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A protist ( ) or protoctist is any that is not an , , or . Protists do not form a natural group, or , but are a grouping of several independent clades that evolved from the last eukaryotic common ancestor.

Protists were historically regarded as a separate kingdom known as Protista or Protoctista. With the advent of analysis and electron microscopy studies, the use of Protista as a formal was gradually abandoned. In modern classifications, protists are spread across several eukaryotic clades called supergroups, such as ( that includes plants), , (which includes fungi and animals), and .

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 as important components of the biogeochemical cycles and . They exist abundantly and ubiquitously in a variety of forms that evolved multiple times independently, such as free-living , and , or as important . Together, they compose an amount of biomass that doubles that of animals. They exhibit varied types of nutrition (such as , or ), sometimes combining them (in ). They present unique adaptations not present in multicellular animals, fungi or land plants. The study of protists is termed .


Definition
There is not a single accepted definition of what protists are. As a assemblage of diverse biological groups, they have historically been regarded as a that includes any eukaryotic organism (i.e., living beings whose cells possess a nucleus) that is not an animal, a or a . Because of this definition by exclusion, protists encompass almost all of the broad spectrum of expected in eukaryotes.

They are generally , eukaryotes that can be purely , which are generally called , or purely , which are traditionally called , but there is a wide range of protists where and coexist. They have different life cycles, , , and .

(2024). 9781405141574, Wiley-Blackwell. .
Some protists can be .

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

  • , which are protists. Traditionally called "protophyta", they are found within most of the big evolutionary lineages or supergroups, intermingled with protists which are traditionally called "". There are many multicellular and colonial examples of algae, including , , some types of , and some lineages of .
  • , which bear eukaryotic . They are found in all lineages, reflecting that the common ancestor of all living eukaryotes was a flagellated .
  • , which usually lack flagella but move through changes in the shape and motion of their to produce . 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 "", amoebae with -supported pseudopodia radiating from the cell, with at least three independent origins. Others, referred to as "", grow a shell around the cell made from organic or inorganic material.
  • , which are amoebae capable of producing stalked reproductive structures that bear spores, often through (numerous amoebae aggregating together). This type of multicellularity has evolved at least seven times among protists.
  • Fungus-like protists, which can produce -like structures and are often . They have evolved multiple times, often very distantly from true fungi. For example, the (water molds) or the .
  • Parasitic protists, such as Plasmodium falciparum, the cause of .

The names of some protists (called protists), because of their mixture of traits similar to both animals and plants or fungi (e.g. and algae like euglenids), have been published under either or both of the ICN and the ICZN codes.


Classification

Modern classification
The evolutionary relationships of protists have been explained through molecular phylogenetics, the of entire and , and electron microscopy studies of the and . New major lineages of protists and novel 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 (phylum, class, order...) and instead only recognize the group that are of related organisms, making the classification more stable in the long term and easier to update. In this new scheme, the protists are divided into wide branches informally named supergroups:

  • , SAR or Harosa – a clade of three highly diverse lineages exclusively containing protists.
    • is a wide clade of photosynthetic and organisms that evolved from a common ancestor with hairs in one of their two flagella. The photosynthetic stramenopiles, called , are a group that acquired chloroplasts from secondary endosymbiosis with a . Among these, the best known are: the unicellular or colonial (>60,000 species), known as diatoms; the filamentous or genuinely multicellular (2,000 species), known as brown algae; and the (>1,200 species). The heterotrophic stramenopiles are more diverse in forms, ranging from fungi-like organisms such as the , and , to various kinds of protozoa such as the flagellates and .
    • contains three of the most well-known groups of protists: , a group with species harmful to humans and animals; , an ecologically important group as a main component of the and a main cause of ; and (4,500 species), the extremely diverse and well-studied group of mostly free-living heterotrophs known as ciliates.
    • 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 and , groups of large and abundant marine amoebae, many of them macroscopic. Much of the rhizarian diversity lies within the phylum , filled with free-living flagellates which usually have pseudopodia, as well as , a group previously considered radiolarian. Other groups comprise various amoebae like or are important parasites like , or .

  • — includes many lineages previously grouped under the paraphyletic "": the , flagellates with bacterial-like mitochondrial genomes; , a free-living flagellate; and the clade, which unites well-known phyla and . Heterolobosea includes amoebae, flagellates and amoeboflagellates with complex life cycles, and the unusual , a group of . Euglenozoa encompasses a clade of algae with chloroplasts of green algal origin and many groups of anaerobic, parasitic or free-living heterotrophs.

