Protist

showing the position of protists, from which all other eukaryotes evolved. Protists are defined as all eukaryotes that are not animals, land plants or fungi, the three traditional "higher" kingdoms of eukaryotes. Because of this definition by exclusion, protists compose a paraphyletic group from which the ancestors of those three kingdoms evolved. As such, there is no unique trait that unifies all protists yet excludes non-protists. Still, together they exhibit a remarkable diversity of life cycles, trophic levels, modes of locomotion, and cellular structures that dwarfs those seen in "higher" eukaryotes. but this disregards the various transitions to multicellularity among protists. were traditionally classified as protists, and some algae (particularly red and green algae) were lumped with plants. According to the current consensus, the label 'protist' specifically excludes animals, embryophytes (land plants) —meaning that all eukaryotic algae fall under this label— and all fungi. Opisthosporidians are considered part of a larger fungal kingdom, although they are studied by protistologists and mycologists alike. == Morphology ==

Protists exist in diverse shapes and sizes. While most are single-celled, others have evolved various forms of multicellularity, ranging from simple colonies of cells to large, complex organisms like giant kelp and slime molds. Single-celled protists are traditionally grouped by their shape and how they move, into four broad categories. • Flagellates are the most common protists, and very likely the most abundant eukaryotes on Earth. They move using one or more whip-like structures called flagella. Most are heterotrophic (known as zooflagellates), feeding on bacteria or other organisms, ranging from filter feeders like choanoflagellates to active predators like provorans. Many are photo- or mixotrophic (known as phytoflagellates) and are studied as algae, like the dinoflagellates. • Amoebae are known for their often flexible shape and ability to form extensions of the cytoplasm known as pseudopodia. These extensions come in various forms, such as lobose (blunt, rounded, as in Amoeba), filose (thin, tapering, as in cercozoans), or reticulose (branching networks, as in foraminifers). Some, called axopodia, take the shape of radiating projections supported by microtubules, characteristic of heliozoa and radiolaria. Amoeboflagellates can produce both pseudopodia and flagella within the same life cycle. some protists have evolved unusual large ciliate-like cells, such as the opalines. • Certain parasitic protists traditionally described as sporozoa are immobile in their adult stage and reproduce through thick-walled spores, such as haplosporidians and apicomplexans. This term also included fungi and animals that have evolved a similar lifestyle—microsporidians and myxozoans, respectively. Other than their lifestyle, they have very little in common, and have evolved in distantly related groups. others are amoeboid, like the reticulose chlorarachniophytes. Multicellularity has evolved numerous times to various degrees among protists, resulting in organisms built either by cells aggregating together (aggregative) or by cells dividing without separating (clonal). Some can reach lengths of several meters. In green algae alone multicellularity is thought to have evolved over 20 separate times, with some groups like the stoneworts developing specialized reproductive organs. == Classification ==

Brief history Starting in the 17th and 18th centuries, after the discovery of microscopic life 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. They were popularly known as "infusion animals" or infusoria, together with bacteria and small invertebrates. Otto Friedrich Müller was the first to introduce microbial protists to the Linnean system of binomial nomenclature. 's 1866 tree of life, with the third kingdom Protista. During the 19th century, after several waves of naturalist studies, or Protista, respectively, to accommodate the predominantly unicellular eukaryotes, and initially bacteria, which were later excluded. and protists (or protoctists) were more popularly accepted as a separate kingdom of eukaryotes. 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. 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. Modern classification and diversity 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). 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. There are also many protists of uncertain position because their DNA has not been sequenced, and consequently their phylogenetic affinities are unknown. 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. ==Biology==

