Protist should not be confused with Protest.
A protist or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancestor excluding land plants, animals, and fungi.
Protists were historically regarded as a separate taxonomic 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 (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 mostly unicellular 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.
Protists are a diverse group of eukaryotes that are primarily single-celled and microscopic and exhibit a wide variety of shapes and life strategies. They have different life cycles, trophic levels, modes of locomotion, and cellular structures.[1] [2] Although most protists are unicellular, there is a considerable range of multicellularity amongst them; some form colonies or multicellular structures visible to the naked eye. The term 'protist' refers to all eukaryotes that are not animals, land plants or fungi, the three traditional eukaryotic kingdoms.[3] Because of this definition by exclusion, protists compose a paraphyletic group that includes the ancestors of those three kingdoms.[4]
The names of some protists (called ambiregnal protists), because of their mixture of traits similar to both animals and land plants or fungi (e.g., slime molds and flagellated algae like euglenids), have been published under either or both of the botanical (ICNafp) and the zoological (ICZN) codes of nomenclature.
Protists exist in diverse shapes and sizes.[5] Most protists are single-celled, but they also include many types of multicellularity, from small aggregations and colonies to giant slime molds and seaweed.[6]
For practical purposes, single-celled protists are typically arranged into four broad categories according to their shapes and locomotion: flagellates, amoebae, ciliates, and sporozoa.[7] These were known by the obsolete term protozoa, due to their usually heterotrophic mode of nutrition, but many protozoan groups include photosynthetic species, and consequently are also studied as algae.[8] [9]
Flagellates are the most common protists, and very likely the most abundant eukaryotes on Earth.[10] They are characterized by at least one whip-like appendage used for movement, known as the eukaryotic flagellum. The standard number of flagella is two (i.e., biflagellate), but it can be reduced to one and has multiplied in several groups. Flagella can be smooth, or exhibit additional structures such as mastigonemesâlateral hair-like extensionsâor scales. Flagellates vary in their modes of movement from gliding to swimming to attached to a substrate. They may be heterotrophic (i.e., zooflagellates), ranging from filter feeders like choanoflagellates to active predators like provorans, or they may be photosynthetic (i.e., phytoflagellates), like dinoflagellates.[11] [8]
Amoebae are characterized by extensions of the cytoplasm known as pseudopodia and often an amorphous, flexible shape. The ability to form pseudopodia has been acquired by numerous groups independently. The most common kinds of pseudopodia are: axopodia, radiating projections supported internally by microtubules, characteristic of heliozoa and radiolaria; lobopodia (or lobose pseudopodia), blunt rounded extensions, as in the archetypal genus Amoeba; filopodia (or filose), thinner extensions, common in many lineages; reticulopodia (or reticulose), elaborated networks with internal microtubules, present in foraminifera and relatives. Some amoebae have more than one type of pseudopodia.[5] Certain groups, known as amoeboflagellates, produce both pseudopodia and flagella within the same life cycle. Some individual amoebae can grow to sizes visible to the naked eye, such as the xenophyophores, up to 20 cm.[12]
Ciliates are distinguished by larger cells with two nuclei and hundreds of flagella, known in them as cilia, arranged in rows across the cell surface. They are among the most familiar protists, usually top of the microbial food web. Although ciliates compose a single lineage,[13] some protists have evolved unusual ciliate-like cells, such as the opalines, with rows of cilia and many nuclei.[14]
Certain pathogenic protists were traditionally described under a separate term: sporozoa.[15] This included protistan parasites that are immobile in their adult stage and usually reproduce through infective thick-walled spores, such as haplosporidians and apicomplexans. It also included fungi and animals that have evolved a similar lifestyleâmicrosporidians and myxozoans, respectively. Other than their lifestyle, they have very little in common, and have evolved in distantly related groups.[16] [7]
Besides the aforementioned phytoflagellates, single-celled algae can also be: coccoid, sperhical and covered by a cell wall, like coccolithophores and diatoms;[17] capsalean, non-motile and embedded in mucilage, like glaucophytes;[18] amoeboid, like chlorarachniophytes.[19]
Multicellularity has evolved numerous times independently among protists, ranging from simple colonies to giant complex organisms with differentiated cell types and tissues. Multicellular protists can be 'aggregative', where many cells merge together to form an organism, or 'clonal', where the organism derives either from a single cell that multiplies (as in animals and plants) or from a large multinucleate cell that divides itself internally by membranes (as in fungi).[6]
Several heterotrophic protists that develop in either of these manners are known as fungus-like protists, traditionally studied by mycologists. They present lifestyles reminiscent of fungi: osmotrophic nutrition, often filamentous shapes that may include segmented hyphae, and the formation of multicellular fruiting bodies (sorocarps if aggregative, sporocarps if clonal)[20] and spores. These include the acrasids, dictyostelids, labyrinthuleans, oomycetes, hyphochytrids, and phytomyxids.[21] [22] [6] Some of these, known as slime molds, live as individual amoebae until they form fruiting bodies.[20] Some slime molds are composed of one large cell with numerous nuclei, and are therefore technically unicellular, but still have a fungus-like lifestyle.[6] This is the case for myxomycetes, which can reach lengths of several meters.[5]
Multicellular algae develop clonally and exhibit a range of complexity. For example, some brown algae are parenchymatous, i.e., show differentiated tissues,[23] like the giant kelp. In green algae alone there may have been over 20 independent origins of multicellularity; among them, stoneworts develop specialized plant-like reproductive organs.[24] [6] Simpler body types include: filaments of connected cells, sometimes branched; palmelloid cells, embedded in a common matrix; coenocytes, composed of one very large multinucleate cell; and pseudoparenchymatous bodies, with partial tissue differentiation.[25] [26]
Besides algae and slime molds, various amoebae and flagellates have developed multicellularity. Several amoebae can connect to each other, forming large reticulate networks. The ciliate Haplozoon is interpreted to have animal-like embryonic development and cell type differentiation. Some heliozoa and ciliates, when building colonies, produce new anatomical features that are not present in the individual specimens.[6] Choanoflagellates, the closest living relatives of animals, include alternating cell types that are interpreted as early stages of animal multicellularity.[27]
Starting in the 17th and 18th centuries, after the discovery of microorganisms by Antonie van Leeuwenhoek, the classification of single-celled protists was largely based on observations under light microscopy. Protists were incorporated into the traditional dichotomy that defined all life as either plant or animal: non-motile algae were considered part of the plant kingdom, and all other protists joined the animal kingdom.[28] They were popularly known as "infusion animals" or infusoria, together with bacteria and small invertebrates. Otto Friedrich MĂŒller was the first to introduce microbial protists to the Linnean system of binomial nomenclature.
