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Protist

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

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

Protists represent an extremely large genetic and ecological diversity in all environments, including extreme habitats. Their 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 algae, amoebae and , or as important . Together, they compose an amount of biomass that doubles that of animals. They exhibit varied types of nutrition (such as phototrophy, phagotrophy or osmotrophy), sometimes combining them (in mixotrophy). They present unique adaptations not present in multicellular animals, fungi or land plants. The study of protists is termed protistology.

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 Dikarya. Because of this definition by exclusion, protists encompass almost all of the broad spectrum of biological trait expected in eukaryotes.

They are generally unicellular, microorganism eukaryotes. Some species can be purely (generally called algae), or purely heterotrophic (traditionally called protozoa), but there is a wide range of mixotrophic protists which exhibit both phagotrophy and phototrophy together. They have different life cycles, , motility, and organelle.

David Harper (2024). 9781405141574, Wiley-Blackwell. . ISBN 9781405141574

Some protists can be .

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

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". There are many multicellular and colonial examples of algae, including kelp, red algae, some types of , and some lineages of green algae.
, which bear eukaryotic flagella. They are found in all lineages, reflecting that the common ancestor of all living eukaryotes was a flagellated heterotroph.
Amoebae, which usually lack flagella but move through changes in the shape and motion of their protoplasm 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 "", 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 multicellularity (numerous amoebae aggregating together). This type of multicellularity has evolved at least seven times among protists.
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 (water molds) or the .
Parasitic protists, such as Plasmodium falciparum, the cause of malaria.

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 algae like euglenids), have been published under either or both of the ICN and the ICZN codes.

Classification

The evolutionary relationships of protists have been explained through molecular phylogenetics, the DNA sequencing of entire and , and electron microscopy studies of the flagellum and cytoskeleton. New major lineages of protists and novel biodiversity continue to be discovered, resulting in dramatic changes to the eukaryotic tree of life. The newest classification systems of eukaryotes, revised in 2019, do not recognize the formal (kingdom, phylum, class, order...) and instead only recognize clades of related organisms, making the classification more stable in the long term and easier to update. In this new cladistic scheme, the protists are divided into various wide branches informally named supergroups:

Archaeplastida — consists of groups that have evolved from a photosynthetic common ancestor that obtained directly through a single event of endosymbiosis with a cyanobacterium:

Picozoa (1 species), non-photosynthetic predators.
Glaucophyta (26 species), unicellular algae found in freshwater and terrestrial environments.
(2024). 9780123914996, Academic Press. ISBN 9780123914996
Rhodophyta (5,000–6,000 species), mostly multicellular marine life algae that lost chlorophyll and only harvest light energy through .
Rhodelphidia (2 species), predators with non-photosynthetic plastid.
Viridiplantae or Chloroplastida, containing both green algae and which are not protists. The green algae comprise many lineages of varying diversity, such as Chlorophyta (7,000), Prasinodermophyta (10), Zygnematophyceae (4,000), Charophyceae (877), Klebsormidiophyceae (48) or Coleochaetophyceae (36).

SAR supergroup, SAR or Harosa – a clade of three highly diverse lineages exclusively containing protists.

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), known as diatoms; the filamentous or genuinely multicellular Phaeophyta (2,000 species), 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.
Alveolate 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 marine life microplankton and a main cause of ; and Ciliophora (4,500 species), the extremely diverse and well-studied group of mostly free-living heterotrophs known as ciliates.
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.

Haptista — includes the Haptophyta algae and the heterotrophic Centrohelida, which are "heliozoan"-type amoebae.

Cryptista — closely related to Archaeplastida, it includes the Cryptophyta algae, with a plastid of red algae origin, and two obscure relatives with two flagella, and Palpitomonas.

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 . 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 anaerobe protozoa, primarily Flagellate. Some are gut symbionts of animals, others are free-living (for example, Paratrimastix pyriformis), and others are well-known Parasitism (for example, Giardia lamblia).

Amorphea — unites two huge clades:

Amoebozoa (2,400 species) is a large group of heterotrophic protists, mostly amoebae. Many lineages are that produce spore-releasing fruiting bodies, such as Myxogastria, Dictyostelia and Protosporangiida, and are often studied by . Within the non-fruiting amoebae, the Tubulinea contain many naked amoebae (such as Amoeba itself) and a well-studied order of testate amoebae known as Arcellinida. Other non-fruiting amoebozoans are Variosea, Discosea and Archamoebae.
Obazoa includes the two kingdoms Metazoa (animals) and Fungi, and their closest protist relatives inside a clade known as Opisthokonta. The opisthokont protists are Nucleariida, Ichthyosporea, Pluriformea, Filasterea, Choanoflagellate and the elusive Tunicaraptor (1 species). They are flagellated or amoeboid heterotrophs of vital importance in the search for the genes that allow animal multicellularity. Sister groups to Opisthokonta are Apusomonadida (28 species) and Breviatea (4 species).

