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68-557: Halvaria is a taxonomic grouping of protists that includes Alveolata and Stramenopiles (Heterokonta). Analyses in 2007 and 2008 revealed that the Stramenopiles and the Alveolata are related, and form a reduced clade of what were seen to be a paraphyletic group, the chromalveolates . The two clades together with the Rhizaria (originally one of the six major eukaryote groups) form

136-498: A model organism ). Free-living ciliates are usually the top heterotrophs and predators in microbial food webs, feeding on bacteria and smaller eukaryotes, present in a variety of ecosystems, although a few species are kleptoplastic . Others are parasitic of numerous animals. Ciliates have a basal position in the evolution of alveolates, together with a few species of heterotrophic flagellates with two cilia collectively known as colponemids . The remaining alveolates are grouped under

204-421: A big portion of the oxygen produced worldwide, and comprising much of the marine phytoplankton ; the brown algae , filamentous or 'truly' multicellular (with differentiated tissues) macroalgae that constitute the basis of many temperate and cold marine ecosystems, such as kelp forests ; and the golden algae , unicellular or colonial flagellates that are mostly present in freshwater habitats. Inside Gyrista,

272-646: A clade dubbed the SAR supergroup . A phylogenomic analysis from 2016 cast doubt on Halvaria, suggesting that Alveolata is the sister group to Rhizaria (making the R + A clade ) through new rhizarian sequence data, and that support for Halvaria might be an artifact of low taxon sampling as well as long branch attraction . However, later analyses from 2021 support Halvaria as a solid clade. Rhizaria [REDACTED] Stramenopiles [REDACTED] Alveolata [REDACTED] Protist A protist ( / ˈ p r oʊ t ɪ s t / PROH -tist ) or protoctist

340-449: A collection of amoebae, flagellates and amoeboflagellates with complex life cycles, among which are some slime molds ( acrasids ). The two clades Euglenozoa and Percolozoa are sister taxa, united under the name Discicristata , in reference to their mitochondrial cristae shaped like discs. The species Tsukubamonas globosa is a free-living flagellate whose precise position within Discoba

408-639: A diverse group (>1,000 living species) of amoebae, often bearing delicate and intricate siliceous skeletons. The forams (Foraminifera) are also diverse (>6,700 living species), and most of them are encased in multichambered tests constructed from calcium carbonate or agglutinated mineral particles. Both groups have a rich fossil record, with tens of thousands of described fossil species. Single-celled Although some prokaryotes live in colonies , they are not specialised cells with differing functions. These organisms live together, and each cell must carry out all life processes to survive. In contrast, even

476-553: A form of phagocytosis . While protozoa reproduce mainly asexually, some protozoa are capable of sexual reproduction. Protozoa with sexual capability include the pathogenic species Plasmodium falciparum , Toxoplasma gondii , Trypanosoma brucei , Giardia duodenalis and Leishmania species. Ciliophora , or ciliates, are a group of protists that utilize cilia for locomotion. Examples include Paramecium , Stentors , and Vorticella . Ciliates are widely abundant in almost all environments where water can be found, and

544-484: A greater range of genetic diversity by combining the DNA of the parents followed by recombination . Metabolic functions in eukaryotes are more specialized as well by sectioning specific processes into organelles. The endosymbiotic theory holds that mitochondria and chloroplasts have bacterial origins. Both organelles contain their own sets of DNA and have bacteria-like ribosomes. It is likely that modern mitochondria were once

612-601: A major cause of harmful algal blooms due to their toxicity; some live as symbionts of corals, allowing the creation of coral reefs. Dinoflagellates exhibit a diversity of cellular structures, such as complex eyelike ocelli, specialized vacuoles, bioluminescent organelles, and a wall surrounding the cell known as the theca . Rhizaria is a lineage of morphologically diverse organisms, composed almost entirely of unicellular heterotrophic amoebae, flagellates and amoeboflagellates, commonly with reticulose (net-like) or filose (thread-like) pseudopodia for feeding and locomotion. It

680-501: A nucleus, or the Golgi apparatus. Prokaryotic cells probably transitioned into eukaryotic cells between 2.0 and 1.4 billion years ago. This was an important step in evolution. In contrast to prokaryotes, eukaryotes reproduce by using mitosis and meiosis . Sex appears to be a ubiquitous and ancient, and inherent attribute of eukaryotic life. Meiosis, a true sexual process, allows for efficient recombinational repair of DNA damage and

