (Amoebozoa) testate amoebae.
After the Gaskiers glaciation of the Ediacaran (~579 Ma), fossils of heterotrophic protists undergo diversification. Some fossils similar to VSMs are interpreted as the oldest fossils of Foraminifera dated at 548 Ma (e.g., Protolagena), but their foraminiferal affinity is doubtful. Other microfossils that are possibly foraminifera include some poorly preserved tubular shells from 716–635 Ma rocks.
Following the Cambrian explosion and rapid diversification of animals, the Precambrian microbe-dominated ecosystems were replaced by primarily benthic and nekto-benthic communities, with most marine organisms (animals, foraminifers, radiolarians) limited to the depths of shallow water environments. Mirroring the animal radiation, there was a radiation of phytoplanktonic protists (i.e., acritarchs) around 520–510 Ma, followed by a decrease in diversity around 500 Ma. Later, the surviving acritarchs expanded in diversity and morphological innovation due to a decrease in predation from benthic animals (particularly and ), which suffered extinction due to various proposed environmental factors such as Anoxic event. Both phytoplankton and zooplankton (e.g., radiolarians) flourished, as signaled by an increase of organic carbon buried in the sediment known as the SPICE event (~497 Ma). This abundant biomass supported a second animal radiation known as the Great Ordovician Biodiversification Event (GOBE), where many animals switched to a planktonic lifestyle and pelagic predators first appeared (e.g., , swimming ). This event is also known as the 'Ordovician Plankton Revolution' due to the significant diversification of planktonic protists, and it spanned from the late Cambrian well into the Ordovician.
The Ordovician also includes the oldest euglenid fossil, known as Moyeria, which is found in rocks spanning from the middle Ordovician (~471 Ma) to the Silurian. There are putative records of calcareous foraminifera from the Early Ordovician to the Silurian, but these are not widely accepted; the oldest trusted and well-known calcaerous foraminifera appear in the Middle Devonian, the next geological period.
In Early Devonian terrestrial ecosystems the first fossils of freshwater arcellinid testate amoebae are found (e.g., Palaeoleptochlamys, Cangweulla), as well as various types of freshwater green algae, including , volvocaceae and , and some putative algal fossils that might represent . During the Devonian some benthic foraminifera acquired the ability of calcifying, and particularly the giant Fusulinida became the dominant fossilizable protists. This time interval is also considered the molecular origin of (~310 Ma) and (397–382 Ma), which did not leave fossil traces until later in the Mesozoic. After the Late Devonian extinction (372 Ma), -like radiolarians appeared for the first time, with a unique body plan among marine protists.
During the Carboniferous period, no new fossilizable protists originated despite the major environmental changes. However, starting in the Late Carboniferous, radiolarian diversity and productivity increased, causing a large amount of biosiliceous sediment (chert) to be accumulated worldwide; this is known as the Radiolarian Optimum Event, which lasted primarily from the Middle Permian until the Early Cretaceous. (2025). 9780478099195, GNS Science. ISBN 9780478099195 Around the Capitanian mass extinction event (262–259 Ma) of the Permian period, genetically diverged from the rest of haptophytes, possibly as a response to a reduction in atmospheric oxygen, and there was a faunal turnover from larger to smaller fusulinids. radiolarians appear in the latest Permian.
Around the Early–Middle Jurassic, after the global Toarcian Oceanic Anoxic Event there was a diversification of dinoflagellates and coccolithophores, in both species and abundance. This interval also saw the completion of a symbiosis between Acantharia radiolarians and lineages of Phaeocystis haptophytes, as well as the appearance of planktonic foraminifera. The period of low atmospheric oxygen ends in the Aptian-Albian boundary during the Early Cretaceous, and the first fossils of diatoms and silicoflagellates appear. Samples of amber from around 100 Ma contain the oldest fossil records of (particularly agents and ), Trypanosomatida, and —particularly mutualistic of cockroaches, representing the earliest record of mutualism between protists and animals.
The diversification of coccolithophores, mixotrophic dinoflagellates, and later diatoms across the Mesozoic era caused an accelerated transfer of primary production into higher trophic levels. This evolutionary radiation of phytoplankton was, in turn, responsible for the animal "Mesozoic marine revolution", characterized by the appearance of widespread predation among most invertebrate phyla. Coccolithophores, dinoflagellates and especially diatoms became the dominating eukaryotic producers in oceans until today, as opposed to cyanobacteria and green algae which dominated earlier.
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