Protist

Protists Temporal range: Paleoproterozoic–present[1] Pha. Proterozoic Archean Had.
Examples of protists. Clockwise from top left: golden alga, dinoflagellate, metamonad, amoeba, slime mold .
Scientific classification (paraphyletic)
Domain:	Eukaryota
Supergroups[2]
Amorphea Amoebozoa fungi ) CRuMs Discoba Metamonada Malawimonadida Ancyromonadida Hemimastigophora Provora[3] Diaphoretickes Archaeplastida (including plants) Cryptista Haptista Telonemia SAR Stramenopiles Alveolata Rhizaria
Cladistically included but traditionally excluded taxa
Animalia Fungi Embryophyta (land plants)

A protist (

Protists were historically regarded as a separate

Protists are a diverse group of

• Amoebae. Characterized by their irregular, flexible shapes, these protists move by extending portions of their cytoplasm, known as pseudopodia, to crawl along surfaces.^[18] Many groups of amoebae are naked, but testate amoebae and foraminifera grow a shell around their cell made from digested material or surrounding debris. Some, known as radiolarians and heliozoans, have special spherical shapes with microtubule-supported pseudopodia radiating from the cell.^[17] Some amoebae are capable of producing stalked multicellular stages that bear spores, often by aggregating together; these are known as slime molds.^[19] The main clades containing amoebae are Amoebozoa (including various slime molds and testate amoebae) and Rhizaria (including famous groups such as foraminifera and radiolarians, as well as a few testate amoebae).^[20][21] Even some individual amoebae can grow to giant sizes visible to the naked eye.^[22][23]
• amoeboflagellates.^[27]
• euglenophytes) to amoeboid cells (chlorarachniophytes) to colonial and multicellular macroscopic forms (e.g., red algae, some green algae, and some ochrophytes such as kelp).^[29]
• Fungus-like protists. Several clades of protists have evolved an appearance similar to
  perkinsozoans, in Alveolata).^[30][31]
• parasitic protists that reproduced via spores (the apicomplexans, microsporidians, myxozoans and ascetosporeans).^[6] Its current use is restricted to the apicomplexans,^[32] such as Plasmodium falciparum, the cause of malaria.^[33]

The

The malawimonads (Malawimonadida) are a small group (three species) of freshwater or marine suspension-feeding bacterivorous flagellates^[53] with typical excavate appearance, closely resembling Jakobida and some metamonads but not phylogenetically close to either in most analyses.^[17]

Representative genera of excavates

Giardia (diplomonad)

Trichomonas (parabasalid)

Trypanosoma (kinetoplastid)

Euglena (euglenid)

Malawimonas (malawimonad)

Representative genera of stramenopiles

Phytophthora (oomycete)

Nitzschia (diatom)

Macrocystis (kelp)

Cafeteria (bicosoecid)

Opalina (opalinid)

The

Representative genera of alveolates

Gregarina (apicomplexan)

Dinovorax (perkinsid)

Peridinium

(dinoflagellate)

Representative genera of rhizarians

Chlorarachnion (chlorarachniophyte)

Euglypha (imbricatean)

Haplosporidium

(ascetosporean)

Haptista and Cryptista are two similar phyla of single-celled protists previously thought to be closely related, and collectively known as Hacrobia.^[85] However, the monophyly of Hacrobia was disproven, as the two groups originated independently.^[86] Molecular analyses place Cryptista next to Archaeplastida, forming the hypothesized "CAM" clade, and Haptista next to the Telonemia and SAR clade (Telonemia may either be the sister group to SAR, forming the hypothesized TSAR clade,^[87] or to Haptista, forming a common sister clade to SAR^[54][39]).^[39][40]

The phylum Haptista includes two distinct clades with mineralized scales: haptophytes and centrohelids.^[17] The haptophytes (Haptophyta) are a group of over 500 living species^[47] 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 coccolithophores, whose calcified scales ('coccoliths') contribute to the formation of sedimentary rocks and the biogeochemical cycles of carbon and calcium. Some species are capable of forming toxic blooms.^[88] The centrohelids (Centroplasthelida) are a small (~95 species)^[89] but widespread group of heterotrophic heliozoan-type amoebae, usually covered in scale-bearng mucous, that form an important component of benthic food webs of aquatic habitats, both marine and freshwater.^[90]

The phylum Cryptista is a clade of three distinct groups of unicellular protists:

