A protist () is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant or fungus. The protists do not form a natural group, or clade, since they exclude certain eukaryotes; but, like algae or , they are often grouped together for convenience. In some systems of biological classification, such as the popular five-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues".
Besides their relatively simple levels of organization, protists do not necessarily have much in common. When used, the term "protists" is now considered to mean a paraphyletic assemblage of similar-appearing but diverse taxa (biological groups); these taxa do not have an exclusive common ancestor beyond being composed of eukaryotes and have different life cycles, , motility and cellular structures.
In cladistic systems (classifications based on common ancestry), there are no equivalents to the taxa Protista or Protoctista, both terms referring to a paraphyletic group that spans the entire eukaryotic tree of life. In cladistic classification, the contents of Protista are distributed among various supergroups (SAR supergroup, such as protozoa and some algae, Archaeplastida, such as land plants and some algae, Excavata, which are a group of unicellular organisms, and Opisthokonta, such as animals and fungi, etc.). "Protista", ''Protoctista'' and "Protozoa" are considered obsolete. However, the term "protist" continues to be used informally as a catch-all term for unicellular eukaryotic microorganisms. For example, the word "protist pathogen" may be used to denote any disease-causing microbe that is not bacteria, virus, viroid, prion, or metazoa.
Some protists, sometimes called ambiregnal protists, have been considered to be both protozoa and algae or fungi (e.g., slime molds and algae), and names for these have been published under either or both of the ICN and the ICZN.Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell. p. 41. . Conflicts, such as these – for example the dual-classification of Euglenids and , which are mixotrophic – is an example of why the kingdom Protista was adopted.
These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics ( relatedness among organisms). Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related phylum. For instance, the water molds are now considered to be closely related to photosynthetic organisms such as Brown algae and Diatoms, the are grouped mainly under Amoebozoa, and the Amoebozoa itself includes only a subset of "Amoeba" group, and significant number of erstwhile "Amoeboid" genera are distributed among Rhizarians and other Phyla.
However, the older terms are still used as informal names to describe the morphology and ecology of various protists. For example, the term protozoa is used to refer to species of protists that do not form filaments.
In 1938, Herbert Copeland resurrected Hogg's label, arguing that Haeckel's term Protista included anucleated microbes such as bacteria, which the term "Protoctista" (literally meaning "first established beings") did not. In contrast, Copeland's term included nucleated such as , green algae and fungi. This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life. The kingdom Protista was later modified to separate into the separate kingdom of Monera, leaving the protists as a group of eukaryotic microorganisms. These five kingdoms remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that neither protists nor monera were single groups of related organisms (they were not Clade).
The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular, that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues (but vegetative cell differentiation may occur restricted to sexual reproduction, alternate vegetative morphology, and quiescent or resistant stages, such as cysts); this definition excludes many brown algae, multicellular red algae and green algae, which may have tissues.
The taxonomy of protists is still changing. Newer classifications attempt to present monophyly groups based on morphological (especially ultrastructure),Pitelka, D. R. (1963). Electron-Microscopic Structure of Protozoa. Pergamon Press, Oxford.Berner, T. (1993). Ultrastructure of Microalgae. Boca Raton: CRC Press. Beckett, A., Heath, I. B., and Mclaughlin, D. J. (1974). An Atlas of Fungal Ultrastructure. Longman, Green, New York. biochemistry (chemotaxonomy)Ragan M.A. & Chapman D.J. (1978). A Biochemical Phylogeny of the Protists. London, New York: Academic Press. Lewin R. A. (1974). "Biochemical taxonomy", pp. 1–39 in Algal Physiology and Biochemistry, Stewart W. D. P. (ed.). Blackwell Scientific Publications, Oxford. and DNA sequence (molecular research) information.Oren, A., & Papke, R. T. (2010). Molecular phylogeny of microorganisms. Norfolk, UK: Caister Academic Press. Horner, D. S., & Hirt, R. P. (2004). "An overview on eukaryote origins and evolution: the beauty of the cell and the fabulous gene phylogenies", pp. 1–26 in Hirt, R.P. & D.S. Horner. Organelles, Genomes and Eukaryote Phylogeny, An Evolutionary Synthesis in the Age of Genomics. New York: CRC Press. However, there are sometimes discordances between molecular and morphological investigations; these can be categorized as two types: (i) one morphology, multiple lineages (e.g. morphological convergence, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages).
Because the protists as a whole are paraphyletic, new systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al. (2005) does not recognize formal ranks (phylum, class, etc.) and instead treats groups as clades of phylogenetically related organisms. This is intended to make the classification more stable in the long term and easier to update. Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox, upper right. Many are thought to be monophyletic, though there is still uncertainty. For instance, the Excavata are probably not monophyletic and the are probably only monophyletic if the and are excluded.
