Protists were historically regarded as a separate
taxonomickingdom 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 (
photoautotrophs that includes land plants),
SAR,
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 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
ecosystems as important components of the
biogeochemical cycles and
trophic webs. They exist abundantly and ubiquitously in a variety of forms that evolved multiple times independently, such as free-living
algae,
amoebae and
slime moulds, or as important
parasites. 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
cells possess a
nucleus) that is not an animal, a
land plant or a
dikaryon fungus. Because of this definition by exclusion, protists encompass almost all of the broad spectrum of
biological characteristics expected in eukaryotes.[3]
Examples of basic protist forms that do not represent evolutionary cohesive lineages include:[7]
Algae, which are
photosynthetic protists. Traditionally called "protophyta", they are found within most of the big evolutionary lineages or
supergroups, intermingled with
heterotrophic protists which are traditionally called "
protozoa".[8] There are many multicellular and colonial examples of algae, including
kelp,
red algae, some types of
diatoms, and some lineages of
green algae.
Amoebae, which usually lack flagella but move through changes in the shape and motion of their
protoplasm[9] to produce
pseudopodia. They have evolved independently several times, leading to major
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, with at least three independent origins. Others, referred to as "
testate amoebae", grow a shell around the cell made from organic or inorganic material.
Slime molds, which are amoebae capable of producing stalked reproductive structures that bear spores, often through
aggregative multicellularity (numerous amoebae aggregating together). This type of multicellularity has evolved at least seven times among protists.[10]
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
oomycetes (water molds) or the
myxomycetes.
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
flagellated algae like
euglenids), have been published under either or both of the ICN and the ICZN codes.[12][13]
The evolutionary relationships of protists have been explained through
molecular phylogenetics, the
sequencing of entire
genomes and
transcriptomes, and
electron microscopy studies of the
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 ranks (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
supergroups:[7][1]
Sar, SAR or Harosa – a clade of three highly diverse lineages exclusively containing protists.
Stramenopiles 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),[18] known as diatoms; the filamentous or genuinely multicellular
Phaeophyta (2,000 species),[19] 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.[7]
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
marinemicroplankton and a main cause of
algal blooms; and
Ciliophora (4,500 species),[20] the extremely diverse and well-studied group of mostly free-living heterotrophs known as ciliates.[7]
Rhizaria is a morphologically diverse lineage mostly comprising heterotrophic amoebae, flagellates and amoeboflagellates, and some unusual algae (
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.[7]
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 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 CRuMs clade, 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]
In the early 19th century, German naturalist
Georg August Goldfuss introduced
Protozoa (meaning 'early animals') as a class within
Kingdom Animalia,[30] to refer to four very different groups:
infusoria (
ciliates),
corals, 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).[31] 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 of
eukaryotic organisms, with "nucleated cells" and the "common organic characters" of plants and animals, although he also included
sponges within protozoa.[8]
Origin of the protist kingdom
In 1860, British
naturalistJohn 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
sponges, at the merging bases of the plant and animal kingdoms.[32][8]
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[33] (Kingdom Protista) as the
third kingdom of life, comprising primitive forms that were "neither animals nor plants". He grouped both bacteria[34] 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.[35][36] 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
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.[8]
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.[8]
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
true plants as those with
chlorophyll a and
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
protozoans.[8][37] This classification was the basis for Whittaker's later definition of Fungi,
Animalia,
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
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
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
The five-kingdom model remained the accepted classification until the development of
molecular phylogenetics 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.[43] Today, protists are not treated as a formal
taxon, but the term is commonly used for convenience in two ways:[44]
Functional definition: protists are essentially those eukaryotes that are never
multicellular,[44] that either exist as independent cells, or if they occur in
colonies, do not show differentiation into tissues.[48] 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).[49]
According to
molecular data, protists dominate
eukaryotic diversity, accounting for a vast majority of
environmental DNA sequences or
operational taxonomic units (OTUs). However, their
species diversity is severely underestimated by traditional methods that differentiate species based on
morphological characteristics. The number of described protistan
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
biomass in both
marine and
terrestrial ecosystems. It has been estimated that protists account for 4
gigatons (Gt) of
biomass in the entire 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.[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
environmental DNA or
RNA. The richest protist communities appear in
soil, followed by
ocean and
freshwater habitats.[54]
Phagotrophic 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).
