protozoan

View more Animal Life videos

(click to enlarge)
Representative protozoans. The phytoflagellate Gonyaulax is one of the dinoflagellates (credit: © Merriam-Webster Inc.)

Any of a group of small (usually microscopic) single-celled protists that are nonfilamentous and heterotrophic (using organic carbon as a source of energy). They are found in most soils, fresh water, and oceans. While most are solitary individuals, various colonial forms exist. The taxonomic relationships of protozoans to one another and to other protists continue to be revised. The smallest known protozoans are tiny blood parasites less than 2 micrometres long; the largest may be 16 mm long and visible to the naked eye. Protozoan shapes vary, but all share such eukaryotic features as lipid-protein membranes and membrane-enclosed vacuoles and organelles ( eukaryote). They show wide variation in modes of movement, nutrition, and reproduction. Various classification systems exist to group the protozoans. Commonly known protozoans include dinoflagellates, amoebas, and paramecia ( paramecium).

For more information on protozoan, visit Britannica.com.

A group of eukaryotic microorganisms traditionally classified in the animal kingdom. Although the name signifies primitive animals, some Protozoa (phytoflagellates and slime molds) show enough plantlike characteristics to justify claims that they are plants.

Protozoa are almost as widely distributed as bacteria. Free-living types occur in soil, wet sand, and in fresh, brackish, and salt waters. Protozoa of the soil and sand live in films of moisture on the particles. Habitats of endoparasites vary. Some are intracellular, such as malarial parasites in vertebrates, which are typical Coccidia in most of the cycle. Other parasites, such as Entamoeba histolytica, invade tissues but not individual cells. Most trypanosomes live in the blood plasma of vertebrate hosts. Many other parasites live in the lumen of the digestive tract or sometimes in coelomic cavities of invertebrates, as do certain gregarines. See also Coccidia; Gregarinia; Trypanosomatidae.

Many Protozoa are uninucleate, others are binucleate or multinucleate, and the number of nuclei also may vary at different stages in a life cycle. Protozoa range in size from 1 to 10⁶ micrometers. Colonies are known in flagellates, ciliates, and Sarcodina. Although marked differentiation of the reproductive and somatic zooids characterizes certain colonies, such as Volvox, Protozoa have not developed tissues and organs.

Morphology

A protozoan may be a plastic organism (ameboid type), but changes in form are often restricted by the pellicle. A protective layer is often secreted outside the pellicle, although the pellicle itself may be strengthened by incorporation of minerals. Secreted coverings may fit closely, for example, the cellulose-containing theca of Phytomonadida and Dinoflagellida, analogous to the cell wall in higher plants. The dinoflagellate theca (Fig. 1a) may be composed of plates arranged in a specific pattern. Tests, as seen in Rhizopodea (Arcellinida, Gromiida, Foraminiferida), may be composed mostly of inorganic material, although organic (chitinous) tests occur in certain species. Siliceous skeletons, often elaborate, characterize the Radiolaria (Fig. 1d). A vase-shaped lorica, from which the anterior part of the organism or its appendages may be extended, occurs in certain flagellates (Fig. 1b) and ciliates (Fig. 1c). Certain marine ciliates (Tintinnida) are actively swimming loricate forms.

dinoflagellate (Peridinium), showing separate plates. (b) Lorica of a colonial chrysomonad, Dinobryon. (c) Two zooids within a lorica of a peritrich, Cothurnia. (d) A radiolarian skeleton, siliceous type. (After L. H. Hyman, The Invertebrates, vol. 1, McGraw-Hill, 1940)">
External coverings of Protozoa. (a) Theca of dinoflagellate (Peridinium), showing separate plates. (b) Lorica of a colonial chrysomonad, Dinobryon. (c) Two zooids within a lorica of a peritrich, Cothurnia. (d) A radiolarian skeleton, siliceous type. (After L. H. Hyman, The Invertebrates, vol. 1, McGraw-Hill, 1940)

Flagella occur in active stages of Mastigophora and flagellated stages of certain Sarcodina and Sporozoa. A flagellum consists of a sheath enclosing a matrix in which an axoneme extends from the cytoplasm to the flagellar tip. In certain groups the sheath shows lateral fibrils (mastigonemes) which increase the surface area and also may modify direction of the thrust effecting locomotion. Although typically shorter than flagella, cilia are similar in structure. See also Cilia and flagella.