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). 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. According to the nutrient source, they can be divided into autotrophs (or phototrophs, producers, traditionally algae), which photosynthesize their own organic molecules, and heterotrophs (consumers, traditionally protozoa), which obtain organic molecules from the environment, either by passive feeding of small particles (i.e., osmotrophs) or by engulfing whole cells or parts of cells of other organisms (phagotrophs). Phagotrophy phagocytosing a Paramecium'' ciliate Phagotrophic protists feed by phagocytosis, a process unique to eukaryotes where food particles or cells are digested into a vacuole, the phagosome. a tract supported by microtubules. '' extracting algal cell content with a pseudopodium (arrow) According to the method of digestion, protists can be divided into filter, raptorial, or diffusion feeders. Filter feeders accumulate small suspended particles into the cytostome by filtering them through pseudopodia or rigid tentacles, like choanoflagellates, or by generating water currents around the cytostome, like ciliates. nematodes, or tissues of larger animals. Probably all eukaryotes are capable of osmotrophy, but some have no alternative of acquiring nutrients. Obligate osmotrophs include the aphagean euglenids, some green algae, the human parasite Blastocystis, some metamonads, and the fungus-like oomycetes and hyphochytrids. as they combine photosynthesis with phagocytosis. While some mixotrophs already have chloroplasts (i.e., algae), others acquire chloroplasts by stealing them from their prey, a process known as kleptoplasty. Kleptoplastic protists may be generalists, able to steal chloroplasts from a variety of prey, like some ciliates, or they may be specialists, only capable of obtaining chloroplasts from very specific prey. Specialists may keep the entire prey inside of their cells, as do many foraminifers and radiolarians, or they may only engulf the plastids and discard the rest. 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. Homeostasis s in Paramecium aurelia Many freshwater protists need to osmoregulate (i.e., remove excess water volume to adjust the ion concentrations) because non-saline water enters in excess from the environment. Mitochondria and respiration The last eukaryotic common ancestor was aerobic, bearing mitochondria that synthesize ATP through oxidative respiration, which requires oxygen. Most protists are aerobes, but many lineages of free-living and parasitic protists have independently adapted to inhabit anaerobic or microaerophilic (low-oxygen) habitats by modifying their mitochondria into organelles collectively known as mitochondrion-related organelles (MROs). These exist in a continuum from lower to higher degrees of reduction. For example, hydrogenosomes have lost the electron transport chain used in respiration, as well as other features of classical mitochondria (their DNA, the Krebs cycle, etc.), but can still generate ATP anaerobically through the fermentation of pyruvate, releasing hydrogen gas as a byproduct. Mitosomes have lost both the respiratory chain and the production of ATP. One group of protists, the genus Monocercomonoides, has lost its mitochondria entirely. In a similar manner, the oxidative peroxisome evolved into a fermentative glycosome in trypanosomatids. Mitochondrial genomes (mitogenomes), typically composed of one circular chromosome, can appear as numerous linear chromosomes in many unrelated protists, such as Amoebidium, with hundreds of chromosomes. The large mitogenome of kinetoplastids is condensed into a kinetoplast, which is physically tied to the flagellar apparatus. The smallest known mitogenome belongs to the symbiotic alga Chromera velia. Mitochondrial cristae, foldings of the inner membrane, have been used to classify protists since the advent of electron microscopy. Cytoskeleton The cytoskeleton of protists generally consists of an array of microtubules and other fibers that radiate from a complex flagellar apparatus. This structure—sometimes known as the mastigont—was present in the ancestor of all eukaryotes, and is fundamental to the structure, movement and division of cells. It is one of the only cellular features that can be compared across all protists, as it is relatively conserved. The basic plan of the flagellar apparatus consists of two basal bodies (B1 and B2), one for each flagellum, followed by four primary microtubular 'roots' (named R1 through R4) and a 'singlet root' (SR) formed by a single microtubule and originating from B1. Attached to the R1 is a multilayered structure, also known as C fiber. Each protist group has modifications or secondary losses of this standard organization. In archaeplastids, the SR and R2 supporting the feeding groove were lost, likely due to their shift to autotrophic nutrition. This connection is often done through different kinds of filamentous structures, variously called rhizoplasts or internal flagellar roots. Sensory perception with an ocelloid (double arrowhead)|alt=An image of a single cell featuring a large nucleus and an ocelloid, which is composed of a roundish "lens" and a darkly pigmented disc-shaped retinal body.|178x178px 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 photoreceptors of varying degrees of complexity, from simple receptors with light antennae (as in the eyespot apparatus of many algae), to receptors with opaque screens, to complex ocelloids with intracellular lenses (as in the dinoflagellate family Warnowiaceae). 