Representative genera of haptists and cryptists

Raphidiophrys (centrohelid)

Coccolithus (haptophyte)

Cryptomonas (cryptomonad)

Roombia (katablepharid)

Palpitomonas

Archaeplastida is the clade containing those photosynthetic groups whose

Representative genera of archaeplastids

Cyanophora (glaucophyte)

Corallina (red alga)

Volvox (chlorophyte)

Spirogyra (zygnematophycean)

Chara (charophycean)

Representative genera of amoebozoans

Hyalosphenia (arcellinid)

Physarum (myxogastrid)

Dictyostelium (dictyostelid)

Entamoeba

(archamoeba)

Representative genera of opisthokont protists

Nuclearia (nucleariid)

Amoebidium (ichthyosporean)

Sphaeroeca (choanoflagellate)

Capsaspora (filasterean)

Syssomonas (pluriformean)

Several smaller lineages do not belong to any of the three main supergroups, and instead have a deep-branching "kingdom-level" position in eukaryote evolution. They are usually poorly known groups with limited data and few species, often referred to as "orphan groups".

Protists display a wide variety of food preferences and feeding mechanisms.[2]^[131] According to the source of their nutrients, they can be divided into autotrophs (producers) and heterotrophs (consumers). Autotrophic protists synthesize their own organic compounds from inorganic substrates through the process of photosynthesis, using light as the source of energy;^[132]: 217 accordingly, they are also known as phototrophs.^[133]

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

Phagotrophic feeding consists of two phases: the concentration of food particles in the environment, and the phagocytosis, which encloses the food particle in a vacuole (the phagosome)^[131] where digestion takes place. In ciliates and most phagotrophic flagellates, digestion occurs at the oral region or cytostome, which is covered by a single membrane from which vacuoles are formed; the phagosomes then may be shuttled to the interior of the cell along the cytopharynx.^[138] In amoebae, phagocytosis takes place anywhere on the cell surface. The average food particle size is around one tenth the size of the protist cell.^[139]

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.,

Rapaza viridis is a species of obligate specialist mixotrophs: it survives through the predation of Tetraselmis algae and acquisition of their chloroplasts. It rejects any other prey cells. Even when well fed, it cannot survive without a light source, as it needs to photosynthesize with those chloroplasts.^[142]

Most autotrophic protists are

Noctiluca).^[145]

Among exclusively heterotrophic protists, variation of nutritional modes is also observed. The

diplonemids, which inhabit deep waters where photosynthesis is absent, can flexibly switch between osmotrophy and bacterivory depending on the environmental conditions.^[147]

Osmoregulation

Many

freshwater protists need to osmoregulate (i.e., remove excess water volume to adjust the ion concentrations) because non-saline water enters in excess by osmosis from the environment^[148] and by endocytosis when feeding.^[138] Osmoregulation is done through active ion transporters of the cell membrane and through contractile vacuoles, specialized organelles 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.^[130]

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).^[138] 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.^[148]

Respiration

The

trypanosomatid protists, the fermentative glycosome evolved from the peroxisome.^[130]

Sensory perception

Light micrograph of an ocelloid-containing dinoflagellate. n: nucleus, double arrowhead: ocelloid, scale bar: 10 μm.^[149]

Many flagellates and probably all motile algae exhibit a positive

geotaxis), and others swim in relation to the concentration of dissolved oxygen in the water.^[130]

Endosymbionts

Protists have an accentuated tendency to include

methanogenic role inside anaerobic ciliates.^[130]

Life cycle and reproduction

colonial stages (blue), and formation of cysts. Each protist group has a different sexual cycle (light purple) as well as different means of exiting the colonial stage.^[150]

Protists exhibit a large range of

strategies involving multiple stages of different morphologies which have allowed them to thrive in most environments. Nevertheless, most of the knowledge concerning protist life cycles concerns model organisms and important parasites. Free-living uncultivated protists represent the majority, but knowledge on their life cycles remains fragmentary.^[150]

Asexual reproduction

Protists typically reproduce asexually under favorable environmental conditions,[151] allowing for rapid exponential population growth with minimal genetic diversification. This asexual reproduction, occurs through mitosis and has historically been regarded as the primary reproductive mode in protists.^[150] This process is also known as vegetative reproduction, as it is only performed by the 'vegetative stage' or individual.^[152]

Unicellular protists often multiply via

propagules composed of numerous cells (e.g., in red algae).^[152]