Phototrophic protists (producers) appear in lower proportions, probably constrained by intense predation. They exist in similar abundance in both oceans and soil. They are mostly
dinophytes in oceans,
chrysophytes in freshwater, and
Archaeplastida in soil.[54]
Mixotrophic marine protists, while not very researched, are present abundantly and ubiquitously in the global oceans, on a wide range of marine habitats. In
metabarcoding analyses, they constitute more than 12% of the
environmental sequences. They are an important and underestimated source of carbon in
eutrophic and
oligotrophic habitats.[56] Their abundance varies
seasonally.[58] 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.[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]
Non-constitutive mixotrophs acquire the ability to photosynthesize by stealing
chloroplasts from their prey. They can be divided into two: generalists, which can use chloroplasts stolen from a variety of prey (e.g.
oligotrichciliates), 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
plastids (e.g. Mesodinium, Dinophysis), and endosymbiotic, those which contain
endosymbionts (e.g. mixotrophic
Rhizaria such as
Foraminifera and
Radiolaria, dinoflagellates like Noctiluca).[59] Both plastidic and generalist non-constitutive mixotrophs have similar biogeographies and low abundance, mostly found in eutrophic coastal waters. Generalist
ciliates can account for up to 50% of ciliate communities in the photic zone. The endosymbiotic mixotrophs are the most abundant non-constitutive type.[56]
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
floods.[60] The main
freshwater producers (
chrysophytes,
cryptophytes and
dinophytes) 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.[54]
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).[54] Soil protists are also morphologically and functionally diverse, with four major categories:[61]
Fungus-like protists are present abundantly in soil. Most environmental sequences belong to the
Oomycetes (Stramenopiles), an
osmotrophic and
saprotrophic group that contains free-living and
parasitic species of other protists, fungi, plants and animals. Another important group in soil are
slime molds (found in
Amoebozoa,
Opisthokonta,
Rhizaria and
Heterolobosea), which reproduce by forming fruiting bodies known as
sporocarps (originated from a single cell) and
sorocarps (from aggregations of cells).[61]
Phagotrophic 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
NH3, making it available to plants and other microbes.[62] Many soil protists are also
mycophagous, and facultative (i.e. non-obligate) mycophagy is a widespread evolutionary feeding mode among soil protozoa.[63] Amoeboflagellates like the
glissomonads and
cercomonads (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.
arcellinids and
euglyphids) have
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.[61]
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[64][65] (the
oomycetePhytophthora infestans causes
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
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)[71] can reduce red fire ant populations by 53–100%.[72] Researchers have also been able to infect
phorid fly
parasitoids of the ant with the protist without harming the flies. This turns the flies into a
vector that can spread the pathogenic protist between red fire ant colonies.[73]
Biology
Physiological adaptations
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]
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 or "
eyespots": (1) receptors with light antennae, found in many
green algae,
dinoflagellates and
cryptophytes; (2) receptors with opaque screens; and (3) complex
ocelloids with intracellular lenses, found in one group of predatory
dinoflagellates, 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.[75]
Endosymbiosis. Protists have an accentuated tendency to include
endosymbionts 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
xenosomes are
bacterial endosymbionts found in ciliates, sometimes with a
methanogenic role inside anaerobic ciliates.[75]
Sexual reproduction
Protists generally
reproduce asexually under favorable environmental conditions, but 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.[77]
Eukaryotes emerged in evolution more than 1.5 billion years ago.[78] 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
pathogenic 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.[79][80] Most of these meiotic genes were likely present in the
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
amoebae. Amoebae have been regarded as
asexual organisms, but the study describes evidence that most
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 Toxoplasma gondii are capable of infecting and undergoing asexual reproduction in a wide variety of animals – which act as secondary or intermediate
host – but can undergo sexual reproduction only in the primary or definitive
host (for example:
felids such as
domestic cats in this case).[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]
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 clocks suggest that LECA originated between 1200 and more than 1800 million years ago (Ma). Based on all molecular predictions, modern eukaryotes reached
morphological and
ecological diversity before 1000 Ma in the form of
multicellularalgae capable of
sexual reproduction, and unicellular protists capable of
phagocytosis and
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 "
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
sterols" for their cell membranes (e.g.
cholesterol and
ergosterol), stem eukaryotes produced "
protosterols", which appear earlier in the
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
osmotic shock during desiccation and rehydration cycles. Crown sterols can also receive
ethyl groups, thus enhancing cohesion between
lipids 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
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
anaerobes 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"
glaciations and the extreme global heat that came afterwards.[91]
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 algaBangiomorpha (from 1050 Ma), and the
chlorophytegreen algaProterocladus (from 1000 Ma).[91] Abundant fossils of
heterotrophic protists appear later, around 900 Ma, with the emergence of
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]
^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]
^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 or true fungi.[21] However, many researchers currently accept those three groups as part of a larger Kingdom Fungi.[1][22][23]
^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
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.
Tsukii, Y. (1996). Protist Information Server (database of protist images). Laboratory of Biology, Hosei University.
Protist Information Server. Updated: March 22, 2016.