During the 19th century, after several waves of naturalist studies, it became clear that these microorganisms were distinct from animals and plants. John Hogg and Ernst Haeckel proposed a separate kingdom of life, named Protoctista[29] or Protista,[30] respectively, to accommodate the predominantly unicellular eukaryotes, and initially bacteria, which were later excluded. The classical framework of protist classification was established, as exemplified by the works of Otto BĂŒtschli, where they were grouped according to morphological and locomotive features, such as Mastigophora (flagellates), Rhizopoda (amoebae), Sporozoa (spore-forming parasites), and Infusoria (ciliates).[28] However, BĂŒtschli retained a division between the Protozoa (animal-like protists) and Protophyta (plant-like). This dogma remained widely accepted throughout the early 20th century.
From the mid-20th century, eukaryotes were firmly split from bacteria (prokaryotes) due to the presence of the cell nucleus,[31] and protists (or protoctists) were more widely accepted as a separate kingdom.[32] [33] [34] The advent of electron microscopy shifted the methods of classification, as it revealed previously unrecognized cellular characteristics (i.e., ultrastructure, particularly of the flagellar apparatus and the cytoskeleton) that suggested evolutionary affinities between superficially disparate lineages. For example, the tripartite flagellar mastigonemes were used to group heterokont algae, oomycetes and opalines into the Stramenopiles; the discovery of cortical alveoli showed affinities between dinoflagellates and ciliates, which now belong to the Alveolata; and disc-shaped mitochondrial cristae were shared by kinetoplastids and euglenids, now united as Euglenozoa. The algae-protozoa dichotomy became obsolete.[35] [28]
Since the 1990s, molecular phylogenetic analyses based primarily on the SSU rRNA gene demonstrated that protists were a paraphyletic assemblage of clades spanning the entire eukaryotic tree of life, from which the other three "kingdoms" (animals, plants, and fungi) had evolved.[36] [28] Beginning in the 2000s, single-cell sequencing and phylogenomics technologies progressively improved the resolution of deeper evolutionary relationships. Altogether, these innovations led to successive revisions of protist classification,[28] such as the ones published by the International Society of Protistologists.[37] [38] Eukaryotes could no longer be divided into four monophyletic kingdoms, and instead are arranged in "supergroups", each often encompassing an unexpected variety of morphologies and lifestyles that do not resemble one another. New deep-branching groups are added to the tree at a rate of nearly one per year.[39] [40] [41]
Protists are currently divided among a number of clades informally named supergroups. Most of these supergroups fall under either of two large clades of eukaryotes: Amorphea and Diaphoretickes. The animals and fungi belong to the Opisthokonta supergroup in the Amorphea clade, along with several other groups of protists (e.g., Amoebozoa). Diaphoretickes contains the diverse supergroups Archaeplastida (including plants), Stramenopiles, Alveolata, Rhizaria, and the less species-rich Cryptista, Haptista, Telonemia, and Disparia.[42] Outside of these larger clades, various groups of protists with primitive cell architecture (Discoba, Metamonada, and Malawimonadida) are collectively known as the excavates or "Excavata". The name 'excavate' refers to the shared characteristic of a ventral groove in the cell used for feeding, which is considered an ancestral trait present in the last eukaryotic common ancestor.[43] [44] [45]
The following table lists estimated numbers of described extant species for all known protist supergroups, and provides an overview of their diversity in terms of morphologies, habitats, and nutritional modes. For large groups, the overview is not exhaustive and only mentions the most characteristic members.
| Clade | Supergroup | Example | Brief description of morphology, lifestyle and habitat | Living species | |
|---|---|---|---|---|---|
| Diaph. | Stramenopiles | Ancestrally flagellates distinguished by two 'heterokont' (unequal) flagella, one with tripartite mastigonemes. Present in virtually all habitats. The most species-rich lineage, the ochrophytes, are algae of diverse morphologies, ranging from flagellates (like golden algae) to walled ornamented cells (like diatoms, pictured) to truly multicellular macroalgae with differentiated tissues (brown algae such as kelp). All other lineages are composed of heterotrophs: bacterivorous flagellates (e.g., bicosoecids, bigyromonads), fungus-like osmotrophs (oomycetes, hyphochytrids, and labyrinthulomycetes), heliozoan amoebae (actinophryids), and ciliate-like obligate symbionts of animals (opalinids).[46] | [47] | ||
| Alveolata | Ancestrally flagellated predators with cortical alveoli. The colponemids represent these ancestral characteristics. The most diverse group are the ciliates (pictured), with large cells covered in rows of cilia, usually at the top of the microbial food chain.[48] The remaining alveolates belong to the clade Myzozoa and are ancestrally photosynthetic; some have retained their photosynthetic ability (chromerids and many dinoflagellates), while others have evolved into parasites of animals and algae (apicomplexans, perkinsozoans, and some dinoflagellates).[49] [50] [51] | [52] | |||
| Rhizaria | Amoebae with filose or reticulose pseudopodia.[53] The most species-rich group is Retaria, home to conspicuous marine amoebae encased in hard skeletons (radiolarians) or multichambered tests (foraminifers, pictured). Secondly is Cercozoa, especially abundant in soils and exhibiting a wide range of morphologies from amoeboflagellates to aggregative slime molds,[54] testate amoebae (e.g., euglyphids),[55] and radiolarian-like organisms (phaeodarians); some are capable of photosynthesis (e.g., chlorarachniophytes). Lastly, Endomyxa contains both free-living predatory amoebae (e.g., vampyrellids) and obligate parasites of animals, plants, and algae (e.g., phytomyxeans and ascetosporeans).[56] | [57] | |||
| Telonemia | Free-living flagellates with a unique cytoskeleton and a combination of cell structures. Present in all marine and freshwater environments feeding on bacteria.[58] | [59] [60] | |||