Sexual reproduction

While asexual reproduction remains the most common strategy among protists,

amoebae are traditionally considered asexual organisms, most asexual amoebae likely arose recently and independently from sexually reproducing amoeboid ancestors.^[160] Even in the early 20th century, some researchers interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoebae as sexual reproduction.^[161]

Basic sexual cycles

Every sexual cycle involves the events of syngamy and meiosis, which increase or decrease the

gametes, which generates a diploid (2n) cell called zygote. The diploid cell then undergoes meiosis to generate haploid cells. Depending on which cells compose the individual or vegetative stage (i.e., the stage that grows by mitosis), there are three distinguishable sexual cycles observed in free-living protists:^[150]

• In the
  Volvocales), many dinoflagellates, some metamonads, and apicomplexans.^[138]
  ^: 26

• In the
  conjugation with another ciliate, and fuse the two nuclei into a new diploid nucleus.^[65]

• In the
  land plants.^[138]: 26 There are three modes of this cycle depending on the relative growth and lifespan of one generation compared to the other: haploid-dominant, diploid-dominant, or equally dominant generations. Brown algae exhibit the full range of these modes.^[162]

Free-living protists tend to reproduce sexually under stressful conditions, such as starvation or heat shock.

DNA damage, also appears to be an important factor in the induction of sex in protists.^[151]

Sexual cycles in pathogenic protists

Pathogenic protists tend to have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually.[163] The stages that feed and multiply inside the host are generally known as trophozoites (from Greek trophos 'nutrition' and zoia 'animals'), but the names of each stage vary depending on the protist group.^[153] For example:

• In apicomplexans, a haploid sporozoite is released into the host, penetrates a host cell, begins the infection and transforms into a meront that grows and asexually divides into numerous merozoites (a schizogony called merogony); each merozoite continues the infection by multiplying. Eventually, the merozoites differentiate (gamogony) into female (macrogametocytes) and male (microgametocytes) that generate gametes, which in turn fuse (sporogony) into a diploid zygote that grows into a sporocyst. The sporocyst then undergoes meiosis to form sporozoites that transmit the infection.^[68][155]

• In phytomyxeans, the diploid primary zoospores enter the host, encyst, and penetrate cells as a uninucleate protoplast or plasmodium. Inside the cells, the protoplast grows into a multinucleate zoosporangium, which then divides into secondary zoospores that infect more cells. These multiply into thick-walled resting spores that begin meiosis and divide into binucleate resting spores; one nucleus is lost, and the spores hatch as primary zoospores.^[164]

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.,

domestic cats).^[165] Others, such as Leishmania, are capable of performing syngamy in the secondary vector.^[166] In apicomplexans, sexual reproduction is obligatory for parasite transmission.^[167]

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.[168]

Ecology

Protists are indispensable to modern

symbioses.^[37]

Habitat diversity

Protists are abundant and diverse in nearly all habitats. They contribute 4 gigatons (Gt) to Earth's biomass—double that of animals (2 Gt), but less than 1% of the total. Combined, protists, animals, archaea (7 Gt), and fungi (12 Gt) make up less than 10% of global biomass, with plants (450 Gt) and bacteria (70 Gt) dominating.

prokaryotes as well as protists.^[171]

Primary producers

Microscopic phototrophic protists (or

arctic oceans and along continental margins.^[173] In freshwater habitats, most phototrophic protists are mixotrophic, meaning they also behave as consumers, while strict consumers (heterotrophs) are less abundant.^[171]

benthic ones, as both food and refuge from predators. Some communities of seaweeds exist adrift on the ocean surface, serving as a refuge and means of dispersal for associated organisms.^[175][176]

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

archaeplastids (green algae). There is also presence of environmental DNA from dinoflagellates and haptophytes in soil, but no living forms have been seen.^[177]

Consumers

Phagotrophic protists are the most diverse functional group in all ecosystems, primarily represented by cercozoans (dominant in freshwater and soils), radiolarians (dominant in oceans), non-photosynthetic stramenopiles (with higher abundance in soils than in oceans), and ciliates.^[171]

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 seasonal abundance^[178] 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 ciliates 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 radiolarians).^[146][145]

In the

vampyrellids, 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 nematodes. Bacterivorous and mycophagous protists amount to similar biomasses.^[140]