Two major types of pseudopodia have been described, the contraction-hydraulic and the two-way flow types. The first are lobopodia with rounded tips and ectoplasm denser than endoplasm. The larger ones commonly contain granular endoplasm and clear ectoplasm. Two-way flow pseudopodia include reticulopodia of Foraminiferida and related types, filoreticulopodia of Radiolaria, and axopodia of certain Heliozoia.

In addition to nuclei, food vacuoles (gastrioles) in phagotrophs, chromatophores and stigma in many phytoflagellates, water-elimination vesicles in many Protozoa, and sometimes other organelles, the cytoplasm may contain mitochondria, Golgi material, pinocytotic vacuoles, stored food materials, endoplasmic reticulum, and sometimes pigments of various kinds.

Nutrition

In protozoan feeding, either phagotrophic (holozoic) or saprozoic (osmotrophic) methods predominate in particular species. In addition, chlorophyll-bearing flagellates profit from photosynthesis; in fact, certain species have not been grown in darkness and may be obligate phototrophs.

Phagotrophic ingestion of food, followed by digestion in vacuoles, is characteristic of Sarcodina, ciliates, and many flagellates. Digestion follows synthesis of appropriate enzymes and their transportation to the food vacuole. Details of ingestion vary. Formation of food cups, or gulletlike invaginations to enclose prey, is common in more or less ameboid organisms, such as various Sarcodina, many flagellates, and at least a few Sporozoa. Entrapment in a sticky reticulopodial net occurs in Foraminiferida and certain other Sarcodina. A persistent cytostome and gullet are involved in phagotrophic ciliates and a few flagellates. Many ciliates have buccal organelles (membranes, membranelies, and closely set rows of cilia) arranged to drive particles to the cytostome. Particles pass through the cytostome into the cytopharynx (gullet), at the base of which food vacuoles (gastrioles) are formed. Digestion occurs in such vacuoles.

By definition saprozoic feeding involves passage of dissolved foods through the cortex. It is uncertain to what extent diffusion is responsible, but enzymatic activities presumably are involved in uptake of various simple sugars, acetate and butyrate. In addition, external factors, for example, the pH of the medium, may strongly influence uptake of fatty acids and phosphates.

Reproduction

Reproduction occurs after a period of growth which ranges, in different species, from less than half a day to several months (certain Foraminiferida). General methods include binary fission, budding, plasmotomy, and schizogony. Fission, involving nuclear division and replication of organelles, yields two organisms similar in size. Budding produces two organisms, one smaller than the other. In plasmotomy, a multinucleate organism divides into several, each containing a number of nuclei. Schizogony, characteristic of Sporozoa, follows repeated nuclear division, yielding many uninucleate buds.

Simple life cycles include a cyst and an active (trophic) stage undergoing growth and reproduction. In certain free-living and parasitic species, no cyst is developed. Dimorphic cycles show two active stages; polymorphic show several. The former include adult and larva (Suctoria); flagellate and ameba (certain Mastigophora and Sarcodina); flagellate and palmella (nonflagellated; certain Phytomonadida); and ameba and plasmodium (Mycetozoia especially).

Parasitic protozoa

Parasites occur in all major groups. Sporozoa are exclusively parasitic, as are some flagellate orders (Trichomonadida, Hypermastigida, and Oxymonadida), the Opalinata, Piroplasmea, and several ciliate orders (Apostomatida, Astomatida, and Entodiniomorphida). Various other groups contain both parasitic and free-living types. Protozoa also serve as hosts of other protozoa, certain bacteria, fungi, and algae.

Relatively few parasites are distinctly pathogenic, causing amebiasis, visceral leishmaniasis (kala azar), sleeping sickness, Chagas' disease, malaria, tick fever of cattle, dourine of horses, and other diseases. See also Ciliophora; Cnidospora; Malaria; Sarcomastigophora; Sporozoa.

