PROTOZOAN

(subkingdom Protozoa), any of a group of small, usually microscopic, unicellular, eukaryotic organisms of the kingdom Protista. Protozoans are found worldwide in most soils, freshwater, and oceans. Although some scientists consider protozoans to form a group only in that they exhibit certain common characteristics, most scientists now classify them as a subphylum of Protista. The taxonomic relationships of protozoans to one another and to other protists continues to change as genetic and biochemical testing establishes new relationships and as previous classifications based primarily on morphological and physiological features are revised. The smallest known protozoans are tiny blood parasites less than 2 microns long; the largest may be 16 mm (0.6 inch) in length and visible to the naked eye. Although protozoan shapes vary, all protozoans share such eukaryotic features as lipid-protein membranes and membrane-bound vacuoles and organelles. When oxygen is used at any stage in metabolism, mitochondria and associated ribosomes are present. All protozoans have at least one nucleus, and many are multinucleate. Some membrane-bound food vacuoles aid in the breakdown of food, and contractile vacuoles maintain the osmotic balance in the cell. Though mostly solitary, protozoans sometimes form colonies that are held together by a matrix. Some protozoans move with the aid of flagellalong, whiplike structures that move either back and forth or spirally. Some move with cilia, which resemble flagella in their infrastructure but are much smaller and more numerous; they are distributed in characteristic locations along the cell body, such as around the mouth (cytosome) or in measured rows called kineties. Cilia move in wavelike patterns that propel the protozoan forward, much like oars. There are many theories to explain how amoebas move, and this movement is largely dependent on the type of pseudopodium displayed by the species; but movement generally means the flow of cytoplasm outward from the cell as contractile pressures are exerted on the cytoplasm. Most protozoans use aerobic respirationi.e., the oxidation of glucose and water to carbon dioxide and water. Although rare, anaerobic processes do take place in certain protozoans, especially parasites. Some protozoans demonstrate purely heterotrophic, or animal-like, nutrition; other protozoans are autotrophs, depending principally or completely on photosynthesis for the manufacture of their foodstuffs. Chlorophyll and other pigments are available to autotrophs for photosynthesis. Other protozoans demonstrate both characteristics (mixotrophy), and they can alter their metabolism to accommodate changes in environment and available nutrients. The most common form of reproduction in protozoans is binary fission, which involves nuclear division, the duplication of organelles, and an approximately equal division of the body. Budding involves the unequal division of the cytoplasm, resulting in organisms of vastly different size. In schizogony, the nucleus undergoes a series of divisions followed by a rapid squeezing off of uninuclear buds. Plasmotomy occurs when a multinucleate organism separates into two or more organisms without undergoing nuclear division first. Protozoans also reproduce sexually. any member of the subkingdom Protozoa, a collection of single-celled eukaryotic (i.e., possessing a well-defined nucleus) organisms. As such, they are among the simplest of all living organisms. Although they comprise a subkingdom, protozoans are not necessarily related to one another. In biological terms, they are not a natural group but simply a collection of organisms. There are more than 65,000 described species, of which over half are fossil. Protozoa are ubiquitous in most soils and in aquatic habitats from the South to the North poles. Most are invisible to the naked eye. Many are symbionts of other organisms, and about one-third of the living species are parasites. The classification of protozoans requires regular revision because modern electron microscopy and new biochemical and genetic techniques provide an ever-increasing pool of knowledge about the relationships of various protistan species and groups, often showing previous assignments to be incorrect. Additional reading John J. Lee, Seymour H. Hutner, and Eugene C. Bovee (eds.), An Illustrated Guide to the Protozoa (1985), includes sections on the different groups, with many excellent illustrations. Michael A. Sleigh, Protozoa and Other Protists (1989), covers the features and physiology of the major groups, with a section on symbiosis and ecology, and is well illustrated. Coverage of the ecology and physiology of ciliates, sarcodines, and