Decomposers

nucleariid amoebae specifically consume the contents of dead or damaged cells, but not healthy cells. However, all these examples are only facultative necrophages that also feed on live prey. In contrast, the algivorous cercozoan family Viridiraptoridae, present in shallow bog waters, are broad-range but sophisticated necrophages that feed on a variety of exclusively dead algae, potentially fulfilling an important role in cleaning up the environment and releasing nutrients for live microbes.^[180]

Parasites and pathogens

Neotropical forest soils, apicomplexans dominate eukaryotic diversity and have an important role as parasites of small invertebrates, while oomycetes are very scarce in contrast.^[181]

Some protists are significant parasites of animals (e.g.; five species of the parasitic genus

late blight in potatoes)^[184] or even of other protists.^[185][186] Around 100 protist species can infect humans.^[177]

Biogeochemical cycles

Marine protists have a fundamental impact on

fix as much carbon as all terrestrial plants combined.^[171] Soil protists, particularly testate amoebae, contribute to the silica cycle as much as forest trees through the biomineralization of their shells.^[177]

History of protist classification

Early classifications

From the start of the 18th century, the popular term "infusion animals" (later

Carl von Linnaeus largely ignored the protists,^[f] his Danish contemporary Otto Friedrich Müller was the first to introduce protists to the binomial nomenclature system.^[188][189]

In the early 19th century, German naturalist

sponges within protozoa.^[28]

In 1860, British

Linnaeus' plant, animal and mineral) which comprised all the lower, primitive organisms, including protophyta, protozoa and sponges, at the merging bases of the plant and animal kingdoms.^[192][28]

In 1866, the 'father of protistology', German scientist

blastula stage of animal development. He also returned the terms Protozoa and Protophyta as subkingdoms of Protista.^[28]

End of the animal-plant dichotomy

Bütschli considered the kingdom to be too

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.^[28]

In 1938, American biologist

Plantae and Protista as the four kingdoms of life.^[198]

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,^[198] recognized the fundamental division of life between the prokaryotic Monera and the eukaryotic kingdoms: Animalia (ingestion), Plantae (photosynthesis), Fungi (absorption) and the remaining Protista.^{[199][200][28]}

In the five-kingdom system of American evolutionary biologist

microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as kelp, red algae, and slime molds.^[201] 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.^[202]

Advances in electron microscopy and molecular phylogenetics

The five-kingdom model remained the accepted classification until the development of

Eukarya) became prevalent.^[203] Today, protists are not treated as a formal taxon, but the term is commonly used for convenience in two ways:^[13]

• land plant or fungus,^[205] thus excluding many unicellular groups like the fungal Microsporidia, Chytridiomycetes and yeasts, and the non-unicellular Myxozoan animals included in Protista in the past.^[206]
• Functional definition: protists are essentially those eukaryotes that are never
  choanoflagellates) or complex multicellularity (e.g., brown algae).^[208]

There is, however, one classification of protists based on traditional ranks that lasted until the 21st century. The British protozoologist

fungal groups Microsporidia, Rozellida and Aphelida are considered protozoans under the phylum Opisthosporidia. This scheme endured until 2021, the year of his last publication.^[9]

Fossil record

Further information:

microfossils

The protist fossil record is mainly represented by protists with fossilizable coverings, such as foraminifera, radiolaria, testate amoebae and diatoms, as well as multicellular algae.

fungi, all eukaryotes were protists. As a result, the early fossil record of protists is equivalent to the early record of eukaryotic life.^[167]

Palæoproterozoic

Mesoproterozoic

Neoproterozoic

Palæozoic

Mesozoic

Cenozoic

International Chronostratigraphic Chart. Legend:
  accepted fossil record (including name of earliest fossil),   putative fossil record,   biochemical signatures,   molecular clock estimate, ^†major extinctions

.

Paleo- and Mesoproterozoic

Further information: Protosterol biota

Modern or

stem-group eukaryotes, extinct lineages preceding LECA. These lineages displayed early eukaryotic traits like flexible cell membranes and complex cell wall ornamentations, which require a flexible endomembrane system, but they lacked crown-group eukaryotes' advanced sterols (e.g., cholesterol), and instead produced simpler protosterols that require less oxygen during biosynthesis.^[211] Examples of these are: Trachyhystrichosphaera and Leiosphaeridia dated at 1100 Ma,^[212] Satka dated at 1300 Ma,^[213] Tappania and Shuiyousphaeridium dated at 1600 Ma,^[214] Grypania dated at 1800–1900 Ma, and Valeria which ranges from 1650 to 700 Ma.^[215]