Single-celled animals, such as amoebas, that are the most primitive form of animal life. In modern biology, they are classified in the kingdom of Protoctista rather than in the animal kingdom. (See Linnean classification.)

Pl. protozoa [Gr.] any member of the Protozoa.

• Heterotrophs - protozoans: animallike, eukaryotic organisms of kingdom Protista, primarily unicellular, colonial, and nonphotosynthetic, categorized by means of motility as ciliates, flagellates, or pseudopods
• Heterotrophs

Leishmania donovani, (a species of protozoa) in a bone marrow cell

'Protozoa', a diverse group of unicellular eukaryotic organisms,^[1] many of which are motile. Originally, protozoa had been defined as unicellular protists with animal-like behavior, e.g., movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behaviour, e.g., photosynthesis.

Contents 1 Terminology 2 Characteristics 2.1 Motility and digestion 2.2 Ecological role 2.3 Life cycle 3 Classification 3.1 Sub-groups 4 Human disease 5 References

Terminology

Following the Greek root of the name cillia, the singular form is protozoon /proʊtəˈzoʊ.ɒn/. Its use has, however, partially been replaced by the word protozoan, which was originally only used as an adjective. In the same manner the plural form protozoans is sometimes being used instead of protozoa.

In general, protozoa are referred to as animal-like protists because of movement (motile). However, both protozoa and protists are paraphyletic groups (not including all genetic relatives of the group). For example, Entamoeba is more closely related to humans than to Euglena. "Protozoa" is considered an outdated classification in more formal contexts. However, the term is still used in children's education.^[2]

While there is no exact definition for the term protozoa, it often refers to a unicellular heterotrophic protist, such as the amoeba and ciliates. The term algae is used for microorganisms that photosynthesize. However, distinction between protozoa and algae is often vague. For example, the algae Dinobryon has chloroplasts for photosynthesis, but it can also feed on organic matter and is motile.

Protozoa is sometimes considered a subkingdom.^[3] It was traditionally considered a phylum under Animalia.^[4]

Characteristics

Protozoa commonly range from 10 to 52 micrometers, but can grow as large as 1 mm, and are seen easily by microscope. The largest protozoa known are the deep-sea dwelling xenophyophores, which can grow up to 20 cm in diameter.

They were considered formerly to be part of the protista family. Protozoa exist throughout aqueous environments and soil, occupying a range of trophic levels.

Motility and digestion

Tulodens are 2 of the slow-moving form of protozoa^{[citation needed]} . They move around with whip-like tails called flagella, hair-like structures called cilia, or foot-like structures called pseudopodia. Others do not move at all.

Protozoa may absorb food via their cell membranes, some, e.g., amoebas, surround food and engulf it, and yet others have openings or "mouth pores" into which they sweep food. All protozoa digest their food in stomach-like compartments called vacuoles.^[5] Protozoa are invertibrates.they reproduce by simply cell division,which is called fission.

Ecological role

As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi. Protozoa are both herbivores and consumers in the decomposer link of the food chain. They also control bacteria populations and biomass to some extent. Protozoa such as the malaria parasites (Plasmodium spp.), trypanosomes and leishmania, are also important as parasites and symbionts of multicellular animals.

Life cycle

Some protozoa have life stages alternating between proliferative stages (e.g., trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites (Greek, tropho = to nourish), they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation.

Protozoa can reproduce by binary fission or multiple fission. Some protozoa reproduce sexually, some asexually, while some use a combination, (e.g., Coccidia). An individual protozoan is hermaphroditic.

Classification

Protozoa were previously often grouped in the kingdom of Protista, together with the plant-like algae and fungus-like slime molds. As a result of 21st-century systematics, protozoa, along with ciliates, mastigophorans, and apicomplexans, are arranged as animal-like protists. With the possible exception of Myxozoa, protozoa are not categorized as Metazoa.^[6] Protozoa are unicellular organisms and are often called the animal-like protists because they subsist entirely on other organisms for food. Most protozoa can move about on their own. Amoebas, Paramecia, and Trypanosomes are all examples of animal-like Protists.