| Haptista | Two groups of different free-living single-celled protists: centrohelidsâpredatory heliozoan amoebae, widespread in aquatic and soil environments[61] âand haptophytesâcoccoid or flagellated photosynthetic algae, mostly marine (e.g., coccolithophores, pictured).[62] Both can produce an outer coat of complex mineralized scales. | ||||
| Pancryptista | Free-living flagellates, except one species of heliozoan amoebae, Microheliella maris.[63] Almost all of the flagellates are distinguished by specialized ribbon-shaped extrusomes known as ejectisomes. Many are photosynthetic, known as cryptomonads (pictured), while the rest are phagotrophs, consumers of bacteria. Present in aquatic environments worldwide.[64] | ||||
| Archaeplastida | Algae with chloroplasts derived from primary endosymbiosis with a cyanobacterium. Found in all environments. Almost entirely photosynthetic, with the exception of two small groups of phagotrophic flagellates, rhodelphids[65] and picozoans.[66] The two major groups, red algae and green algae (pictured), exhibit diverse morphologies, ranging from single cellsâcoccoid, palmelloid, sarcinoid, flagellatedâto colonies, simple filaments, and macroscopic thalli with varying degrees of complexity (e.g., coralline algae, sea lettuce, stoneworts). Also included are glaucophytes, rare blue-green algae found in surface waters.[67] | [68] | |||
| Disparia | Three lineages of free-living predatory flagellates with unique cytoskeletons. These are: Hemimastigophora, with two rows of flagella, present in soils and aquatic sediments;[69] Provora, fast-swimming predators of other protists through a strong feeding apparatus resembling jaws, found in low abundance in marine environments globally;[70] [71] and Caelestes (pictured), rare inhabitants of the marine benthos whose cells protrude arms or stalks used for movement or prey capture.[72] | ||||
| Amorph. | Amoebozoa | Amoebae of diverse morphologies, with blunt (lobose) or fine (filose) pseudopodia, and sometimes with flagella. Most are free-living phagotrophs found across terrestrial and aquatic environments, such as the archetypal genus Amoeba itself,[73] [74] [75] or the testate amoebae Arcellinida, one of the most conspicuous groups of protists.[76] Numerous groups have independently evolved fungus-like fruiting bodies,[77] such as myxomycetes (pictured).[78] Some of the free-living amoebae are important vectors of pathogenic bacteria or are pathogenic themselves (e.g., Acanthamoeba).[79] Others are anaerobic intestinal symbionts (e.g., Entamoeba). | [80] | ||
| Breviatea | Anaerobic free-living amoeboflagellates with fine pseudopodia and modified mitochondria. Present only in low-oxygen marine and brackish sediments, their growth depends on mutualistic interactions with prokaryotes.[81] | [82] | |||
| Apusomonadida | Free-living flagellates distinguished by a proboscis, a sleeve-like structure that envelops one of their two flagella. Found gliding on wet soil and aquatic sediments worldwide. | [83] | |||
| Opisthokonta | Flagellates distinguished by a single posterior flagellum, many with complex life cycles and varying degrees of multicellularity. Some are entirely amoeboid, with fine pseudopodia (e.g., filastereans and nucleariids, including slime molds),[84] [85] while others become amoeboid temporarily (e.g., choanoflagellates, pictured).[86] Most species are free-living filter-feeders or predators,[87] [88] [89] but some lineages (e.g., ichthyosporids) evolved into osmotrophic parasites of animals.[90] [91] | ||||
| Excavat. | Discoba | Flagellates with very different lifestyles, present in aquatic and terrestrial environments, ranging from aerobes to anaerobes. The most diverse group, Euglenozoa, includes free-living osmotrophs, phagotrophs, phototrophs (euglenophytes, pictured), and pathogens (kinetoplastids).[92] The less diverse Heterolobosea are primarily amoeboflagellates, and include some slime molds (acrasids) and well-known opportunistic parasites (e.g., Naegleria fowleri).[93] The smallest group, Jakobida, consume bacteria by suspension feeding.[94] | [95] [96] | ||
| Metamonada | Anaerobic or microaerophilic flagellates, amoebae, or amoeboflagellates, with reduced or completely lost[97] mitochondria. A few are free-living, found in aquatic hypoxic sediments, but most species are obligate parasites (e.g., Giardia, pictured) or commensals in animal intestines (e.g., parabasalids). Many have a high number of flagella.[98] [99] | ||||
| Malawimonadida | Free-living bacterivorous flagellates that feed by suspension feeding, present in marine or fresh waters.[100] | ||||
| Other | Ancyromonadida | Tiny free-living aquatic flagellates composed of flattened cells with an inflexible pellicle and a lateral rostrum with extrusomes. Found in most aquatic habitats. | [101] | ||
| CRuMs | Free-living flagellates and filose amoebae with a pellicle underneath the cell membrane. Almost all flagellated members can produce filose pseudopodia. Found in aquatic environments. | ||||
| |||||
There are also many genera of uncertain affiliation among eukaryotes because their DNA has not been sequenced, and consequently their phylogenetic affinities are unknown. One enigmatic heliozoan species is so large that it does not match the description of any known genus, and was consequently transferred to a separate genus Berkeleyaesol with an unclear position, although it probably belongs to Diaphoretickes along with all other heliozoa.[102] The organism Parakaryon is harder to place, as it shares traits from both prokaryotes and eukaryotes.[103]
The total species diversity of protists is severely underestimated by traditional methods that differentiate species based on morphological characteristics. The number of described protist species is very low (ranging from 26,000 to over 76,000) in comparison to the diversity of land plants, animals and fungi, which are historically and biologically well-known and studied. The predicted number of species also varies greatly, ranging from 140,000 to 1,600,000, and in several groups the number of predicted species is arbitrarily doubled. Most of these predictions are highly subjective. Molecular techniques such as environmental DNA barcoding have revealed a vast diversity of undescribed protists that accounts for the majority of eukaryotic sequences or operational taxonomic units (OTUs), dwarfing those from land plants, animals and fungi. As such, it is considered that protists dominate eukaryotic diversity.
