any member of the kingdom Protista, a group of eukaryotic, predominantly unicellular microscopic organisms. They may share certain morphological and physiological characteristics with animals or plants, or both. The protists comprise what have traditionally been called protozoa, algae, and lower fungi. From the time of Aristotle, near the end of the 4th century BC, until well after the middle of the 20th century, the entire biotic world was generally considered divisible into just two great kingdoms, the plants and the animals. The separation was based on the assumption that plants are pigmented (basically green), nonmotile (most commonly from being rooted in the soil), photosynthetic and therefore capable solely of self-contained (autotrophic) nutrition, and unique in possessing cellulosic walls around their cells. By contrast, animals are without photosynthetic pigments (colourless), actively motile, nutritionally phagotrophic (and therefore required to capture or absorb important nutrients), and without walls around their cells. When microscopy arose as a science in its own right, botanists and zoologists discovered evidence of the vast diversity of life mostly invisible to the unaided eye. With rare exception, authorities of the time classified such microscopic forms as minute plants (called algae) and minute animals (called first animals, or protozoa). Such taxonomic assignments went essentially unchallenged for many years, despite the fact that the great majority of these minute forms of lifenot to mention certain macroscopic ones, various parasitic forms, and the entire group known as the fungidid not possess the cardinal characteristics on which the plants and animals had been differentiated and thus had to be forced to fit into those kingdom categories. An authority who took exception to the imposition of the plant and animal categories on the protists was the German zoologist Ernst Haeckel. In 1866 he proposed a third kingdom, the Protista, to embrace such lower organisms, but his conception failed to gain widespread support during his lifetime. Some 8090 years later, Herbert F. Copeland, an American botanist, attempted a revival of the protist concept, but again without much success. The basis for a major change in the systematics of these lower forms came through an advancement in the concept of the composition of the biotic world. About 1960, resurrecting and embellishing an idea originally conceived 20 years earlier by the French marine biologist Edouard Chatton but universally overlooked, R.Y. Stanier, C.B. Van Niel, and their colleagues formally proposed the division of all living things into two great groups, the prokaryotes and the eukaryotes. (Prokaryotesbacteria and other Moneraare unicellular organisms that differ from eukaryotes in nuclear and morphological characteristics and are typically of much smaller size.) This organization was based on characteristicssuch as the presence or absence of a true nucleus, the simplicity or complexity of the DNA (deoxyribonucleic acid) molecules constituting the chromosomes, and the presence or absence of intracellular membranes (and of specialized organelles apart from ribosomes) in the cytoplasmthat revealed a long phylogenetic separation of the two assemblages. The concept of protists originally embraced all the microorganisms in the biotic world. The entire assemblage thus included the protists as defined below plus the bacteria, the latter considered at that time to be lower protists. The great evolutionary boundary between the prokaryotes and the eukaryotes, however, has meant a major taxonomic boundary restricting the protists to eukaryotic microorganisms (but occasionally including relatively macroscopic organisms) and the bacteria to prokaryotic microorganisms. During the 1970s and '80s, attention was redirected to the problem of possible high-level systematic subdivisions within the eukaryotes. The American biologists R.H. Whittaker and Lynn Margulis, as well as others, became involved in such challenging questions. A major outcome was widespread support among botanists and zoologists for considering living organisms as constituting five separate kingdoms, four of which are placed in what may be thought of as the superkingdom Eukaryota (Protista, Plantae, Animalia, and Fungi); the fifth kingdom, Monera, constitutes the superkingdom Prokaryota. This article discusses the kingdom Protista in general terms. For discussion of the differences and similarities among the four kingdoms of the superkingdom Eukaryota, as well as the Prokaryota, see taxonomy. For a generally more detailed treatment of the members of the Protista, see protozoa and algae. (kingdom Protista; sometimes called Protoctista), any