748-422: A process called budding , where most of the cytoplasm is held by the mother cell. Saccharomyces cerevisiae ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread. S. cerevisiae is also an important model organism, since it is a eukaryotic organism that is easy to grow. It has been used to research cancer and neurodegenerative diseases as well as to understand

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816-432: A sexual process referred to as natural genetic transformation . Transformation is a bacterial process for transferring DNA from one cell to another, and is apparently an adaptation for repairing DNA damage in the recipient cell. In addition, plasmids can be exchanged through the use of a pilus in a process known as conjugation . The photosynthetic cyanobacteria are arguably the most successful bacteria, and changed

884-473: A small group (three species) of freshwater or marine suspension-feeding bacterivorous flagellates with typical excavate appearance, closely resembling Jakobida and some metamonads but not phylogenetically close to either in most analyses. Diaphoretickes includes nearly all photosynthetic eukaryotes. Within this clade, the TSAR supergroup gathers a colossal diversity of protists. The most basal branching member of

952-592: A species similar to Rickettsia , with the parasitic ability to enter a cell. However, if the bacteria were capable of respiration, it would have been beneficial for the larger cell to allow the parasite to live in return for energy and detoxification of oxygen. Chloroplasts probably became symbionts through a similar set of events, and are most likely descendants of cyanobacteria. While not all eukaryotes have mitochondria or chloroplasts, mitochondria are found in most eukaryotes, and chloroplasts are found in all plants and algae. Photosynthesis and respiration are essentially

1020-474: A variety of algae. In addition, two smaller groups, Haptista and Cryptista , also belong to Diaphoretickes. The stramenopiles, also known as Heterokonta, are characterized by the presence of two cilia, one of which bears many short, straw-like hairs ( mastigonemes ). They include one clade of phototrophs and numerous clades of heterotrophs, present in virtually all habitats. Stramenopiles include two usually well-supported clades, Bigyra and Gyrista , although

1088-420: A wide variety of animals – which act as secondary or intermediate host – but can undergo sexual reproduction only in the primary or definitive host (for example: felids such as domestic cats in this case). 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

1156-565: Is paraphyletic , with some analyses placing the root of the eukaryote tree within Metamonada. Discoba includes three major groups: Jakobida , Euglenozoa and Percolozoa . Jakobida are a small group (~20 species) of free-living heterotrophic flagellates, with two cilia, that primarily eat bacteria through suspension feeding; most are aquatic aerobes, with some anaerobic species, found in marine, brackish or fresh water. They are best known for their bacterial-like mitochondrial genomes. Euglenozoa

1224-440: Is a rich (>2,000 species) group of flagellates with very different lifestyles, including: the free-living heterotrophic (both osmo- and phagotrophic) and photosynthetic euglenids (e.g., the euglenophytes , with chloroplasts originated from green algae); the free-living and parasitic kinetoplastids (such as the trypanosomes ); the deep-sea anaerobic symbiontids ; and the elusive diplonemids . Percolozoa (~150 species) are

1292-443: 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 polyphyletic grouping of several independent clades that evolved from the last eukaryotic common ancestor . Protists were historically regarded as a separate taxonomic kingdom known as Protista or Protoctista . With the advent of phylogenetic analysis and electron microscopy studies,

1360-445: Is not yet settled, but is probably more closely related to Discicristata than to Jakobida. The metamonads (Metamonada) are a phylum of completely anaerobic or microaerophilic protozoa, primarily flagellates . Some are gut symbionts of animals such as termites , others are free-living, and others are parasitic. They include three main clades: Fornicata , Parabasalia and Preaxostyla . Fornicata (>140 species) encompasses

1428-500: Is termed protistology . Protists are a diverse group of eukaryotes (organisms whose cells possess a nucleus ) that are primarily single-celled and microscopic but exhibit a wide variety of shapes and life strategies. They have different life cycles , trophic levels , modes of locomotion , and cellular structures . Although most protists are unicellular , there is a considerable range of multicellularity amongst them; some form colonies or multicellular structures visible to

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1496-402: Is the cause of amebic dysentery. Entamoeba histolytica appears to be capable of meiosis . Unicellular algae are plant-like autotrophs and contain chlorophyll . They include groups that have both multicellular and unicellular species: Unicellular fungi include the yeasts . Fungi are found in most habitats, although most are found on land. Yeasts reproduce through mitosis, and many use