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. Others, like Hemimastigophora (10 species) and Provora (7 species), appear to be related to or within Diaphoretickes, 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:

History

Early concepts

From the start of the 18th century, the popular term "infusion animals" (later infusoria) referred to protists, bacteria and small invertebrate 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 Protozoa (meaning 'early animals') as a class within Kingdom Animalia, to refer to four very different groups: infusoria (ciliates), , 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). 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.

Origin of the protist kingdom

In 1860, British naturalist John 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 , at the merging bases of the plant and animal kingdoms.

In 1866 the 'father of protistology', German scientist Ernst Haeckel, addressed the problem of classifying all these organisms as a mixture of animal and vegetable characters, and proposed Protistenreich ( Kingdom Protista) as the third kingdom of life, comprising primitive forms that were "neither animals nor plants". He grouped both bacteria and eukaryotes, both unicellular and multicellular organisms, as Protista. He retained the Infusoria in the animal kingdom, until German zoologist Otto Butschli demonstrated that they were unicellular. At first, he included sponges and fungi, but in later publications he explicitly restricted Protista to predominantly unicellular organisms or colonies incapable of forming tissues. He clearly separated Protista from metazoa on the basis that the defining character of protists was the absence of sexual reproduction, while the defining character of animals was the blastula stage of animal development. He also returned the terms protozoa and protophyta as subkingdoms of Protista.

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.

In 1938, American biologist Herbert Copeland resurrected Hogg's label, arguing that Haeckel's term Protista included anucleated microbes such as bacteria, which the term Protoctista (meaning "first established beings") did not. Under his four-kingdom classification (Monera, Protoctista, Plantae, Animalia), the protists and bacteria were finally split apart, recognizing the difference between anucleate (prokaryotic) and nucleate (eukaryotic) organisms. To firmly separate protists from plants, he followed Haeckel's blastular definition of true animals, and proposed defining embryophyta as those with chlorophyll a and chlorophyll b, carotene, xanthophyll and production of starch. He also was the first to recognize that the unicellular/multicellular dichotomy was invalid. Still, he kept fungi within Protoctista, together with red algae, brown algae and . This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life.

In the popular five-kingdom scheme published by American plant ecologist Robert Whittaker in 1969, Protista was defined as eukaryotic "organisms which are unicellular or unicellular-colonial and which form no tissues". Just as the prokaryotic/eukaryotic division was becoming mainstream, Whittaker, after a decade from Copeland's system, recognized the fundamental division of life between the prokaryotic Monera and the eukaryotic kingdoms: Animalia (ingestion), Plantae (photosynthesis), Fungi (absorption) and the remaining Protista.

In the five-kingdom system of American evolutionary biologist Lynn Margulis, the term "protist" was reserved for microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as kelp, red algae, and .

(2009). 9780080920146, Academic Press. . ISBN 9780080920146

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. . ISBN 9783319281476

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. Today, protists are not treated as a formal taxon, but the term is commonly used for convenience in two ways:

Phylogenetic definition: protists are a paraphyletic group. A protist is any eukaryote that is not an animal, land plant or fungus, thus excluding many unicellular groups like the fungal Microsporidia, Chytridiomycetes and , and the non-unicellular animals included in Protista in the past.
Functional definition: protists are essentially those eukaryotes that are never multicellular, 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. brown algae).
(2024). 9780429351907, CRC Press. ISBN 9780429351907

Kingdoms Protozoa and Chromista

There is, however, one classification of protists based on traditional ranks that lasted until the 21st century. The British protozoologist Thomas Cavalier-Smith, since 1998, developed a six-kingdom model: Bacteria, Animalia, Plantae, Fungi, Protozoa and Chromista. In his context, paraphyletic groups take preference over clades: both protist kingdoms Protozoa and Chromista contain paraphyletic phylum such as Apusozoa, Eolouka or Opisthosporidia. Additionally, red algae and green algae are considered true plants, while the fungal groups Microsporidia, Rozellida and Aphelida are considered protozoans under the phylum Opisthosporidia. This scheme endured until 2021, the year of his last publication.

Diversity

Species diversity

According to phylogenetics, 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 to 74,400 as of 2012) in comparison to the biodiversity 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 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 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 marine life 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.

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.

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). 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 Archaeplastida in soil.