In general, protists have typical eukaryotic cells that follow the same principles of biology described for those cells within the "higher" eukaryotes (animals, fungi and land plants).[104] However, many have evolved a variety of unique physiological adaptations that do not appear in the remaining eukaryotes, and in fact protists encompass almost all of the broad spectrum of biological characteristics expected in eukaryotes.
Protists display a wide variety of food preferences and feeding mechanisms.[105] According to the source of their nutrients, they can be divided into autotrophs (producers, traditionally algae) and heterotrophs (consumers, traditionally protozoa). Autotrophic protists synthesize their own organic compounds from inorganic substrates through the process of photosynthesis, using light as the source of energy;[106] accordingly, they are also known as phototrophs.[107]
Heterotrophic protists obtain organic molecules synthesized by other organisms, and can be further divided according to the size of their nutrients. Those that feed on soluble molecules[106] or macromolecules under 0.5 ÎŒm in size are called osmotrophs,[105] and they absorb them by diffusion, ciliary pits, transport proteins of the cell membrane, and a type of endocytosis (i.e., invagination of the cell membrane into vacuoles, called endosomes) known as pinocytosis or fluid-phase endocytosis.[105] Those that feed on organic particles over 0.5 ÎŒm in size or entire cells are called phagotrophs, and they ingest them through a type of endocytosis known as phagocytosis.[105] [106] Endocytosis is considered one of the most important adaptations in the origin of eukaryotes because it increased the potential food supply, and phagocytosis allowed the endosymbiosis and development of mitochondria and chloroplasts. In both osmotrophs and phagotrophs, endocytosis is often restricted to a specific region of the cell membrane, known as the cytostome, which may be followed by a cytopharynx, a specialized tract supported by microtubules.[105]
Osmotrophic protists acquire soluble nutrients through membrane channels and carriers, but also through pinocytosis: the nutrients are trapped in vesicles that merge into a digestive vacuole or endosome where digestion takes place.[108] [105] Some osmotrophs, called saprotrophs or lysotrophs, perform external digestion by releasing enzymes into the environment and decomposing organic matter into simpler molecules that can be absorbed. This external digestion has a distinct advantage: it allows greater control over the substances that are allowed to enter the cell, thus minimizing the intake of harmful substances or infection.[109]
Probably all eukaryotes are capable of osmotrophy, but some have no alternative of acquiring nutrients. Obligate osmotrophs and saprotrophs include some euglenids, some green algae, the human parasite Blastocystis, some metamonads, the parasitic trypanosomatids,[110] and the fungus-like oomycetes and hyphochytrids.[109]
Phagotrophic feeding consists of two phases: the concentration of food particles in the environment, and the phagocytosis, which encloses the food particle in a vacuole (the phagosome)[105] where digestion takes place. In ciliates and most phagotrophic flagellates, digestion occurs at the oral region or cytostome, which is covered by a single membrane from which vacuoles are formed; the phagosomes then may be shuttled to the interior of the cell along the cytopharynx.[111] In amoebae, phagocytosis takes place anywhere on the cell surface. The average food particle size is around one tenth the size of the protist cell.[112]
Phagotrophic protists can be further classified according to how they approach the nutrients. The filter feeders acquire small, suspended food particles or prokaryotic cells and accumulate them by filtration into the cytostome (e.g., choanoflagellates, some chrysomonads, most ciliates); filter-feeding flagellates accumulate particles by propelling them with a flagellum through a collar of rigid tentacles or pseudopodia that act as a filter, while filter-feeding ciliates generate water currents through cilia and membranelle zones surrounding the cytostome. The raptorial feeders (e.g., bicosoecids, chrysomonads, kinetoplastids, some euglenids, many dinoflagellates and ciliates), instead of retaining all particles in bulk, capture each particle individually.[8] Among raptorial protists, the grazers search and ingest prey from surfaces covered with potential food items such as bacterial lawns, while the predators actively pursue scarce prey. Predators that feed on filamentous algae or fungal hyphae either swallow the filaments entirely or penetrate the cell wall and ingest the cytoplasm (e.g., Viridiraptoridae). Predators may have adaptations to hunt prey, such as 'toxicysts' that immobilize prey cells. Certain ciliates have developed a specialized kind of raptorial feeding called histophagy, where they attack damaged but live animals (e.g., annelids and small crustaceans), enter the wounds, and ingest animal tissue. Large raptorial amoebae enclose their prey in a "food cup" of pseudopodia, prior to the formation of the food vacuole.[8] Lastly, diffusion feeders (e.g., heliozoa, foraminifera and many other amoebae, suctorian ciliates) engulf prey that happen to collide with their pseudopods or, in the case of ciliates, tentacles that carry toxicysts or extrusomes to immobilize the prey.[8]
Consumers of prokaryotes are popularly called bacterivores (e.g., most amoebae), while consumers (including osmotrophic parasites) of eukaryotes are known as eukaryovores. In particular, eukaryovores that feed on unicellular protists are cytotrophs (e.g., colponemids, colpodellids, many amoebae, some ciliates); those that feed on fungi are mycophages or mycotrophs (e.g., the ciliate family Grossglockneriidae of obligate mycophages); those that prey on nematodes are nematophages;[113] and those that feed on algae are phycotrophs (e.g., vampyrellids).
Most autotrophic protists are mixotrophs[114] and combine photosynthesis with phagocytosis. They are classified into various functional groups or 'mixotypes'. Constitutive mixotrophs have the innate ability to photosynthesize through already present chloroplasts, and have diverse feeding behaviors, as some require phototrophy, others phagotrophy, and others are obligate mixotrophs (e.g., nanoflagellates such as some haptophytes and dinoflagellates). Non-constitutive mixotrophs acquire the ability to photosynthesize by stealing chloroplasts from their prey, a process known as kleptoplasty. Non-constitutives can be divided into two: generalists, which can steal chloroplasts from a variety of prey (e.g., oligotrich ciliates), or specialists, which can only acquire chloroplasts from a few specific prey (e.g., Rapaza viridis can only steal from Tetraselmis cells).[115] The specialists are further divided into two types: plastidic, which only engulf the plastids (e.g., Mesodinium, Dinophysis), and endosymbiotic, which keep the entire prey (e.g., mixotrophic Rhizaria such as Foraminifera and Radiolaria, dinoflagellates like Noctiluca).