1564-402: 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

1632-408: Is widespread among multicellular eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes. The main reason for this view was that sex appeared to be lacking in certain pathogenic protists whose ancestors branched off early from the eukaryotic family tree. However, several of these "early-branching" protists that were thought to predate

1700-499: The bicosoecids , phagotrophic flagellates that consume bacteria, and the closely related Placidozoa , which consists of several groups of heterotrophic flagellates (e.g., the deep-sea halophilic Placididea ) as well as the intestinal commensals known as Opalinata (e.g., the human parasite Blastocystis , and the highly unusual opalinids , composed of giant cells with numerous nuclei and cilia, originally misclassified as ciliates). The alveolates (Alveolata) are characterized by

1768-655: The bigyromonads , a group of bacterivorous or eukaryovorous phagotrophs. A small group of heliozoan-like heterotrophic amoebae, Actinophryida , has an uncertain position, either within or as the sister taxon of Ochrophyta. The little studied phylum Bigyra is an assemblage of exclusively heterotrophic organisms, most of which are free-living. It includes the labyrinthulomycetes , among which are single-celled amoeboid phagotrophs, mixotrophs, and fungus-like filamentous heterotrophs that create slime networks to move and absorb nutrients, as well as some parasites. Also included in Bigyra are

1836-531: The cell cycle . Furthermore, research using S. cerevisiae has played a central role in understanding the mechanism of meiotic recombination and the adaptive function of meiosis . Candida spp . are responsible for candidiasis , causing infections of the mouth and/or throat (known as thrush) and vagina (commonly called yeast infection). Most unicellular organisms are of microscopic size and are thus classified as microorganisms . However, some unicellular protists and bacteria are macroscopic and visible to

1904-547: The diplomonads , with two nuclei (e.g., Giardia , genus of well-known parasites of humans), and several smaller groups of free-living, commensal and parasitic protists (e.g., Carpediemonas , retortamonads ). Parabasalia (>460 species) is a varied group of anaerobic, mostly endobiotic organisms, ranging from small parasites (like Trichomonas vaginalis , another human pathogen) to giant intestinal symbionts with numerous flagella and nuclei found in wood-eating termites and cockroaches . Preaxostyla (~140 species) includes

1972-532: The heterotrophic protists, known as protozoa , were considered part of the animal kingdom , while the phototrophic ones, called algae , were studied as part of the plant kingdom . Even after the creation of a separate protist kingdom, some minuscule animals (the myxozoans ) and 'lower' fungi (namely the aphelids , rozellids and microsporidians , collectively known as Opisthosporidia ) were studied as protists, and some algae (particularly red and green algae ) remained classified as plants. According to

2040-431: The monophyly of Bigyra is being questioned. Branching outside both Bigyra and Gyrista is a single species of enigmatic heterotrophic flagellates, Platysulcus tardus . Much of the diversity of heterotrophic stramenopiles is still uncharacterized, known almost entirely from lineages of genetic sequences known as MASTs (MArine STramenopiles), of which only a few species have been described. The phylum Gyrista includes

2108-470: The nucleoid . Most prokaryotes have a single, circular chromosome , which is in contrast to eukaryotes, which typically have linear chromosomes. Nutritionally, prokaryotes have the ability to utilize a wide range of organic and inorganic material for use in metabolism, including sulfur, cellulose, ammonia, or nitrite. Prokaryotes are relatively ubiquitous in the environment and some (known as extremophiles) thrive in extreme environments. Bacteria are one of

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2176-480: The sequencing of entire genomes and transcriptomes , and electron microscopy studies of the flagellar apparatus and cytoskeleton . New major lineages of protists and novel biodiversity continue to be discovered, resulting in dramatic changes to the eukaryotic tree of life. The newest classification systems of eukaryotes do not recognize the formal taxonomic ranks (kingdom, phylum, class, order...) and instead only recognize clades of related organisms, making

2244-521: The TSAR clade is Telonemia , a small (seven species) phylum of obscure phagotrophic predatory flagellates, found in marine and freshwater environments. They share some cellular similarities with the remaining three clades: Rhizaria , Alveolata and Stramenopiles , collectively known as the SAR supergroup . Another highly diverse clade within Diaphoretickes is Archaeplastida , which houses land plants and

