Protist

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For the journal, see Protist (journal).

Protists
Examples of protists. Clockwise from top left: red algae, kelp, ciliate, golden alga, dinoflagellate, metamonad, amoeba, slime mold.
Examples of protists. Clockwise from top left:
golden alga, dinoflagellate, metamonad, amoeba, slime mold
.
Scientific classificationEdit this classification
(paraphyletic)
Domain: Eukaryota
Supergroups[1]

fungi and animals
)
Apusomonadida

land plants
)
Breviatea
CRuMs
Cryptista
Discoba

Haptista
Hemimastigophora
Malawimonadida

Metamonada

Provora[2]
SAR supergroup
Telonemia

Cladistically included but traditionally excluded taxa

Animalia

Fungi

Embryophyta
(land plants)

A protist (

last eukaryotic common ancestor
.

Protists were historically regarded as a separate

photoautotrophs that includes land plants), SAR, Obazoa (which includes fungi and animals), Amoebozoa and Excavata
.

Protists represent an extremely large

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

biological characteristics expected in eukaryotes.[3]

They are generally

phagotrophy and phototrophy together.[3] They have different life cycles, trophic levels, modes of locomotion, and cellular structures.[4][5] Some protists can be pathogens.[6]

Examples of basic protist forms that do not represent evolutionary cohesive lineages include:[7]

The names of some protists (called

slime molds and flagellated algae like euglenids), have been published under either or both of the ICN and the ICZN codes.[12][13]

Classification

Phylogenomic tree of eukaryotes, as regarded in 2020. Supergroups are in color.
Further information: Taxonomy of Protista and Eukaryote § Phylogeny

The evolutionary relationships of protists have been explained through

cladistic scheme, the protists are divided into various wide branches informally named supergroups:[7][1]
  • Sar, SAR or Harosa – a clade of three highly diverse lineages exclusively containing protists.
  • Acrasida, a group of slime molds. Euglenozoa encompasses a clade of algae with chloroplasts of green algal origin and many groups of anaerobic, parasitic or free-living heterotrophs.[7]

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

Malawimonadida and Ancyromonadida, appear to be related to Amorphea.[7] Others, like Hemimastigophora (10 species)[26] and Provora (7 species), appear to be related to or within Diaphoretickes, a clade that unites SAR, Archaeplastida, Haptista and Cryptista.[2]

Although the root of the tree is still unresolved, one possible topology of the eukaryotic tree of life is:[27][2]

History

Early concepts

Goldfuss' system of life, introducing the Protozoa within animals.

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

Carl von Linnaeus largely ignored the protists,[c] his Danish contemporary Otto Friedrich Müller was the first to introduce protists to the binomial nomenclature system.[28][29]

In the early 19th century, German naturalist

sponges within protozoa.[8]

Origin of the protist kingdom

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

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.[32][8]

Haeckel's 1866 tree of life, with the third kingdom Protista.

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.[8]

Butschli 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.[8]

In 1938, American biologist

Plantae and Protista as the four kingdoms of life.[38]

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,[38] recognized the fundamental division of life between the prokaryotic Monera and the eukaryotic kingdoms: Animalia (ingestion), Plantae (photosynthesis), Fungi (absorption) and the remaining Protista.[39][40][8]

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.[41] 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.[42]

Phylogenetics and modern concepts

Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes

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

Eukarya) became prevalent.[43] Today, protists are not treated as a formal taxon, but the term is commonly used for convenience in two ways:[44]

Kingdoms Protozoa and Chromista

Further information: 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

fungal groups Microsporidia, Rozellida and Aphelida are considered protozoans under the phylum Opisthosporidia. This scheme endured until 2021, the year of his last publication.[21]

Diversity

Species diversity

Difference between morphological (A) and genetic (B) view of total eukaryotic diversity. Protists dominate DNA barcoding analyses, but constitute a minority of catalogued species.[51]

According to

species is very low (ranging from 26,000[52] to 74,400[51] as of 2012) in comparison to the 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 1.4×105 to 1.6×106, and in several groups the number of predicted species is arbitrarily doubled. Most of these predictions are highly subjective.[51]

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 markers need to be used to survey the vast protistan diversity, because there is no universal marker that can be applied to all lineages.[51]

Biomass

Protists make up a large portion of the

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.[53]

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

freshwater habitats.[54]

chrysophytes in freshwater, and Archaeplastida in soil.[54]

Marine

Further information: Marine protists
Marine diatoms are important oxygen producers.

trophic networks as part of the plankton.[56]

cryptophytes.[57]

Prymnesium, a constitutive mixotroph that participates in toxic algal blooms.

biogeographies
:

  • Constitutive mixotrophs, also called '
    phagotrophy, and others are obligate mixotrophs.[56] They are responsible for harmful algal blooms. They dominate the eukaryotic microbial biomass in the photic zone, in eutrophic and oligotrophic waters across all climate zones, even in non-bloom conditions. They account for significant, often dominant predation of bacteria.[59]
Noctiluca, a specialist non-constitutive mixotroph that photosynthesizes through endosymbionts
.