Many protists are flagellate, for example, and filter feeding can take place where flagellates find prey. Other protists can engulf bacteria and other food particles, by extending their cell membrane around them to form a vacuole and digesting them internally in a process termed phagocytosis.
|+ Nutritional types in protist metabolism|
|Sunlight||Organic compounds or carbon fixation||Most algae|
|Organic compounds||Organic compounds||Apicomplexa, Trypanosomes or Amoeboid|
For most important cellular structures and functions of animal and plants, it can be found a heritage among protists.Plattner, H. (2018). Evolutionary cell biology of proteins from protists to humans and plants. J. Eukaryot. Microbiol. 65(2): 255-289.
Some species, for example Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually. However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protists may be clonal lines that rarely exchange genes with other members of their species.
emerged in evolution more than 1.5 billion years ago. The earliest eukaryotes were likely protists. Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that sex appeared to be lacking in certain protists whose ancestors branched off early from the eukaryotic family tree. However, several of these protists are now known to be capable of, or to recently have had the capability for, meiosis and hence sexual reproduction. For example, the common intestinal parasite Giardia lamblia was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, G. lamblia was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes. These results suggested that G. lamblia is capable of meiosis and thus sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of sex, was also found in G. lamblia.
Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al. tested for 29 genes that function in meiosis, they found 27 to be present, including 8 of 9 genes specific to meiosis in model eukaryotes. These findings suggest that T. vaginalis may be capable of meiosis. Since 21 of the 29 meiotic genes were also present in G. lamblia, it appears that most of these meiotic genes were likely present in a common ancestor of T. vaginalis and G. lamblia. These two species are descendants of protist lineages that are highly divergent among eukaryotes, leading Malik et al. to suggest that these meiotic genes were likely present in a common ancestor of all eukaryotes.
This view was further supported by a study of amoebae by Lahr et al. Amoeba have generally been regarded as asexual protists. However, these authors describe evidence that most amoeboid lineages are anciently sexual, and that the majority of asexual groups likely arose recently and independently. Early researchers (e.g., Calkins) have interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoeboid organisms as sexual reproduction.
Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.Bernstein H, Bernstein C, Michod RE (2012). "DNA repair as the primary adaptive function of sex in bacteria and eukaryotes". Chapter 1: pp. 1–49 in DNA Repair: New Research, Sakura Kimura and Sora Shimizu (eds.). Nova Sci. Publ., Hauppauge, N.Y. Oxidative stress, which is associated with the production of reactive oxygen species leading to DNA damage, also appears to be an important factor in the induction of sex in protists.
Some commonly found Protist pathogens such as Toxoplasma gondii are capable of infecting and undergoing asexual reproduction in a wide variety of animals – which act as secondary or intermediate host – but can undergo sexual reproduction only in the primary or definitive host (for example: such as in this case).
Recent papers have proposed the use of viruses to treat infections caused by protozoa.
Researchers from the Agricultural Research Service are taking advantage of protists as pathogens to control red imported fire ant ( Solenopsis invicta) populations in Argentina. Spore-producing protists such as Kneallhazia solenopsae (recognized as a sister clade or the closest relative to the fungus kingdom now) can reduce red fire ant populations by 53–100%. Researchers have also been able to infect Phoridae fly of the ant with the protist without harming the flies. This turns the flies into a vector that can spread the pathogenic protist between red fire ant colonies.Durham, Sharon (January 28, 2010) ARS Parasite Collections Assist Research and Diagnoses. Ars.usda.gov. Retrieved 2014-03-20.
Others are relatively common in the fossil record, Why Is The Museum On The Web?. Ucmp.berkeley.edu. Retrieved 2014-03-20. as the , Fossil Record of Diatoms. Ucmp.berkeley.edu. Retrieved 2014-03-20. golden algae, Introduction to the Chrysophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. (coccoliths), Introduction to the Prymnesiophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. , (ciliates), , Fossil Record of the Dinoflagellata. Ucmp.berkeley.edu. Retrieved 2014-03-20. green algae, Systematics of the "Green Algae", Part 1. Ucmp.berkeley.edu. Retrieved 2014-03-20. red algae, Fossil Record of the Rhodophyta. Ucmp.berkeley.edu. Retrieved 2014-03-20. , , Fossil Record of the Radiolaria. Ucmp.berkeley.edu. Retrieved 2014-03-20. , Fossil Record of Foraminifera. Ucmp.berkeley.edu. Retrieved 2014-03-20. and testate amoebae (, ). Introduction to the Testaceafilosea. Ucmp.berkeley.edu. Retrieved 2014-03-20. Some are even used as paleoecological indicators to reconstruct ancient environments.
More probable eukaryote fossils begin to appear at about 1.8 billion years ago, the , spherical fossils of likely algal protists. Fossil Record of the Eukaryota. Ucmp.berkeley.edu. Retrieved 2014-03-20. Another possible representant of early fossil eukaryotes are the Gabonionta.