Marine

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

marine protists located in the photic zone as phytoplankton are vital in the oceanic systems. They carbon fixation 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 heterotrophic fraction of marine picoplankton encompasses primarily early-branching stramenopiles (e.g. and labyrinthulomycetes), as well as , and ; protists of lower frequency include and cryptophytes.

Mixotrophic marine protists, while not very researched, are present abundantly and ubiquitously in the global oceans, on a wide range of marine habitats. In DNA barcoding, they constitute more than 12% of the environmental sequences. They are an important and underestimated source of carbon in eutrophic and oligotrophic habitats. Their abundance varies season. 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. They are responsible for harmful . They dominate the eukaryotic microbial biomass in the photic zone, in eutrophic and oligotrophic waters across all climate zones, even in non-algal bloom conditions. They account for significant, often dominant predation of bacteria.

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. oligotrich ), 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. Mesodinium, Dinophysis), and endosymbiotic, those which contain (e.g. mixotrophic Rhizaria such as Foraminifera and Radiolaria, dinoflagellates like Noctiluca). 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

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 (chrysophytes, cryptophytes and ) 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.

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

soil protists, or algae, are as abundant as their marine life counterparts. Given the importance of marine algae, soil algae may provide a larger contribution to the global carbon cycle than previously thought, but the magnitude of their carbon fixation has yet to be quantified. Most soil algae belong to the supergroups Stramenopiles (, Xanthophyceae and Eustigmatophyceae) and Archaeplastida (Chlorophyceae and Trebouxiophyceae). There is also the presence of environmental DNA from and in soil, but no living forms have been seen.

Fungus-like protists are present abundantly in soil. Most environmental sequences belong to the Oomycetes (Stramenopiles), an and saprotrophic group that contains free-living and parasitic species of other protists, fungi, plants and animals. Another important group in soil are (found in Amoebozoa, Opisthokonta, Rhizaria and Heterolobosea), 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 bacterial grazers, they have a significant role in the foodweb: they excrete nitrogen in the form of ammonia, making it available to plants and other microbes. Many soil protists are also mycophagous, and facultative (i.e. non-obligate) mycophagy is a widespread evolutionary feeding mode among soil protozoa. Amoeboflagellates like the and (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. and ) have protist shell 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.

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

Parasitic

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

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 (the oomycete Phytophthora infestans causes late blight in potatoes)Campbell, N. and Reese, J. (2008) Biology. Pearson Benjamin Cummings; 8 ed. . pp. 583, 588 or even of other protists.Lauckner, G. (1980). "Diseases of protozoa". In: Diseases of Marine Animals. Kinne, O. (ed.). Vol. 1, p. 84, John Wiley & Sons, Chichester, UK.Cox, F.E.G. (1991). "Systematics of parasitic protozoa". In: Kreier, J.P. & J. R. Baker (ed.). Parasitic Protozoa, 2nd ed., vol. 1. San Diego: Academic Press.

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

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) can reduce red fire ant populations by 53–100%. Researchers have also been able to infect Phoridae 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 physiology and biochemistry 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. Freshwater protists without are able to osmoregulation through contractile vacuoles, specialized 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.

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

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 with intracellular lenses, found in one group of predatory , 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.

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 Chlorella; others more transient. Many protists contain captured chloroplasts, chloroplast-mitochondrial complexes, and even eyespots from algae. The are bacterial endosymbionts found in ciliates, sometimes with a methanogenic 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. Oxidative stress, which leads to DNA damage, also appears to be an important factor in the induction of sex in protists.

(2024). 9781621008088, Nova Sci. Publ.. ISBN 9781621008088

emerged in evolution more than 1.5 billion years ago. 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 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. 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. Amoebae have been regarded as , but the study describes evidence that most amoeboid 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 (chromatin granules free in the cytoplasm) 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 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.

Fossil record

Mesoproterozoic

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. 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 multicellular algae capable of sexual reproduction, and unicellular protists capable of phagocytosis and cell locomotion. 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 "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 " for their cell membranes (e.g. cholesterol and ergosterol), 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 osmotic shock during desiccation and rehydration cycles. Crown sterols can also receive Ethyl group 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 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 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 subaerial 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 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" and the extreme global heat that came afterwards.

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 alga Bangiomorpha (from 1050 Ma), and the chlorophyte green alga Proterocladus (from 1000 Ma). Abundant fossils of heterotrophic protists appear later, around 900 Ma, with the emergence of fungi. For example, the oldest fossils of Amoebozoa are vase-shaped microfossils resembling modern testate amoebae, found in 800 million-year-old rocks. 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.

Protist

( Obsolete Eukaryote Taxa )

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Protist ( Obsolete Eukaryote Taxa )

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Protist

( Obsolete Eukaryote Taxa )