  • — a clade of completely protozoa, primarily flagellates. Some are gut symbionts of animals, others are free-living, and others are well-known parasites (for example, ).

Many lineages do not belong to any of these supergroups, and are usually poorly known groups with limited data. Some, such as the clade, and , appear to be related to Amorphea. Others, like (10 species) and (7 species), appear to be related to or within , a clade that unites SAR, Archaeplastida, Haptista and Cryptista.

Although the root of the tree is still unresolved, one possible topology of the eukaryotic tree of life is:


Historical classifications

Early concepts
From the start of the 18th century, the popular term "infusion animals" (later ) referred to protists, and small animals. In the mid-18th century, while Swedish scientist Carl von Linnaeus largely ignored the protists, his Danish contemporary Otto Friedrich Müller was the first to introduce protists to the binomial nomenclature system.

In the early 19th century, German naturalist Georg August Goldfuss introduced (meaning 'early animals') as a class within , to refer to four very different groups: (), , phytozoa (such as ) and . Later, in 1845, Carl Theodor von Siebold was the first to establish as a phylum of exclusively unicellular animals consisting of two classes: Infusoria (ciliates) and (, ). 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 was the first to define Protozoa as a separate kingdom of organisms, with "nucleated cells" and the "common organic characters" of plants and animals, although he also included within protozoa.


Origin of Kingdom Protista or Protoctista
In 1860, British John Hogg proposed Protoctista (meaning 'first-created beings') as the name for a fourth kingdom of nature (the other kingdoms being ' plant, animal and mineral) which comprised all the lower, primitive organisms, including protophyta, protozoa and , at the merging bases of the plant and animal kingdoms.

In 1866 the 'father of protistology', German scientist , 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 in the animal kingdom, until German zoologist demonstrated that they were unicellular. At first, he included 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 on the basis that the defining character of protists was the absence of sexual reproduction, while the defining character of animals was the stage of animal development. He also returned the terms protozoa and protophyta as subkingdoms of Protista.

Butschli considered the kingdom to be too 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 and solidified it as a branch of study independent from and .

In 1938, American biologist 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 (, , , ), the protists and bacteria were finally split apart, recognizing the difference between anucleate () and nucleate () organisms. To firmly separate protists from plants, he followed Haeckel's blastular definition of true animals, and proposed defining as those with and , , and production of . He also was the first to recognize that the unicellular/multicellular dichotomy was invalid. Still, he kept fungi within Protoctista, together with , and . This classification was the basis for Whittaker's later definition of Fungi, , 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 , the term "protist" was reserved for microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as , , and .

(2009). 9780080920146, Academic Press. .
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.
(2024). 9783319281476, Springer International Publishing. .


Molecular phylogenetics and modern definitions
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 group from which animals, fungi and plants evolved, and the three-domain system (Bacteria, , ) became prevalent. Today, protists are not treated as a formal , but the term is commonly used for convenience in two ways:
  • definition: protists are a group. A protist is any that is not an animal, or , thus excluding many unicellular groups like the fungal , and , and the non-unicellular animals included in Protista in the past.
  • Functional definition: protists are essentially those eukaryotes that are never , that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues. 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. ).
    (2024). 9780429351907, CRC Press.


Two-kingdom system of protists
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, , , Fungi, and . In his context, paraphyletic groups take preference over clades: both protist kingdoms Protozoa and Chromista contain paraphyletic such as , or . Additionally, and are considered true plants, while the groups , and are considered protozoans under the phylum . This scheme endured until 2021, the year of his last publication.


Diversity

Species diversity
According to , protists dominate 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 to 74,400 as of 2012) in comparison to the of plants, animals and fungi, which are historically and biologically well-known and studied. The predicted number of species also varies greatly, ranging from 1.4×10 to 1.6×10, and in several groups the number of predicted species is arbitrarily doubled. Most of these predictions are highly subjective.

Molecular techniques such as 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 need to be used to survey the vast protistan diversity, because there is no universal marker that can be applied to all lineages.


Biomass
Protists make up a large portion of the biomass in both and terrestrial ecosystems. It has been estimated that protists account for 4 (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, (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.


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 . The richest protist communities appear in , followed by and habitats.

protists (consumers) are the most diverse functional group in all ecosystems, with three main taxonomical groups of phagotrophs: (mainly in freshwater and soil habitats, and in oceans), ciliates (most abundant in freshwater and second most abundant in soil) and non-photosynthetic (third most represented overall, higher in soil than in oceans). protists (producers) appear in lower proportions, probably constrained by intense predation. They exist in similar abundance in both oceans and soil. They are mostly in oceans, in freshwater, and in soil.