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.—or their nitrogen fixation. Others maintain only the chloroplasts of algae they ingest, and dispose of the remaining cellular structures (i.e., kleptoplasty). Several groups of protists host non-photosynthetic prokaryotes, often maintaining an anaerobic lifestyle through the metabolism of their symbionts. Xenosomes are bacterial endosymbionts with a methanogenic role, found in anaerobic ciliates. Similarly, breviates have hydrogen-oxidizing epibiotic bacteria. Metamonads, particularly parabasalids and oxymonads found in the hindgut of termites, typically host methanogenic archaea as epi- or endobionts. Some rare associations involve prokaryotes that defend the protist host against potential predators, namely in symbiontids and in the ciliate Euplotidium, where the epibionts are verrucomicrobia that eject genetic material as a defense mechanism. There are also some species of oxymonads whose epibionts function as chemosensors, providing their host with information on the surrounding chemical gradient. Besides algae, occurrence of mutualistic eukaryotic symbionts is rare among protists. In the genus Neoparamoeba, some species have endosymbionts that resemble Perkinsela amoebae, a species of trypanosomatids. Although no benefits are yet known from this association, their evolution matches almost perfectly, suggesting that the symbionts are inherited. == Life cycle and reproduction ==

(n) at each stage. Purple arrows represent a full haplo-diploid cycle, while the green and blue arrows represent the variations present in the haploid (meiosis immediately after syngamy) and the diploid (syngamy immediately after meiosis) cycles, respectively. Vegetative reproduction is shown in pink. Protists exhibit a large variability of life cycles and strategies involving multiple stages of different morphologies which have allowed them to thrive in most environments. Nevertheless, most research concerning protist life cycles corresponds to model organisms and important parasites; knowledge on the life cycles of the free-living majority remains fragmentary. Asexual reproduction Protists typically reproduce asexually under favorable environmental conditions, Sexual reproduction Sexual reproduction is a fundamental characteristic of eukaryotes. which likely had the ability to reproduce sexually on a facultative (non-obligate) basis. Even protists that no longer reproduce sexually still retain a core set of meiosis-related genes, reflecting their descent from sexual ancestors. For example, although amoebae are traditionally considered asexual organisms, most asexual amoebae likely arose recently and independently from sexually reproducing amoeboid ancestors. Even in the early 20th century, some researchers interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoebae as sexual reproduction. Three distinguishable sexual cycles are observed in protists depending on the ploidy of the individual or vegetative stage: Zygnematophytes, a group of green algae, fuse vegetative cells directly by conjugation instead of producing gametes. bridge. • Diploid cycle (as in animals): the individual is diploid and undergoes meiosis to generate haploid gametes, which fuse into a zygote that develops as a new individual. • Haplo-diploid cycle (as in land plants): there are two alternating generations of individuals. One, the diploid agamont (or sporophyte), undergoes meiosis to generate haploid cells (called spores) that develop into the other generation, the haploid gamont (or gametophyte). The gamont then generates gametes by mitosis, which fuse to form the diploid zygote that develops into the agamont. Many red algae have a three-generational cycle with a carposporophyte, whose spores germinate into a tetrasporophyte, whose spores develop into the gametophyte. Factors inducing sexual cycles 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. Several protists synchronize their life cycles (namely the formation or release of gametes) according to environmental factors such as nutrient or light levels, resulting in synchronization with the day-night cycle, the lunar cycle, or the seasons. The malaria agent Plasmodium falciparum synchronizes its life cycle with the host's levels of melatonin. Cycles in pathogenic protists Pathogenic protists tend to have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually. 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 (e.g., sporozoites and merozoites in apicomplexans; primary and secondary zoospores in phytomyxeans). Some pathogenic protists undergo asexual reproduction in a wide variety of organisms – which act as secondary or intermediate hosts – but can undergo sexual reproduction only in the primary or definitive host (e.g., Toxoplasma gondii in felids such as domestic cats). Others, such as Leishmania, are capable of performing syngamy in the secondary vector. In apicomplexans, sexual reproduction is obligatory for parasite transmission. 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. == Habitats ==