Among exclusively heterotrophic protists, variation of nutritional modes is also observed. The diplonemids, which inhabit deep waters where photosynthesis is absent, can flexibly switch between osmotrophy and bacterivory depending on the environmental conditions.[116]
Many freshwater protists need to osmoregulate (i.e., remove excess water volume to adjust the ion concentrations) because non-saline water enters in excess by osmosis from the environment[117] and by endocytosis when feeding.[111] Osmoregulation is done through active ion transporters of the cell membrane and through contractile vacuoles, specialized organelles that periodically excrete fluid high in potassium and sodium through a cycle of diastole and systole. The cycle stops when the cells are placed in a medium with different salinity, until the cell adapts.
The contractile vacuoles are surrounded by the spongiome, an array of cytoplasmic vesicles or tubes that slowly collect fluid from the cytoplasm into the vacuole. The vacuoles then contract and discharge the fluid outside of the cell through a pore. The contractile mechanism varies depending on the protist: in ciliates, the spongiome is composed of irregular tubules and actin filaments wind around the pore and over the vacuole surface, together with microtubules; in most flagellates and amoebae, the spongiome is composed of both vesicles and tubules; in dinoflagellates, a flagellar rootlet branches to form a contractile sheath around the vacuole (known as pusule).[111] The location and amount also varies: unicellular flagellated algae (cryptomonads, euglenids, prasinophytes, golden algae, haptophytes, etc.) typically have a single contractile vacuole in a fixed position; naked amoebae have numerous small vesicles that fuse into one vacuole and then split again after excretion. Marine or parasitic protists (e.g., metamonads), as well as those with rigid cell walls, lack these vacuoles.[117]
The last eukaryotic common ancestor was aerobic, 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 hydrogenosomes, 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.
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.
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.
Protists exhibit a large range of life cycles and strategies involving multiple stages of different morphologies which have allowed them to thrive in most environments. Nevertheless, most of the knowledge concerning protist life cycles concerns model organisms and important parasites. Free-living uncultivated protists represent the majority, but knowledge on their life cycles remains fragmentary.
Protists typically reproduce asexually under favorable environmental conditions, allowing for rapid exponential population growth with minimal genetic diversification. This asexual reproduction, occurs through mitosis and has historically been regarded as the primary reproductive mode in protists. This process is also known as vegetative reproduction, as it is only performed by the 'vegetative stage' or individual.
Unicellular protists often multiply via binary fission, similarly to bacteria. They can also divide through budding, similarly to yeasts, or through multiple fissions, a process known as schizogony. In multicellular protists, vegetative reproduction can take the form of fragmentation of body parts, or specialized propagules composed of numerous cells (e.g., in red algae).[118]
While asexual reproduction remains the most common strategy among protists, sexual reproduction is also a fundamental characteristic of eukaryotes.[119] Sexual reproduction involves meiosis (a specialized nuclear division enabling genetic recombination) and syngamy (the fusion of nuclei from two parents). These processes are thought to have been present in the last eukaryotic common ancestor,[120] which likely had the ability to reproduce sexually on a facultative (non-obligate) basis.[121] Even protists that no longer reproduce sexually still retain a core set of meiosis-related genes, reflecting their descent from sexual ancestors.[122] [123] For example, although amoebae are traditionally considered asexual organisms, most asexual amoebae likely arose recently and independently from sexually reproducing amoeboid ancestors.[124] Even in the early 20th century, some researchers interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoebae as sexual reproduction.[125]
Every sexual cycle involves the events of syngamy and meiosis, which increase or decrease the ploidy (i.e., number of chromosome sets, represented by the letter n), respectively. Syngamy implies the fusion of two haploid (1n) reproductive cells, known as gametes, which generates a diploid (2n) cell called zygote. The diploid cell then undergoes meiosis to generate haploid cells. Depending on which cells compose the individual or vegetative stage (i.e., the stage that grows by mitosis), there are three distinguishable sexual cycles observed in free-living protists:
Free-living protists 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.[128]
Pathogenic protists tend to have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually.[129] The stages that feed and multiply inside the host are generally known as trophozoites, but the names of each stage vary depending on the protist group[130] (e.g., sporozoites and merozoites in apicomplexans;[131] [132] primary and secondary zoospores in phytomyxeans).[133]
Some pathogenic protists undergo asexual reproduction in a wide variety of organisms â which act as secondary or intermediate hosts â but can undergo sexual reproduction only in the primary or definitive host (e.g., Toxoplasma gondii in felids such as domestic cats).[134] Others, such as Leishmania, are capable of performing syngamy in the secondary vector.[135] In apicomplexans, sexual reproduction is obligatory for parasite transmission.[136]
Despite undergoing sexual reproduction, it is unclear how frequently there is genetic exchange between different strains of pathogenic protists, as most populations may be clonal lines that rarely exchange genes with other members of their species.[137]
Protists are abundant in nearly all habitats. They contribute an estimated 4 gigatons (Gt) to Earth's biomassâdouble that of animals (2 Gt), but less than 1% of the total. Combined, protists, animals, archaea (7 Gt), and fungi (12 Gt) make up less than 10% of global biomass, with plants (450 Gt) and bacteria (70 Gt) dominating.
Protist diversity, as detected through environmental DNA surveys, is vast in every sampled environment, but it is mostly undescribed. The richest protist communities appear in soils, followed by oceanic and lastly freshwater habitats, mostly as part of the plankton. Freshwater 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.[138]
Soil-dwelling protist communities are ecologically the richest, possibly 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.