flagellates may be found in Johanna Laybourn-Parry, A Functional Biology of Free-Living Protozoa (1984); and Tom Fenchel, The Ecology of Protozoa (1987). Richard R. Kudo, Protozoology, 5th ed. (1966, reissued 1971), although slightly out-of-date, is nonetheless a useful book covering the major groups in detail. Michael Levandowsky and Seymour H. Hutner (eds.), Biochemistry and Physiology of Protozoa, 2nd ed., 4 vol. (197981), contains articles on all aspects of protozoan physiology and biochemistry, including structure, locomotion, metabolism, mating behaviour, and immunology of parasitic protozoan diseases. J.P. Kreier and J.R. Baker, Parasitic Protozoa (1987), is an easy-to-read overview covering physiology, biochemistry, and life cycles. R.S. Phillips, Malaria (1983), a small, easy-to-read book, discusses pathology, immunology, and control methods. John R. Haynes, Foraminifera (1981), covers the fossil record and the importance of these sarcodines, with many illustrations. O. Roger Anderson, Radiolaria (1983), covers the biology of this marine group. F.J.R. Taylor (ed.), The Biology of Dinoflagellates (1987), contains articles on the physiology and other aspects of these flagellates. Lynda J. Goff (ed.), Algal Symbiosis (1983), contains articles on endosymbionts of some protozoans and their role. John O. Corliss, The Ciliated Protozoa: Characterization, Classification, and Guide to the Literature, 2nd ed. (1979), is a specialized text providing further references. Lynn Margulis, Symbiosis and Cell Evolution: Life and Its Environment on the Early Earth (1981), is an interesting text on the evolution of cellular structures. Johanna E.M. Laybourn-Parry Classification General principles Until the 1970s the general view was that the protozoans were animals, and as such they were placed in the animal kingdom as the phylum Protozoa. Under this system of classification, zoologists placed the coloured flagellates in the phylum Protozoa despite their obvious plant affinities. Botanists, on the other hand, classified the same organisms as algae, which were regarded as plants. Protozoans are now regarded as a phylum or subkingdom of the kingdom Protista (sometimes called the Protoctista), which also includes the algae. The Society of Protozoologists, which periodically reviews the systematics of the group, favours the subkingdom level for the Protozoa. Protozoan systematics remains a subject of debate and change. Protozoans comprise a large, unwieldy assemblage, and assignments of species to particular taxa change as new biochemical techniques and electron microscopy studies provide more details on the affinities of various species. The subkingdom Protozoa essentially represents a level of organizationthat is, single-celled eukaryote organismsand does not necessarily indicate single major branches of an evolutionary tree comparable with the kingdoms Plantae or Animalia. Thus, while affinities between some of the groups exist, this is not the case with all the phyla within the subkingdom. Moreover, certain groups (e.g., the Sarcodina or amoebas) are considered by some researchers to have had many different ancestors and lineages (polyphyletic). The Labyrinthomorpha, Myxozoa, and Microspora have no clear affinities either with one another or with other protozoan groups; they are placed in Protozoa until more information is available. Diagnostic features The subkingdom Protozoa has been reviewed and divided by a committee of the Society of Protozoologists into six phyla. A major review was undertaken in 1980 by a committee headed by N.D. Levine, and a series of changes have been made subsequently as outlined in a major work on protozoan systematics published by the society in 1985. The six phyla include the Sarcomastigophora (flagellates and amoebas), the Ciliophora (ciliates), and the entirely parasitic Apicomplexa, all of which are discussed in this article. At least some of these groups show affinities in having common amoeboid or flagellate ancestors early in their evolutionary history. The remaining three phyla are totally unrelated to one another and to the phyla discussed here. In this assemblage of organisms, the only common feature is a single-celled level of organization. Such a situation invariably means that within the whole group there are considerable differences in structure physiology, life mode, and life cycles. In addition, the classification of protists generally continues to be debated, and a standard outline of the kingdom has not been established. The differences between the classification of protozoa given below and that given in the article protist reflect taxonomic variations that arise from individual interpretations.