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. 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. The major parasites in land are the animal-associated apicomplexans and the plant-associated oomycetes and plasmodiophorids. 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. Radiolarians are widespread as the most dominant marine consumers. In freshwater phytoplankton, golden algae, cryptophytes and dinoflagellates are the most abundant groups. In extremely cold habitats, like snow and the Arctic Ocean, diatoms and green algae are the dominant phototrophs. 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 also 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 represented by ciliates, resist up to pH 10.48, higher than the most alkalophilic bacterium. == Ecological roles ==

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. Macroalgae support numerous herbivorous animals, especially benthic ones, as both food and refuge from predators. Consumers near the roots, phyllosphere on the leaves), predatory protists such as cercomonads regulate the populations of bacteria and fungi, indirectly improving plant health and growth. They can also have a more direct impact by releasing proteins with antimicrobial activity. Decomposers 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. Parasites and pathogens Parasitic protists compose around 15–20% of all environmental DNA samples in marine and soil ecosystems, but only around 5% in freshwater systems, where chytrid fungi likely fill that ecological niche. and even of other protists. In plants, oomycetes are the most economically important pathogens (e.g., potato blight), but other lesser studied lineages are known to infect plants, such as phytomyxids (e.g., clubroot), labyrinthulids, and trypanosomatids of the genus Phytomonas. Parasitic protists are among the most well-known human pathogens, causing diseases such as malaria, toxoplasmosis, amoebic meningoencephalitis, sleeping sickness, leishmaniasis, and several diarrheal illnesses like amoebiasis, cryptosporidiosis, and giardiasis. Several amoebae are amphizoic, normally free-living but capable of infection. While parasitic protists are largely studied as protozoa, some are algae, such as the green alga Cephaleuros virescens which infects plant leaves. Hundreds of red algae species parasitize on other red algae, usually closely related species. Certain non-parasitic protists can still be toxic to aquatic animals during periods of excessive growth, either by the release of potent toxins, the depletion of oxygen in the water, or mechanical damage to gills from piercing structures (like the skeletons of silicoflagellates). These phenomena are known as harmful algal blooms, sometimes causing water discoloration (red tides). The most common agents are diatoms and dinoflagellates. When toxins are involved, they can reach human consumption, leading to fish or shellfish poisoning like ciguatera. Mutualists and commensals Many protists live as endosymbionts in a non-parasitic association, providing their hosts with nutritional advantages. Microalgae, namely zoochlorellae (green algae) and zooxanthellae (dinoflagellates, haptophytes and ochrophytes), are widespread endobionts of other protists, especially foraminifers, radiolarians and ciliates, but also of animals. The association of zooxanthellae with corals is extensively studied and valued for its importance in reef ecosystems. Climate warming leads to the loss of zooxanthellae, which manifests as coral bleaching and death of the coral hosts. Some molluscs like the giant clams also harbor zooxanthellae. Sloths are the only mammals with green algae as epibionts on their fur; the hypothesized benefits range from camouflage to sunscreen-like protection or nutritional supplements. Heterotrophic protists are prevalent members of the gut microbiome of animals, although research has focused almost exclusively on gut bacteria. The giant metamonad flagellates found in the hindgut of termites and cockroaches allow them to digest wood. This is an obligate mutualism, as termites will starve if cleaned of these protists. However most gut protists are commensals, such as the ciliates abundantly present in the rumen of ruminants, or the ciliate-like opalinids that inhabit amphibian and reptile guts. Biogeochemical cycles Marine protists have a fundamental impact on biogeochemical cycles, particularly the carbon cycle. As phytoplankton, they fix as much carbon as all terrestrial plants combined. or sand stars, the remains of star-shaped foraminifers. Soil protists, particularly testate amoebae, contribute to the silica cycle as much as forest trees through the biomineralization of their shells. ==Evolution and fossil record==