Marine
protists are highly diverse, have a fundamental impact on biogeochemical cycles (particularly, the ) and are at the base of the marine as part of the .

marine protists located in the as phytoplankton are vital in the oceanic systems. They as all terrestrial plants together. The smallest fractions, the picoplankton (<2 μm) and nanoplankton (2–20 μm), are dominated by several different algae (, , ); fractions larger than 5 μm are instead dominated by and . The fraction of marine picoplankton encompasses primarily early-branching (e.g. and labyrinthulomycetes), as well as , and ; protists of lower frequency include and .

marine protists, while not very researched, are present abundantly and ubiquitously in the global oceans, on a wide range of marine habitats. In , they constitute more than 12% of the environmental sequences. They are an important and underestimated source of carbon in and habitats. Their abundance varies . Planktonic protists are classified into various functional groups or 'mixotypes' that present different :

  • Constitutive mixotrophs, also called ' that eat', have the innate ability to . They have diverse feeding behaviors: some require , others , and others are obligate mixotrophs. They are responsible for harmful . They dominate the eukaryotic microbial biomass in the , in eutrophic and oligotrophic waters across all climate zones, even in non- conditions. They account for significant, often dominant predation of .

  • Non-constitutive mixotrophs acquire the ability to photosynthesize by stealing from their prey. They can be divided into two: generalists, which can use chloroplasts stolen from a variety of prey (e.g. ), 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 (e.g. , ), and endosymbiotic, those which contain (e.g. mixotrophic such as and , dinoflagellates like ). Both plastidic and generalist non-constitutive mixotrophs have similar biogeographies and low abundance, mostly found in eutrophic coastal waters. Generalist can account for up to 50% of ciliate communities in the photic zone. The endosymbiotic mixotrophs are the most abundant non-constitutive type.


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 . The main freshwater producers (, and ) behave alternatively as consumers (). At the same time, strict consumers (non-photosynthetic) are less abundant in freshwater, implying that the consumer role is partly taken by these mixotrophs.


Soil
protist communities are ecologically the richest. This may be due to the complex and highly dynamic distribution of water in the , 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 as well as protists. Only a small fraction of the detected diversity of soil-dwelling protists has been described (8.1% as of 2017). Soil protists are also morphologically and functionally diverse, with four major categories:

  • -like protists are present abundantly in soil. Most environmental sequences belong to the (Stramenopiles), an and group that contains free-living and species of other protists, fungi, plants and animals. Another important group in soil are (found in , , and ), which reproduce by forming fruiting bodies known as sporocarps (originated from a single cell) and (from aggregations of cells).

  • protists are abundant and essential in soil ecosystems. As grazers, they have a significant role in the foodweb: they excrete in the form of , making it available to plants and other microbes. Many soil protists are also , and facultative (i.e. non-obligate) mycophagy is a widespread evolutionary feeding mode among soil protozoa. Amoeboflagellates like the and (in ) are among the most abundant soil protists: they possess both flagella and pseudopodia, a morphological variability well suited for foraging between soil particles. (e.g. and ) have that protect against desiccation and predation, and their contribution to the through the biomineralization of shells is as important as that of forest trees.

  • soil protists (in ) are diverse, ubiquitous and have an important role as parasites of soil-dwelling animals. In forests, environmental DNA from the apicomplexan dominates protist diversity.


Parasitic
protists represent around 15–20% of all environmental DNA in marine and soil systems, but only around 5% in freshwater systems, where fungi likely fill that . In oceanic systems, (i.e. those which kill their hosts, e.g. ) are more abundant. In soil ecosystems, true parasites (i.e. those which do not kill their hosts) are primarily animal-hosted () and plant-hosted () and (). In freshwater ecosystems, parasitoids are mainly and (Alveolata), as well as the fungal . True parasites in freshwater are mostly , and .

Some protists are significant parasites of animals (e.g.; five species of the parasitic genus cause in humans and many others cause similar diseases in other vertebrates), plants (the Phytophthora infestans causes 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. Two papers from 2013 have proposed , the use of viruses to treat infections caused by .

Researchers from the Agricultural Research Service are taking advantage of protists as pathogens to control red imported fire ant ( Solenopsis invicta) populations in . Spore-producing protists such as Kneallhazia solenopsae (recognized as a or the closest relative to the now) can reduce red fire ant populations by 53–100%. Researchers have also been able to infect fly 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.Durham, Sharon (January 28, 2010) ARS Parasite Collections Assist Research and Diagnoses. Ars.usda.gov. Retrieved 2014-03-20.