member of a kingdom of eukaryotic, almost exclusively unicellular organisms, comprising the algae, protozoans, and lower fungi. The concept of the protists was developed to accommodate intermediate organisms, primarily the single-celled-animal-like forms and plantlike forms (protozoans and algae, respectively). The term was first suggested in 1866 by the German zoologist Ernst Haeckel. Protists are predominantly unicellular, although coenocytic, colonial, and single-tissue forms exist. They exhibit all forms of nutrition as well as sexual and asexual reproduction. They are frequently motile, primarily using flagella, cilia, or pseudopodia. Many protists have affinities with both plants and animals, which created problems of placing them somewhat arbitrarily into one or the other of the traditional kingdoms. Furthermore, many previously established relationships were called into question around the mid-20th century with the advent of biochemical and genetic testing. It is postulated that some groups are not as closely related to one another as once thought. As a result, the arrangement or classification of protists, while of working convenience, is regarded as not entirely satisfactory because of the taxonomic variations that arise from individual interpretations. Additional reading Most modern works on protists are to be found in specialized biological journals, and very few books have appeared that are concerned with protists overall. To begin to understand the protist perspective, the following publications are especially recommended for perusal: Herbert F. Copeland, The Classification of Lower Organisms (1956); John O. Corliss, Progress in Protistology During the First Decade Following Reemergence of the Field as a Respectable Interdisciplinary Area in Modern Biological Research, Progress in Protistology, 1:1164 (1986); Lynn Margulis et al. (eds.), Handbook of Protoctista: The Structure, Cultivation, Habitats, and Life Histories of the Eukaryotic Microorganisms and Their Descendants Exclusive of Animals, Plants, and Fungi (1990); Mark A. Ragan and David J. Chapman, A Biochemical Phylogeny of the Protists (1978); Michael A. Sleigh, Protozoa and Other Protists (1989); F.J.R. Taylor, Problems in the Development of an Explicit Hypothetical Phylogeny of the Lower Eukaryotes, BioSystems, 10(1/2):6789 (1978); and R.H. Whittaker and Lynn Margulis, Protist Classification and the Kingdoms of Organisms, BioSystems, 10(1/2):318 (1978).Works on more specific topics include G.C. Ainsworth, F.K. Sparrow, and A.S. Sussman (eds.), The Fungi: An Advanced Treatise, 4 vol. (196573), especially vol. 4; Constantine J. Alexopoulos and Charles W. Mims, Introductory Mycology, 3rd ed. (1979); O. Roger Anderson, Comparative Protozoology: Ecology, Physiology, Life History (1988); R.S.K. Barnes (ed.), A Synoptic Classification of Living Organisms (1984); Harold C. Bold and Michael J. Wynne, Introduction to the Algae: Structure and Reproduction, 2nd ed. (1985); T. Cavalier-Smith, The Kingdom Chromista: Origin and Systematics, Progress in Phycological Research, 4:309348 (1987); John O. Corliss, A Puddle of Protists: There's More to Life than Animals and Plants, The Sciences, 23(3):3439 (May/June 1983), The Kingdom Protista and Its 45 Phyla, BioSystems, 17(2):87126 (1984), Protista Phylogeny and Eukaryogenesis, International Review of Cytology, 100:319370 (1987), and The Protozoon and the Cell: A Brief Twentieth-Century Overview, Journal of the History of Biology, 22(2):307324 (1989); Tom Fenchel, Ecology of Protozoa: The Biology of Free-living Phagotrophic Protists (1987); Joseph G. Gall (ed.), The Molecular Biology of Ciliated Protozoa (1986); J. Grain, The Cytoskeleton in Protists: Nature, Structure, and Functions, International Review of Cytology, 104:153185 (1986); J.C. Green, B.S.C. Leadbeater, and W.L. Diver (eds.), The Chromophyte Algae: Problems and Perspectives (1989); A.M. Johnson, Phylogeny and Evolution of Protozoa, Zoological Science, 7(Suppl.):179188 (1990); Bryce Kendrick, The Fifth Kingdom (1985); J.P. Kreier and J.R. Baker, Parasitic Protozoa (1987); Jrgen Kristiansen and Robert A. Andersen (eds.), Chrysophytes: Aspects and Problems (1986); Johanna Laybourn-Parry, A Functional Biology of Free-living Protozoa (1984); John J. Lee, Seymour H. Hutner, and Eugene C. Bovee (eds.), An Illustrated Guide to the Protozoa (1985); N.D. Levine et al., A Newly Revised Classification of the Protozoa, The Journal of Protozoology, 27(1):3758 (1980); D.H. Lynn and E.B. Small, An Update on the Systematics of the Phylum Ciliophora Doflein, 1901: The Implications of Kinetic Diversity, BioSystems, 21(3/4):317322 (1988); Lynn Margulis, The Classification and Evolution of Prokaryotes and Eukaryotes, in Robert C. King (ed.), Handbook of Genetics, vol. 1, Bacteria, Bacteriophages, and Fungi (1974), pp. 141; Lynn Margulis and Karlene V. Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 2nd ed. (1988); Lindsay S. Olive, The Mycetozoans (1975); Sybil P. Parker (ed.), Synopsis and Classification of Living Organisms, 2 vol. (1982); D.J. Patterson, J. Larsen, and John O. Corliss, The Ecology of Heterotrophic Flagellates and Ciliates Living in Marine Sediments, Progress in Protistology, 3:185277 (1989); Hayden N. Pritchard and Patricia T. Bradt, Biology of Nonvascular Plants (1984); L.J. Rothschild and P. Heywood, Protistan Phylogeny and Chloroplast Evolution: Conflicts and Congruence, Progress in Protistology, 2:168 (1987); John McNeill Sieburth, Sea Microbes (1979); Mitchell L. Sogin et al., Phylogenetic Meaning of the Kingdom Concept: An Unusual Ribosomal RNA from Giardia lamblia, Science, 243(4887):7577 (Jan. 6, 1989); Helen Tappan, The Paleobiology of Plant Protists (1980); and C.R. Woese, Bacterial Evolution, Microbiological Reviews, 51(2): 221271 (1987). John O. Corliss Classification Macrosystems of protist classification There are essentially three broad options with respect to treating protists within classification systems that embrace all living things. One is to recognize that a single kingdom, Protista, is evolutionarily and taxonomically justifiable, as is done in this article. Protists, by virtue of sharing many common characteristics, do seem to manifest an overall taxonomic unity or integrity of their own. Yet, if this approach is taken, a series of major problems remains: what is an acceptable definition of such an assemblage; exactly what does it include (i.e., what are its boundaries); and what are the phylogenetic interrelationships of the high-level subgroups specifically included within it? A popular alternative among evolutionary biologists is to consider the protists as only a structural grade of organization, a temporary state of transition in the evolution of the higher eukaryotic kingdoms from a prokaryote ancestry. While this view has appeal, it leaves confusion in its wake: if the protists belong to distinct taxonomic units at lower levels in the classificational hierarchy, then what phyla or kingdoms are to be identified for them at the top levels in the macrosystem? The fact that certain protists served as evolutionary gap-bridgers in eukaryogenesis and that others have played an ancestor-descendant role in the origin of plants, animals, and fungi by itself does not forbid the recognition of separate taxonomic distinctiveness for the protists as a group. Furthermore, many present-day protist taxa do not appear to have led anywhere evolutionarily. The last of the three options proposes that there are more than four eukaryotic kingdoms and that the protists are scattered throughout them, sometimes sharing a particular kingdom with some plant, fungal, or animal groups. In this option, there is generally no specific kingdom bearing the name (or concept) Protista. For example, in the late 1980s the biochemical cytologist Tom Cavalier-Smith argued, based on his interpretation of a number of facts mostly ultrastructural in nature, that within the Eukaryota there are six kingdoms: Archezoa, Protozoa, Chromista, Plantae, Fungi, and Animalia. The organisms treated as protists in this article appear in all his kingdoms except the Animalia, although only a few are in his Fungi. The huge and diverse group of heterokonts (mostly algal protists in this article) comprise the bulk of his Chromista; all the red and green algae are placed in his kingdom Plantae. Admittedly, the green algae, especially, are closely related to plants and are likely their direct progenitor group. Cavalier-Smith's kingdom Protozoa includes the typical nonpigmented, motile, heterotrophic protists long claimed by protozoologists, but not all such protozoa are included in his kingdom bearing that name. (For example, some are distributed among several other eukaryotic kingdoms, including what he has called Chromista and, especially, Archezoa, the latter containing groups considered in this article as the phyla Metamonadea, Karyoblastea, and Microsporidia.) A scheme of classification is an effort to set up discrete units containing a great diversity of living organisms that have been evolving gradually over hundreds of millions of years, an evolution that does not necessarily show taxonomically convenient breaks in the succession of forms. The challenge is to recognize major lines of evolution within the diverse assemblage and to organize them into named groups and ranks with minimal violation of their probable phylogenetic interrelationships. The single greatest