Prior to the existence of animals, land plants and fungi, all eukaryotes were protists. As such, questions regarding their origin and evolution are questions on protists. Because of the scarce fossil record prior to ca. 750 Mya, dating of early events in eukaryotic evolution relies primarily on molecular clock reconstructions. Eukaryotic cells first evolved from archaea some time during the late Archean eon (ca. 3 billion years ago), forming a lineage that eventually gave rise to the last eukaryotic common ancestor (LECA) with the traits associated with crown-group (modern) eukaryotes, namely mitochondria and a complex endomembrane system. Estimations on the date of LECA range from 2.4 to 1.6 billion years ago, during the Paleoproterozoic, First fossils and stem eukaryotes Fossils before 1 billion years ago are limited and cannot be confidently assigned to modern eukaryotic groups. As such, they are interpreted as potential stem-group eukaryotes, based on their large cell sizes and complex shapes that would still require diagnostic eukaryotic features such as a cytoskeleton and an endomembrane system. The earliest potential stem eukaryotes reach back to the late Paleoproterozoic (2–1.6 billion years ago). There are also two fossils of putative red algae, Ramathallus and Rafatazmia. Following LECA, a series of ecological and evolutionary innovations took place in the span of 300 million years, between the Paleoproterozoic and Mesoproterozoic, resulting in a rapid radiation that originated all major eukaryotic supergroups. Below is a consensus phylogenetic tree of eukaryotes, including all major supergroups that originated from this time period. The oldest fossils of opisthokonts are Ourasphaira giraldae, interpreted as the earliest fungus, Heterotrophic protists appear abundantly throughout the Tonian, as exemplified by the vase-shaped microfossils (780–720 Mya), interpreted as marine testate amoebae Other microfossils include some poorly preserved tubular shells from 716–635 Mya rocks. As oxygen levels rose, crown eukaryotes may have outcompeted stem eukaryotes, expanding into oxygen-rich marine environments that supported an aerobic metabolism enabled by their mitochondria. Stem eukaryotes may have gone extinct due to competition and the extreme climatic changes of the Cryogenian glaciations (720–635 Mya) and subsequent global warming, cementing the dominance of crown eukaryotes which began to appear abundantly in this era, fueled by the proliferation of algae. but their foraminiferal affinity is doubtful. Proterocladus antiquus, Nanfen Formation, Xihe Group (North China).jpg|Proterocladus, the oldest fossil of green algae Bangiomorpha pubescens, Hunting Formation (Sumerset Island, Nunavut, Canada).jpg|Bangiomorpha, the oldest fossil of red algae Phanerozoic diversifications and major events At the very start of the Paleozoic era, the first definitive fossils of radiolarian and foraminiferal Following the Cambrian explosion of animals, the Precambrian microbe-dominated ecosystems were replaced by primarily benthic and nekto-benthic communities, with most marine organisms limited to the depths of shallow water environments. Mirroring the animal radiation, there was a radiation of phytoplanktonic protists (acritarchs) around 520–510 Ma, followed by a decrease in diversity around 500 Ma. The surviving acritarchs expanded in diversity and morphological innovation 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 Mya). In the Devonian period, the first fossils of freshwater arcellinid testate amoebae are found (e.g., Palaeoleptochlamys, Cangweulla), as well as various types of freshwater green algae, including charophytes, volvocaceans and desmids, and some fossils that might represent glaucophytes. Some benthic foraminifera acquired the ability of calcifying, and particularly the giant fusulinids became the dominant fossilizable protists. This period also includes the molecular origin of haptophytes (~310 Mya) and silicoflagellates (397–382 Mya), which did not leave fossil traces until later in the Mesozoic. After the Late Devonian extinction (372 Mya), nassellarian-like radiolarians appeared for the first time, with a unique body plan among marine protists. Around the Capitanian mass extinction event (262–259 Mya) 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. Around the Early–Middle 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. and metamonads—particularly mutualistic parabasalids of cockroaches, representing the earliest record of mutualism between protists and animals. Across the Mesozoic era, coccolithophores, dinoflagellates and later diatoms became the dominating eukaryotic producers in oceans until today, as opposed to cyanobacteria and green algae which dominated earlier. Their diversification caused an accelerated transfer of primary production into higher trophic levels, which in turn caused the animal "Mesozoic marine revolution", characterized by the appearance of widespread predation among most invertebrate phyla. The Cenozoic era began with another extinction event (~66 Ma) that caused the replacement of mesozoic forms of dinoflagellates, foraminifers, coccolithophores, and silicoflagellates with forms 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 (42–33.7 Ma). 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. == See also ==