2312-472: The anaerobic and endobiotic oxymonads , with modified mitochondria , and two genera of free-living microaerophilic bacterivorous flagellates Trimastix and Paratrimastix , with typical excavate morphology. Two genera of anaerobic flagellates of recent description and unique cell architecture, Barthelona and Skoliomonas , are closely related to the Fornicata. The malawimonads (Malawimonadida) are

2380-501: The apicomplexans ( Apicomplexa ) are a large (>6,000 species) and highly specialized group of obligate parasites who have all secondarily lost their photosynthetic ability (e.g., Plasmodium falciparum , cause of malaria ). Their adult stages absorb nutrients from the host through the cell membrane, and they reproduce between hosts via sporozoites, which exhibit an organelle complex (the apicoplast ) evolved from non-photosynthetic chloroplasts. The other branch of Myzozoa contains

2448-450: The botanical ( ICN ) and the zoological ( ICZN ) codes of nomenclature . Protists display a wide range of distinct morphologies that have been used to classify them for practical purposes, although most of these categories do not represent evolutionary cohesive lineages or clades and have instead evolved independently several times. The most recognizable types are: In general, protists are typical eukaryotic cells that follow

2516-675: The cilia beat rhythmically in order to propel the organism. Many ciliates have trichocysts , which are spear-like organelles that can be discharged to catch prey, anchor themselves, or for defense. Ciliates are also capable of sexual reproduction, and utilize two nuclei unique to ciliates: a macronucleus for normal metabolic control and a separate micronucleus that undergoes meiosis. Examples of such ciliates are Paramecium and Tetrahymena that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions. The Amebozoa utilize pseudopodia and cytoplasmic flow to move in their environment. Entamoeba histolytica

2584-399: The clade Myzozoa , whose common ancestor acquired chloroplasts through a secondary endosymbiosis from a red alga. One branch of Myzozoa contains the apicomplexans and their closest relatives, a small clade of flagellates known as Chrompodellida where phototrophic and heterotrophic flagellates, called chromerids and colpodellids respectively, are evolutionarily intermingled. In contrast,

2652-470: The classification more stable in the long term and easier to update. In this new cladistic scheme, the protists are divided into various branches informally named supergroups . Most photosynthetic eukaryotes fall under the Diaphoretickes clade, which contains the supergroups Archaeplastida (which includes plants) and TSAR (including Telonemia , Stramenopiles , Alveolata and Rhizaria ), as well as

2720-446: The common characteristic of a ventral groove in the cell used for suspension feeding , which is considered to be an ancestral trait present in the last eukaryotic common ancestor . The Excavata is composed of three clades: Discoba , Metamonada and Malawimonadida , each including 'typical excavates' that are free-living phagotrophic flagellates with the characteristic ventral groove. According to most phylogenetic analyses, this group

2788-499: The current consensus, the term 'protist' specifically excludes animals, embryophytes (land plants) —meaning that all algae fall under this category— and all fungi, although lower fungi are often studied by protistologists and mycologists alike. The names of some protists (called ambiregnal protists), because of their mixture of traits similar to both animals and plants or fungi (e.g. slime molds and flagellated algae like euglenids ), have been published under either or both of

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2856-761: The dinoflagellates and their closest relatives, the perkinsids ( Perkinsozoa ), a small group (26 species) of aquatic intracellular parasites which have lost their photosynthetic ability similarly to apicomplexans. They reproduce through flagellated spores that infect dinoflagellates, molluscs and fish . In contrast, the dinoflagellates ( Dinoflagellata ) are a highly diversified (~4,500 species) group of aquatic algae that have mostly retained their chloroplasts, although many lineages have lost their own and instead either live as heterotrophs or reacquire new chloroplasts from other sources, including tertiary endosymbiosis and kleptoplasty . Most dinoflagellates are free-living and compose an important portion of phytoplankton, as well as

2924-627: The early atmosphere of the earth by oxygenating it. Stromatolites , structures made up of layers of calcium carbonate and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records. The existence of stromatolites gives an excellent record as to the development of cyanobacteria, which are represented across the Archaean (4 billion to 2.5 billion years ago), Proterozoic (2.5 billion to 540 million years ago), and Phanerozoic (540 million years ago to present day) eons. Much of