handicap to the successful production of an ideal macrosystem for the protists is the scarcity of unambiguous data about the comparative morphology, biochemistry, and molecular biology of practically any taxon of these lower eukaryotes above the level of genus or species. Problems arise when the same group or part of a particular taxon of organisms has been treated quite differently systematically at the higher levels by workers of different scientific backgrounds or training. Application of a protist perspective, taxonomically mixing algal, protozoan, and fungal groups to the degree required by their phylogenetic interrelationships, would mean the dropping of such groups and their formal nomenclatural designations as Protozoa, Algae, Phytomastigophora, Zoomastigophora, Sarcomastigophora, and the like. The phyla and the classes listed in the following working high-level classification of the kingdom Protista are themselves grouped into sections, supraphyletic assemblages given only vernacular names because they do not have an official nomenclatural rank. This is done in order to indicate, in a general way, the supposed phylogenetic closeness of some protist taxa to others. Section I, for example, contains a dozen phyla sharing basic characteristics while also showing major differences that allow them to remain separate at the high level of phylum. It may be noted that one of these phyla has been claimed taxonomically as fungi in the past; three as protozoa only; four as algae only; and four, wholly or partially, as bothsimultaneouslyprotozoa and algae. Only one section is composed solely of algae (the one containing only the unique rhodophytes); seven, all with nonpigmented members, are purely protozoan in nature; four contain mixtures of algal and protozoan phyla; and one contains protozoan and fungal groups (as indicated by their former classifications). It is this commingling of phyla formerly assigned to widely separated assemblages of organisms that makes impossible any recognitionat a formal taxonomic levelof distinct and discrete protozoan protists, algal protists, or fungal protists. The order or the arrangement of the 16 sections below has no particular phylogenetic significance; in fact, a number of biologists today consider the most primitive protists to be members of Sections IX and X. Neighbouring sections may sometimes be closer phylogenetically than more distant ones, but not always (particularly in view of the vast ignorance of most intersectional affinities). In some publications, dinoflagellates and ciliates are postulated as being rather closely related; but, partly in an attempt to keep (former) algal groups close together, the dinoflagellates, in the scheme below, are in Section VI, while the ciliates form Section XVI. Diagnostic characterization Eukaryotic organisms possessing, at most, one tissuetissue being an aggregation of similar cells and their products forming a definite, specialized kind of structural materialprotistan species are predominantly unicellular in organization and microscopic in size. The relatively few syncytial (coenocytic), coenobial, or multicellular forms, which generally appear as filaments, colonies, coenobia, or thalli, still do not exhibit a true multitissue organization in the active (vegetative) stage. Macroscopic sizes are attained by species of a few groups (notably the brown algae). There are no truly vascular protists. All eukaryotic modes of nutrition are shown by the kingdom, with both phototrophic and heterotrophic types being common. Cysts or spores occur widely. Motility is frequently exhibited, principally via flagella, cilia, or pseudopodia; in general, motility in at least one stage of the life cycle is more common among the protists than are completely nonmotile forms. Both intracellular and extracellular elaborations (such as the organelles and the skeleton) show considerable complexity in protists. The diversity that exists among the numerous characteristics of the group supports the hypothesis that protists were ancestral to the other three eukaryotic kingdoms. For example, the distribution of the protists is ubiquitous and cosmopolitan; they show all modes of nutrition, and some species may exhibit only aerobic respiration and others only anaerobic respiration; in aerobic groups, the mitochondrial cristae are tubular, vesicular, lamellar (flattened), or discoidal; and mitotic and meiotic mechanisms and types are diverse. The total number of acceptably described species, extinct and extant, may be estimated to reach at least 120,000, with another 80,000 (mostly fossil forms) on record but of questionable validity.
Meaning of PROTIST in English
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