Protists can survive a broad range of extreme conditions, including extreme temperatures (thermophiles or psychrophiles), salinity (halophiles), and pH (alkaliphiles or acidophiles). Most of the extremophilic eukaryotes are algae, specifically chlorophytes, followed by fungi. Other extremophile-abundant groups are heterolobose amoebae, red algae, stramenopiles, and ciliates.
Eukaryotic algae are well-known to withstand high temperatures; for example, the red alga Cyanidioschyzon merolae persists up to 60°C, similarly to the most extreme thermophilic fungi. Lesser-known thermophilic amoebae and amoeboflagellates (e.g., Echinamoeba thermarum) are repeatedly found in hot environments, including artificially heated systems. While less successful than algae or amoebae, ciliates have also been found in hydrothermal vents up to 52°C. This is still lower than prokaryotes, some of which grow above 80°C.[139]
In terms of pH and salinity, protists can withstand similar extremes relative to prokaryotes and fungi, and also persist in polyextreme environments (polyextremophiles). The record for acidophily is the red alga C. merolae, with an observed minimum growth of pH 0. Besides red algae, some species of green algae and amoeboflagellates are found in high-temperature, low-pH geothermal springs. Alkaliphilic protists, primarily ciliates, resist up to pH 10.48, higher than the most alkalophilic bacterium.[139]
Protists are remarkably successful in extreme salinity due to their salt-out strategy, which consists of accumulating organic solutes in the cell instead of ions to counterbalance the hypertonic environment. Examples include the alga Dunaliella salina and the flagellate Halocafeteria seosinensis, which is able to tolerate up to 36.3% salinity, higher than the maximum reported in bacteria (35%) and fungi (30%).[139]
Protists are indispensable to modern ecosystems worldwide. They also have been the only eukaryotic component of all ecosystems for much of Earth's history, which allowed them to evolve a vast functional diversity that explains their critical ecological significance. They are essential as primary producers, as intermediates in multiple trophic levels, as key regulating parasites or parasitoids, and as partners in diverse symbioses.
Microscopic phototrophic protists (or microalgae) are the main contributors to the biomass and primary production in nearly all aquatic environments, where they are collectively known as phytoplankton (together with cyanobacteria). In marine phytoplankton, the smallest fractions, the picoplankton (<2 ÎŒm) and nanoplankton (2â20 ÎŒm), are dominated by several different algae (prymnesiophytes, pelagophytes, prasinophytes); fractions larger than 5 ÎŒm are instead dominated by diatoms and dinoflagellates. In freshwater phytoplankton, golden algae, cryptophytes and dinoflagellates are the most abundant groups. Altogether, they are responsible for almost half of the global primary production. They are the main providers of much of the energy and organic matter used by bacteria, archaea, and higher trophic levels (zooplankton and fish), including essential nutrients such as fatty acids.[140] Their abundance in the oceans depends mostly on the availability of inorganic nutrients, rather than temperature or sunlight; they are most abundant in coastal waters that receive nutrient-rich run-off from land, and areas where nutrient-rich deep ocean water reaches the surface, namely the upwelling zones in the Arctic Ocean and along continental margins.[141] In freshwater habitats, most phototrophic protists are mixotrophic, meaning they also behave as consumers, while strict consumers (heterotrophs) are less abundant. In extremely cold habitats, like snow and the arctic ocean, diatoms and green algae are the dominant phototrophs.[139]
Macroalgae (namely red algae, green algae and brown algae), unlike phytoplankton, generally require a fixation point, which limits their marine distribution to coastal waters, and particularly to rocky substrates. They support numerous herbivorous animals, especially benthic ones, as both food and refuge from predators. Some communities of seaweeds exist adrift on the ocean surface, serving as a refuge and means of dispersal for associated organisms.[142] [143]
Phototrophic protists are as abundant in soils as their aquatic counterparts. Given the importance of aquatic algae, soil algae may provide a larger contribution to the global carbon cycle than previously thought, but the magnitude of their carbon fixation has yet to be quantified. Most soil algae are stramenopiles (diatoms, xanthophytes and eustigmatophytes) and archaeplastids (green algae). There is also presence of environmental DNA from dinoflagellates and haptophytes in soil, but no living forms have been seen.
Phagotrophic protists are the most diverse functional group in all ecosystems, primarily represented by cercozoans (dominant in freshwater and soils), radiolarians (dominant in oceans), non-photosynthetic stramenopiles (with higher abundance in soils than in oceans), and ciliates.
Contrary to the common division between phytoplankton and zooplankton, much of the marine plankton is composed of mixotrophic protists, which pose a largely underestimated importance and abundance (around 12% of all marine environmental DNA sequences). Mixotrophs have varied presence due to seasonal abundance and depending on their specific type of mixotrophy. Constitutive mixotrophs are present in almost the entire range of oceanic conditions, from eutrophic shallow habitats to oligotrophic subtropical waters but mostly dominating the photic zone, and they account for most of the predation of bacteria. They are also responsible for harmful algal blooms. Plastidic and generalist non-constitutive mixotrophs have similar biogeographies and low abundance, mostly found in eutrophic coastal waters, with generalist ciliates dominating up to half of ciliate communities in the photic zone. Lastly, endosymbiotic mixotrophs are by far the most widespread and abundant non-constitutive type, representing over 90% of all mixotroph sequences (mostly radiolarians).
In the trophic webs of soils, protists are the main consumers of both bacteria and fungi, the two main pathways of nutrient flow towards higher trophic levels. Amoeboflagellates like the glissomonads and cercomonads 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 are also acclimated to the soil environment, as their shells protect against desiccation. As bacterial grazers, they have a significant role in the foodweb: they excrete nitrogen in the form of NH, making it available to plants and other microbes. Traditionally, protists were considered primarily bacterivorous due to biases in cultivation techniques, but many (e.g., vampyrellids, cercomonads, gymnamoebae, testate amoebae, small flagellates) are omnivores that feed on a wide range of soil eukaryotes, including fungi and even some animals such as nematodes. Bacterivorous and mycophagous protists amount to similar biomasses.