2992-448: The early, harsh conditions that life was likely exposed to . Examples of these Archaean extremophiles are as follows: Methanogens are a significant subset of archaea and include many extremophiles, but are also ubiquitous in wetland environments as well as the ruminant and hindgut of animals. This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of adenosine triphosphate . They are

3060-451: 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

3128-413: The external environment. For example, an early RNA replicator ribozyme may have replicated other replicator ribozymes of different RNA sequences if not kept separate. Such hypothetic cells with an RNA genome instead of the usual DNA genome are called ' ribocells ' or 'ribocytes'. When amphiphiles like lipids are placed in water, the hydrophobic tails aggregate to form micelles and vesicles , with

3196-765: The fossilized stromatolites of the world can be found in Western Australia . There, some of the oldest stromatolites have been found, some dating back to about 3,430 million years ago. Clonal aging occurs naturally in bacteria , and is apparently due to the accumulation of damage that can happen even in the absence of external stressors. Hydrothermal vents release heat and hydrogen sulfide , allowing extremophiles to survive using chemolithotrophic growth. Archaea are generally similar in appearance to bacteria, hence their original classification as bacteria, but have significant molecular differences most notably in their membrane structure and ribosomal RNA. By sequencing

3264-484: The hydrophilic ends facing outwards. Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment. Because of their simplicity and ability to self-assemble in water, it is likely that these simple membranes predated other forms of early biological molecules. Prokaryotes lack membrane-bound organelles, such as mitochondria or a nucleus . Instead, most prokaryotes have an irregular region that contains DNA, known as

3332-440: The kingdom Protozoa: Euglenozoa , Amoebozoa , Choanozoa sensu Cavalier-Smith , Loukozoa , Percolozoa , Microsporidia and Sulcozoa . Protozoa, like plants and animals, can be considered heterotrophs or autotrophs. Autotrophs like Euglena are capable of producing their energy using photosynthesis, while heterotrophic protozoa consume food by either funneling it through a mouth-like gullet or engulfing it with pseudopods,

3400-535: The meiosis undertaken in the trypanosomes. The 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 74,400 as of 2012) in comparison to the diversity of plants, animals and fungi, which are historically and biologically well-known and studied. The predicted number of species also varies greatly, ranging from 1.4×10 to 1.6×10 , and in several groups

3468-403: The naked eye. The term 'protist' is defined as a paraphyletic group of all eukaryotes that are not animals , plants or fungi . Because of this definition by exclusion, protists encompass almost all of the broad spectrum of biological characteristics expected in eukaryotes. The distinction between protists and the other three eukaryotic kingdoms has been difficult to settle. Historically,

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3536-860: 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 plants, animals and fungi. As such, it is considered that protists dominate eukaryotic diversity. Stramenopiles Alveolata Rhizaria Telonemia Haptista Microhelida Cryptista Archaeplastida Provora Hemimastigophora Meteora sporadica Discoba Metamonada Ancyromonadida Malawimonadida CRuMs Amoebozoa Breviatea Apusomonadida Opisthokonta The evolutionary relationships of protists have been explained through molecular phylogenetics ,

3604-445: The only known organisms capable of producing methane. Under stressful environmental conditions that cause DNA damage , some species of archaea aggregate and transfer DNA between cells. The function of this transfer appears to be to replace damaged DNA sequence information in the recipient cell by undamaged sequence information from the donor cell. Eukaryotic cells contain membrane bound organelles. Some examples include mitochondria,

3672-442: The photosynthetic Ochrophyta or Heterokontophyta (>23,000 species), which contain chloroplasts originated from a red alga . Among these are many lineages of algae that encompass a wide range of structures and morphologies. The three most diverse ochrophyte classes are: the diatoms , unicellular or colonial organisms encased in silica cell walls ( frustules ) that exhibit widely different shapes and ornamentations, responsible for

3740-572: The phyla Cryptista and Haptista . The animals and fungi fall into the Amorphea supergroup, which contains the phylum Amoebozoa and several other protist lineages. Various groups of eukaryotes with primitive cell architecture are collectively known as the Excavata. Excavata is a group that encompasses diverse protists, mostly flagellates, ranging from aerobic and anaerobic predators to phototrophs and heterotrophs. The common name 'excavate' refers to

3808-456: The precursors to today's unicellular organisms. Although the origin of life is largely still a mystery, in the currently prevailing theory, known as the RNA world hypothesis , early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication. Compartmentalization was necessary for chemical reactions to be more likely as well as to differentiate reactions with