Biology

Physiological adaptations
While, in general, protists are typical and follow the same principles of and described for those cells within the "higher" eukaryotes (animals, fungi or plants), they have evolved a variety of unique physiological adaptations that do not appear in those eukaryotes.

  • Osmoregulation. protists without are able to through contractile vacuoles, specialized that periodically excrete fluid high in and 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.

  • Energetic adaptations. The last eukaryotic common ancestor was , bearing for oxidative metabolism. Many lineages of free-living and parasitic protists have independently evolved and adapted to inhabit anaerobic or habitats, by modifying the early mitochondria into , organelles that generate ATP anaerobically through of . In a parallel manner, in the microaerophilic protists, the fermentative evolved from the .

  • Sensory adaptations. Many flagellates and probably all motile algae exhibit a positive (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 , and ; (2) receptors with opaque screens; and (3) complex with intracellular lenses, found in one group of predatory , the . Additionally, some orient themselves in relation to the Earth's gravitational field while moving (), and others swim in relation to the concentration of dissolved in the water.

  • Endosymbiosis. Protists have an accentuated tendency to include in their cells, and these have produced new physiological opportunities. Some associations are more permanent, such as Paramecium bursaria and its endosymbiont ; others more transient. Many protists contain captured chloroplasts, chloroplast-mitochondrial complexes, and even eyespots from algae. The are endosymbionts found in ciliates, sometimes with a role inside anaerobic ciliates.


Sexual reproduction
Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock. , which leads to , also appears to be an important factor in the induction of sex in protists.
(2024). 9781621008088, Nova Sci. Publ..

emerged in evolution more than 1.5 billion years ago. The earliest eukaryotes were protists. Although sexual reproduction is widespread among 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 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 and Trichomonas vaginalis) are now known to descend from ancestors capable of and meiotic recombination, because they have a set core of meiotic genes that are present in sexual eukaryotes. Most of these meiotic genes were likely present in the common ancestor of all eukaryotes, which was likely capable of facultative (non-obligate) sexual reproduction.

This view was further supported by a 2011 study on . Amoebae have been regarded as , but the study describes evidence that most lineages are ancestrally sexual, and that the majority of asexual groups likely arose recently and independently. Even in the early 20th century, some researchers interpreted phenomena related to chromidia ( granules free in the ) in amoebae as sexual reproduction.


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: such as in this case).

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. 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.

The parasitic protists of the genus have been shown to be capable of a sexual cycle in the invertebrate vector, likened to the meiosis undertaken in the trypanosomes.


Fossil record

Mesoproterozoic
By definition, all before the existence of , and are considered protists. For that reason, this section contains information about the deep ancestry of all eukaryotes.

All living , including protists, evolved from the last eukaryotic common ancestor (LECA). Descendants of this ancestor are known as "" or "modern" eukaryotes. 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 capable of sexual reproduction, and unicellular protists capable of and . However, the fossil record of modern eukaryotes is very scarce around this period, which contradicts the predicted diversity.

Instead, the fossil record of this period contains " 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 fossil record (1650–1000 Ma). They present defining eukaryote characteristics such as complex ornamentation and 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 " for their cell membranes (e.g. and ), stem eukaryotes produced "protosterols", which appear earlier in the biosynthetic pathway.

Crown sterols, while metabolically more expensive, may have granted several evolutionary advantages for LECA's descendants. Specific unsaturation patterns in crown sterols protect against during desiccation and rehydration cycles. Crown sterols can also receive groups, thus enhancing cohesion between 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 . 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 of their time, diversifying into that experienced cycles of desiccation and rehydration, daily extremes of high and low temperatures, and elevated UV radiation (such as , rivers, agitated shorelines and soil).

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 that thrived in waters, which at the time were low on oxygen. Eventually, during the period ( 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 "" and the extreme global heat that came afterwards.


Neoproterozoic
Modern eukaryotes began to appear abundantly in the period (1000–720 Ma), fueled by the proliferation of . The oldest fossils assigned to modern eukaryotes belong to two protists: the multicellular (from 1050 Ma), and the (from 1000 Ma). Abundant fossils of protists appear later, around 900 Ma, with the emergence of . For example, the oldest fossils of are vase-shaped microfossils resembling modern , found in 800 million-year-old rocks. shells are found abundantly in the fossil record after the period (~500 Ma), but more recent paleontological studies are beginning to interpret some fossils as the earliest evidence of radiolarians.


See also


Footnotes

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

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