Freshwater

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.[54]

Soil

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).[54] Soil protists are also morphologically and functionally diverse, with four major categories:[61]

Dictyostelids are fungus-like protists present in soil.
phagotrophic
protists in soil.

Parasitic

Stramenopiles
) is a prevalent intestinal parasite in humans.

oomycetes, Apicomplexa and Ichthyosporea.[54]

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

late blight in potatoes)[66] or even of other protists.[67][68]

Around 100 protist species can infect humans.[61] Two papers from 2013 have proposed virotherapy, the use of viruses to treat infections caused by protozoa.[69][70]

Researchers from the

vector that can spread the pathogenic protist between red fire ant colonies.[73]

Biology

Physiological adaptations

Paramecium aurelia with contractile vacuoles

While, in general, protists are typical

eukaryotic cells and follow the same principles of physiology and biochemistry described for those cells within the "higher" eukaryotes (animals, fungi or plants),[74] they have evolved a variety of unique physiological adaptations that do not appear in those eukaryotes.[75]
Light micrograph of an ocelloid-containing dinoflagellate. n: nucleus, double arrowhead: ocelloid, scale bar: 10 μm.[76]

Sexual reproduction

Two similar-looking but sexually distinct Coleps partners connected at their front ends exchange genetic material via a plasma bridge.

Protists generally

DNA damage, also appears to be an important factor in the induction of sex in protists.[77]

common ancestor of all eukaryotes,[81] which was likely capable of facultative (non-obligate) sexual reproduction.[82]

This view was further supported by a 2011 study on

amoeboid lineages are ancestrally sexual, and that the majority of asexual groups likely arose recently and independently.[83] Even in the early 20th century, some researchers interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoebae as sexual reproduction.[84]

Sex in pathogenic protists

Some commonly found protist pathogens such as

domestic cats in this case).[85][86][87]

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.[88] 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.[89]

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.[90]

Fossil record

Main article: Protists in the fossil record
Further information:
microfossils

Mesoproterozoic

Further information: Protosterol biota

By definition, all

fungi
are considered protists. For that reason, this section contains information about the deep ancestry of all eukaryotes.

All living

locomotion. However, the fossil record of modern eukaryotes is very scarce around this period, which contradicts the predicted diversity.[91]

Instead, the fossil record of this period contains "

biosynthetic pathway.[91]

Crown sterols, while metabolically more expensive, may have granted several evolutionary advantages for LECA's descendants. Specific unsaturation patterns in crown sterols protect against

UV light. A more speculative role of these sterols is their protection against the Proterozoic changing oxygen levels. It is theorized that all of these sterol-based mechanisms allowed LECA's descendants to live as extremophiles of their time, diversifying into ecological niches that experienced cycles of desiccation and rehydration, daily extremes of high and low temperatures, and elevated UV radiation (such as mudflats, rivers, agitated shorelines and subaerial soil).[91]

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

glaciations and the extreme global heat that came afterwards.[91]

Neoproterozoic

Modern eukaryotes began to appear abundantly in the

fungi.[91] For example, the oldest fossils of Amoebozoa are vase-shaped microfossils resembling modern testate amoebae, found in 800 million-year-old rocks.[92][93] Radiolarian 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.[94][95][96]

See also

Footnotes

  1. ^ According to some classifications,[14] all of Archaeplastida is treated as Kingdom Plantae, but others consider the algae (or non-terrestrial "plants") to be protists.[7]
  2. Eumycota or true fungi.[21] However, many researchers currently accept those three groups as part of a larger Kingdom Fungi.[1][22][23]
  3. Carl von Linnaeus did not mention a single protist genus until the tenth edition of Systema Naturae of 1758, where Volvox was recorded.[28]
  4. ^ In 2015, Cavalier-Smith's initial six-kingdom model was revised into a seven-kingdom model after the inclusion of Archaea.[50]

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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
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