3876-539: The presence of cortical alveoli , cytoplasmic sacs underlying the cell membrane of unknown physiological function. Among them are three of the most well-known groups of protists: apicomplexans, dinoflagellates and ciliates. The ciliates ( Ciliophora ) are a highly diverse (>8,000 species) and probably the most thoroughly studied group of protists. They are mostly free-living microbes characterized by large cells covered in rows of cilia and containing two kinds of nuclei, micronucleus and macronucleus (e.g., Paramecium ,

3944-475: The remaining eukaryotes. 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. Eukaryotes emerged in evolution more than 1.5 billion years ago. The earliest eukaryotes were protists. Although sexual reproduction

4012-437: The reverse of one another, and the advent of respiration coupled with photosynthesis enabled much greater access to energy than fermentation alone. Protozoa are largely defined by their method of locomotion, including flagella , cilia , and pseudopodia . While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under

4080-473: The ribosomal RNA, it was found that the Archaea most likely split from bacteria and were the precursors to modern eukaryotes, and are actually more phylogenetically related to eukaryotes. As their name suggests, Archaea comes from a Greek word archaios, meaning original, ancient, or primitive. Some archaea inhabit the most biologically inhospitable environments on earth, and this is believed to in some ways mimic

4148-423: The same principles of physiology and biochemistry described for those cells within the "higher" eukaryotes (animals, fungi or plants): they are aerobic organisms that consume oxygen to produce energy through mitochondria , and those with chloroplasts perform carbon fixation through photosynthesis in chloroplasts . However, many have evolved a variety of unique physiological adaptations that do not appear in

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4216-562: The simplest multicellular organisms have cells that depend on each other to survive. Most multicellular organisms have a unicellular life-cycle stage. Gametes , for example, are reproductive unicells for multicellular organisms. Additionally, multicellularity appears to have evolved independently many times in the history of life. Some organisms are partially unicellular, like Dictyostelium discoideum . Additionally, unicellular organisms can be multinucleate , like Caulerpa , Plasmodium , and Myxogastria . Primitive protocells were

4284-479: The sister clade to Ochrophyta are the predominantly osmotrophic and filamentous Pseudofungi (>1,200 species), which include three distinct lineages: the parasitic oomycetes or water moulds (e.g., Phytophthora infestans , the agent behind the Irish Potato Famine ), which encompass most of the pseudofungi species; the less diverse non-parasitic hyphochytrids that maintain a fungus-like lifestyle; and

4352-717: The study of environmental DNA and is still in the process of being fully described. They are present in all ecosystems as important components of the biogeochemical cycles and trophic webs . They exist abundantly and ubiquitously in a variety of forms that evolved multiple times independently, such as free-living algae , amoebae and slime moulds , or as important parasites . Together, they compose an amount of biomass that doubles that of animals. They exhibit varied types of nutrition (such as phototrophy , phagotrophy or osmotrophy ), sometimes combining them (in mixotrophy ). They present unique adaptations not present in multicellular animals, fungi or land plants. The study of protists

4420-550: 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

4488-479: The world's oldest forms of life, and are found virtually everywhere in nature. Many common bacteria have plasmids , which are short, circular, self-replicating DNA molecules that are separate from the bacterial chromosome. Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance. Bacteria predominantly reproduce asexually through a process called binary fission . However, about 80 different species can undergo

4556-580: Was further supported by a 2011 study on amoebae . Amoebae have been regarded as asexual organisms , but the study describes evidence that most amoeboid lineages are ancestrally sexual, and that the majority of asexual groups likely arose recently and independently. Even in the early 20th century, some researchers interpreted phenomena related to chromidia ( chromatin granules free in the cytoplasm ) in amoebae as sexual reproduction. Some commonly found protist pathogens such as Toxoplasma gondii are capable of infecting and undergoing asexual reproduction in

4624-480: Was the last supergroup to be described, because it lacks any defining characteristic and was discovered exclusively through molecular phylogenetics . Three major clades are included, namely the phyla Cercozoa , Endomyxa and Retaria . Retaria contains the most familiar rhizarians: forams and radiolarians , two groups of large free-living marine amoebae with pseudopodia supported by microtubules , many of which are macroscopic. The radiolarians (Radiolaria) are

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