Necrophagy (the degradation of dead biomass) among microbes is mainly attributed to bacteria and fungi, but protists have a still poorly recognized role as decomposers with specialized lytic enzymes. In soils, fungus-like protists and slime molds (e.g., oomycetes, myxomycetes, acrasids) are present abundantly as osmotrophs and saprotrophs. In marine and estuarine environments, the well-studied thraustochytrids (a type of labyrinthulomycetes) are relevant saprotrophs that decompose various substrates, including dead plant and animal tissue. Various ciliates and testate amoebae scavenge on dead animals. Some nucleariid amoebae specifically consume the contents of dead or damaged cells, but not healthy cells. However, all these examples are of facultative necrophages that also feed on live prey. In contrast, the cercozoan family Viridiraptoridae, present in shallow bog waters, are necrophages that only feed on dead algae, potentially fulfilling an important role in cleaning up the environment and releasing nutrients for other microbes.[144]
Parasitic protists compose around 15â20% of all environmental DNA samples in marine and soil systems, but only around 5% in freshwater systems, where chytrid fungi likely fill that ecological niche. In oceanic systems, parasitoids (i.e. those which kill their hosts, e.g. Syndiniales) are more abundant. In freshwater ecosystems, parasitoids are mainly Perkinsea and Syndiniales (Alveolata), while true parasites (i.e. those which do not kill their hosts) in freshwater are mostly oomycetes, Apicomplexa and Ichthyosporea. In soil ecosystems, true parasites are primarily animal-hosted apicomplexans and plant-hosted oomycetes and plasmodiophorids. In Neotropical forest soils, apicomplexans dominate eukaryotic diversity and have an important role as parasites of small invertebrates, while oomycetes are very scarce in contrast.[145]
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), land plants[146] [147] (the oomycete Phytophthora infestans causes potato blight)[148] or even of other protists.[149] [150] Around 100 protist species can infect humans.
Marine protists have a fundamental impact on biogeochemical cycles, particularly the carbon cycle.[19] As phytoplankton, they fix as much carbon as all terrestrial plants combined. Soil protists, particularly testate amoebae, contribute to the silica cycle as much as forest trees through the biomineralization of their shells.
Before the existence of land plants, animals and fungi, all eukaryotes were protists. As a result, the early fossil record of protists is equivalent to the early record of eukaryotic life.[136] The protist fossil record is mainly represented by protists with fossilizable coverings, such as foraminifera, radiolaria, testate amoebae and diatoms, as well as multicellular algae.
Modern or crown-group eukaryotes originated from the last eukaryotic common ancestor (LECA) and emerged between 1600 and 2400 million years ago (Ma), during the Paleoproterozoic and Mesoproterozoic eras. However, the fossil record through this time is scarce and dominated by stem-group eukaryotes, extinct lineages preceding LECA. These lineages displayed early eukaryotic traits like flexible cell membranes and complex cell wall ornamentations, which require a flexible endomembrane system, but they lacked crown-group eukaryotes' advanced sterols (e.g., cholesterol), and instead produced simpler protosterols that require less oxygen during biosynthesis. Examples of these are: Trachyhystrichosphaera and Leiosphaeridia dated at 1100 Ma,[151] Satka dated at 1300 Ma,[152] Tappania and Shuiyousphaeridium dated at 1600 Ma,[153] Grypania dated at 1800â1900 Ma, and Valeria which ranges from 1650 to 700 Ma.[154]
Crown-group eukaryotes achieved significant morphological and ecological diversity before 1000 Ma, with multicellular algae capable of sexual reproduction and unicellular protists exhibiting modern phagocytosis and locomotion. Their advanced but metabolically expensive sterols likely provided numerous evolutionary advantages due to the increased membrane flexibility, including resilience to osmotic shock during desiccation and rehydration cycles, extreme temperatures, UV light exposure, and protection against changing oxygen levels. These adaptations allowed crown-group eukaryotes to colonize diverse and harsh environments (e.g., mudflats, rivers, agitated shorelines and land). In contrast, stem-group eukaryotes occupied the low-oxygen marine waters as anaerobes. The oldest definitive crown-group eukaryotic fossils include Rafatazmia and Ramathallus, both putative red algae, dated at 1600 Ma.
As oxygen levels rose during the Tonian period, crown-group eukaryotes outcompeted stem-group eukaryotes, expanding into oxygen-rich marine environments that supported an aerobic metabolism enabled by their mitochondria. Stem-group eukaryotes may have gone extinct due to competition and the extreme climatic changes of the Cryogenian glaciations and subsequent global warming, cementing the dominance of crown-group eukaryotes. Crown-group eukaryotes began to appear abundantly in this era, fueled by the proliferation of red algae. The oldest fossils firmly assigned to existing protist groups include three multicellular algae: the rhodophyte Bangiomorpha (1047 Ma),[155] the chlorophyte Proterocladus (1000 Ma),[156] and the xanthophyte Paleovaucheria (1000 Ma).[157] [158] Also included are the oldest fossils of Opisthokonta: Ourasphaira giraldae (1010â890 Ma), interpreted as the earliest fungus,[156] and Bicellum brasieri (1000 Ma), the earliest holozoan, showing traits associated with complex multicellularity.[159]
Abundant fossils of heterotrophic protists appear significantly later, parallel to the emergence of fungi.[156] Vase-shaped microfossils (VSMs), widespread rocks dated at 780â720 Ma (Tonian to Cryogenian), have been described as a variety of organisms across the decades (e.g., algae, chitinozoans, tintinnids), but current scientific consensus relates most VSMs to marine testate amoebae. As such, VSMs comprise the oldest known fossils of both filose (Cercozoa) and lobose (Amoebozoa) testate amoebae.