Crown-group eukaryotes achieved significant

anaerobes.^[211] The oldest definitive crown-group eukaryotic fossils include Rafatazmia and Ramathallus, both putative red algae, dated at 1600 Ma.^[1]

Neoproterozoic

As oxygen levels rose during the

multicellularity.^[219]

Abundant fossils of

chitinozoans, tintinnids), but current scientific consensus relates most VSMs to marine testate amoebae.^[220] As such, VSMs comprise the oldest known fossils of both filose (Cercozoa) and lobose (Amoebozoa) testate amoebae.^[221][222]

After the Gaskiers glaciation of the Late 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),^[220] but their foraminiferal affinity is doubtful. Other microfossils that are possibly foraminifera include some poorly preserved tubular shells from 716–635 Ma rocks.^[223]

Paleozoic

Radiolarian shells appear abundantly in the fossil record since the Cambrian, with the first definitive radiolarian fossils found at the very start of this period (~540 Ma) together with the first small shelly fauna.^[224] Radiolarian records from older Precambrian rocks have been disregarded due to the lack of reliable fossils.^{[225][226][227]} Around this time, between 540 and 510 Ma, the oldest Foraminifera shells appear, first multi-chambered and later tubular.^{[228][210][223]}

Following the

benthic and nekto-benthic communities, with most marine organisms (animals, foraminifers, radiolarians) limited to the depths of shallow water environments.^[229] Mirroring the animal radiation, there was a radiation of phytoplanktonic protists (i.e., acritarchs)^[230] around 520–510 Ma, followed by a decrease in diversity around 500 Ma.^[231] Later, the surviving acritarchs expanded in diversity and morphological innovation^[230] due to a decrease in predation from benthic animals (particularly trilobites and brachiopods), which suffered extinction due to various proposed environmental factors such as anoxia.^[232] 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).^[229][232] 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., cephalopods, swimming arthropods). 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.^[229]

The Ordovician also includes the oldest

silicoflagellates (397–382 Ma), which did not leave fossil traces until later in the Mesozoic. After the Late Devonian extinction (372 Ma), nassellarian-like radiolarians appeared for the first time, with a unique body plan among marine protists.^[210]

During the Carboniferous, no new fossilizable protists originated despite the major environmental changes. Around the Capitanian mass extinction event (262–259 Ma), coccolithophores 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.^[210]

See also

• Evolution of sexual reproduction
• Protist locomotion
• Chromista
• Protozoa

Footnotes

1. ^
   mycologists alike.^[2][11][12]
2. prokaryotic flagella.^[24][25][26]
3. ^ A 2007 report on protist diversity included a table listing the described number of species for protist and fungal groups. The total sum of the listed species, excluding fungi, is 76,144.^[36]
4. ^
   Acrasida and Schizopyrenida. The name Percolozoa encompasses these and other related single-celled protists, not just the 'true' heteroloboseans.^[9]
5. ^ The terms "mixotroph" and "mixoplankton" almost exclusively refer to protists that perform photosynthesis and phagocytosis (photo-phagotrophs). Osmotrophy is always present, but not taken into account. As such, "pure" phototrophs (incapable of phagocytosis) and "pure" phagotrophs (incapable of photosynthesis) are technically mixotrophic due to their innate ability for osmotrophy, but are not usually reported in this sense.^[144]
6. Carl von Linnaeus did not mention a single protist genus until the tenth edition of Systema Naturae of 1758, where Volvox was recorded.^[188]
7. ^ In 2015, Cavalier-Smith's initial six-kingdom model was revised into a seven-kingdom model after the inclusion of Archaea.^[209]

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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^{[permanent dead link‍]}, Charophyta, vol. 2, Chrysomonadida, Coccolithophorida, Charophyta, Diatomacea & Pyrrhophyta), part C (Sarcodina, Chiefly "Thecamoebians" and Foraminiferida) and part D^{[permanent dead link‍]} (Chiefly Radiolaria and Tintinnina). Boulder, Colorado: Geological Society of America; & Lawrence, Kansas: University of Kansas Press.

External links

Wikispecies has information related to Protista.

Wikispecies has information related to Protoctista.

Wikimedia Commons has media related to Protista.

• UniEuk Taxonomy App
• Tree of Life: Eukaryotes
• Tsukii, Y. (1996). Protist Information Server (database of protist images). Laboratory of Biology, Hosei University. Protist Information Server. Updated: March 22, 2016.

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