After the Gaskiers glaciation of the Late Ediacaran (~579 Ma), fossils of heterotrophic protists undergo diversification. Some fossils similar to VSMs are interpreted as the oldest fossils of Foraminifera dated at 548 Ma (e.g., Protolagena),[160] but their foraminiferal affinity is doubtful. Other microfossils that are possibly foraminifera include some poorly preserved tubular shells from 716â635 Ma rocks.[161]
Radiolarian shells appear abundantly in the fossil record since the Cambrian, with the first definitive radiolarian fossils found at the very start of this period (~540 Ma) together with the first small shelly fauna. Radiolarian records from older Precambrian rocks have been disregarded due to the lack of reliable fossils.[162] [163] [164] Around this time, between 540 and 510 Ma, the oldest Foraminifera shells appear, first multi-chambered and later tubular.[165] [166] [161]
Following the Cambrian explosion and rapid diversification of animals, the Precambrian microbe-dominated ecosystems were replaced by primarily benthic and nekto-benthic communities, with most marine organisms (animals, foraminifers, radiolarians) limited to the depths of shallow water environments.[167] Mirroring the animal radiation, there was a radiation of phytoplanktonic protists (i.e., acritarchs)[168] around 520â510 Ma, followed by a decrease in diversity around 500 Ma.[169] Later, the surviving acritarchs expanded in diversity and morphological innovation[168] due to a decrease in predation from benthic animals (particularly trilobites and brachiopods), which suffered extinction due to various proposed environmental factors such as anoxia.[170] Both phytoplankton and zooplankton (e.g., radiolarians) flourished, as signaled by an increase of organic carbon buried in the sediment known as the SPICE event (~497 Ma).[167] [170] This abundant biomass supported a second animal radiation known as the Great Ordovician Biodiversification Event (GOBE), where many animals switched to a planktonic lifestyle and pelagic predators first appeared (e.g., cephalopods, swimming arthropods). This event is also known as the 'Ordovician Plankton Revolution' due to the significant diversification of planktonic protists, and it spanned from the late Cambrian well into the Ordovician.[167]
The Ordovician also includes the oldest euglenid fossil, known as Moyeria, which is found in rocks spanning from the middle Ordovician (~471 Ma) to the Silurian.[171] There are putative records of calcareous foraminifera from the Early Ordovician to the Silurian, but these are not widely accepted; the oldest trusted and well-known calcaerous foraminifera appear in the Middle Devonian, the next geological period.[166] [172]
In Early Devonian terrestrial ecosystems the first fossils of freshwater arcellinid testate amoebae are found (e.g., Palaeoleptochlamys, Cangweulla),[173] as well as various types of freshwater green algae, including charophytes, volvocaceans and desmids,[174] and some putative algal fossils that might represent glaucophytes.[175] During the Devonian some benthic foraminifera acquired the ability of calcifying, and particularly the giant fusulinids became the dominant fossilizable protists. This time interval is also considered the molecular origin of haptophytes (~310 Ma) and silicoflagellates (397â382 Ma), which did not leave fossil traces until later in the Mesozoic. After the Late Devonian extinction (372 Ma), nassellarian-like radiolarians appeared for the first time, with a unique body plan among marine protists.[166]
During the Carboniferous period, no new fossilizable protists originated despite the major environmental changes. However, starting in the Late Carboniferous, radiolarian diversity and productivity increased, causing a large amount of biosiliceous sediment (chert) to be accumulated worldwide; this is known as the Radiolarian Optimum Event, which lasted primarily from the Middle Permian until the Early Cretaceous.[176] [177] [178] Around the Capitanian mass extinction event (262â259 Ma) of the Permian period, coccolithophores genetically diverged from the rest of haptophytes, possibly as a response to a reduction in atmospheric oxygen, and there was a faunal turnover from larger to smaller fusulinids.[166] Spumellarian radiolarians appear in the latest Permian.[176]
The Permian-Triassic extinction event (~251.9 Ma) caused the extinction of many radiolarians, which manifests as a gap in the chert record.[176] The extinction is hypothesized as resulting in the molecular origin of diatoms and modern coccolithophores.[166] The Middle to Late Triassic period saw the acceleration of radiolarian diversity[176] and the appearance of several groups of calcaerous nannofossils. First, various nannofossils, some of which belonged to dinocysts, appeared early at around 235 Ma. Later originated the oldest identifiable coccolithophore, Crucirhabdus minutus (205â201 Ma), as well as the oldest fossils of Phaeodaria.[166] There's a variety of protozoa, including soft-bodied ciliates, and filamentous algae found in amber from the Late Triassic (220â230 Ma).[179]
Around the EarlyâMiddle Jurassic, after the global Toarcian Oceanic Anoxic Event there was a diversification of dinoflagellates and coccolithophores, in both species and abundance. This interval also saw the completion of a symbiosis between Acantharia radiolarians and lineages of Phaeocystis haptophytes, as well as the appearance of planktonic foraminifera.[166] The period of low atmospheric oxygen ends in the Aptian-Albian boundary during the Early Cretaceous, and the first fossils of diatoms and silicoflagellates appear.[166] Samples of amber from around 100 Ma contain the oldest fossil records of apicomplexans (particularly malarian agents and gregarines), trypanosomes,[180] and metamonadsâparticularly mutualistic parabasalids of cockroaches, representing the earliest record of mutualism between protists and animals.[181] [182]
The diversification of coccolithophores, mixotrophic dinoflagellates, and later diatoms across the Mesozoic era caused an accelerated transfer of primary production into higher trophic levels. This evolutionary radiation of phytoplankton was, in turn, responsible for the animal "Mesozoic marine revolution", characterized by the appearance of widespread predation among most invertebrate phyla. Coccolithophores, dinoflagellates and especially diatoms became the dominating eukaryotic producers in oceans until today, as opposed to cyanobacteria and green algae which dominated earlier.[183]
The Cretaceous-Paleogene extinction event (~66 Ma) caused the extinction of many marine dinoflagellates, foraminifers, coccolithophores, and silicoflagellates; mesozoic types of these groups were substituted with types that dominate marine habitats today. Right after this event, putative ebridians begin appearing in the fossil record (e.g., Ammodochium), but the oldest reliable ebridian fossils belong to the upper middle Eocene (42â33.7 Ma).[166] Around this time, the oldest fossils of Synurophyceae appear (~49â40 Ma).[184] Following the Middle Eocene Climatic Optimum (~40 Ma), diatoms became the dominant agents of marine silicon precipitation as opposed to radiolarians, and the fossil record shows the first raphid diatoms and collodarians.[166]