Sub-groups

The classification of protozoa has been and remains a problematic area of taxonomy. Where they are available DNA sequences are used as the basis for classification but for the majority of described protozoa such material is not available. They have been and still are mostly on the basis of the their morphology and for the parasitic species their hosts.

Protozoa have been divided traditionally^{[citation needed]} on the basis of their means of locomotion.

• Flagellates (e.g., Giardia lamblia)
• Amoeboids (e.g., Entamoeba histolytica)
• Sporozoans (e.g., Plasmodium knowlesi)
  • Apicomplexa
  • Myxozoa
  • Microsporidia
• Ciliates (e.g., Balantidium coli)

As a phylum the Protozoa had been divided into four subphyla^[7] reflecting the means of locomotion:

• Subphylum Sarcomastigophora
  • Superclass Mastigophora (includes flagellates)
  • Superclass Sarcodinagdffgferclass Opalinata
• Subphylum Sporozoa (includes apicomplexans)
• Subphylum Cnidospora
  • Class Myxosporidea
  • Class Microsporidea
• Subphylum Ciliophora (includes ciliates)

These classification systems are no longer considered to be valid.

Human disease

Main article: Protozoan infection

Some protozoa are human parasites, causing diseases.

Examples of human diseases caused by protozoa:

• Malaria
• Amoebiasis
• Giardiasis
• Toxoplasmosis
• Cryptosporidiosis
• Trichomoniasis
• Chagas disease
• Leishmaniasis
• Sleeping Sickness
• Dysentery

References

Wikispecies has information related to: Protista

Wikisource has the text of the 1911 Encyclopædia Britannica article Protozoa.

1. ^ I. Edward Alcamo; Jennifer M. Warner (28 August 2009). Schaum's Outline of Microbiology. McGraw Hill Professional. pp. 144–. ISBN 978-0-07-162326-1. http://books.google.com/books?id=nzvthLbcH6EC&pg=PA144. Retrieved 14 November 2010. 
2. ^ Michelle Gunter (1 January 2008). Passing the North Carolina 8th Grade End of Grade Test of Science. American Book Company, Inc.. pp. 196–. ISBN 978-1-59807-186-3. http://books.google.com/books?id=pfUqvi1AJmMC&pg=PA196. Retrieved 14 November 2010. 
3. ^ "Protozoa" at Dorland's Medical Dictionary
4. ^ Prof. R.L.Kotpal. Modern Text Book of Zoology: Invertebrates. Rastogi Publications. pp. 151–. ISBN 978-81-7133-903-7. http://books.google.com/books?id=o7x7Zztp-5AC&pg=PA151. Retrieved 14 November 2010. 
5. ^ "Protozoa". MicrobeWorld. American Society for Chemistry. 2006. Archived from the original on 19 May 008. http://web.archive.org/web/20080519091815/http://www.microbeworld.org/microbes/protista/protozoa.aspx. Retrieved 15 June 2008. 
6. ^ Smothers et al, Science, 16 September 1994, "Molecular evidence that the myxozoan protists are metazoans", 8 June 2010
7. ^ Honigberg, B. M.; W. Balamuth, E. C. Bovee, J. O. Corliss, M. Gojdics, R. P. Hall, R. R. Kudo, N. D. Levine, A. R. Lobblich, J. Weiser (1964). "A Revised Classification of the Phylum Protozoa". Journal of Eukaryotic Microbiology 11 (1): 7–20. doi:10.1111/j.1550-7408.1964.tb01715.x. 

v t e Infectious disease and microbiology Disciplines/ pathogens/ major diseases Bacteriology Bacteria: Tuberculosis · Leprosy · Cholera · Diptheria · Syphilis Virology Viruses: AIDS · Influenza · Polio · Measles · Smallpox Prions: Creutzfeldt–Jakob disease Mycology Fungi: Candidiasis · Tinea · Aspergillosis Parasitology Protozoan infection/Helminths: Malaria · Schistosomiasis · Hookworm · Amoebic dysentery Medical entomology Ectoparasites: Scabies · Lice People Ignaz Semmelweis · Louis Pasteur · Robert Koch · Alexander Fleming Special topics Vaccination · Antibiotics · Pandemic · Eradication · Zoonosis · Transmission (Horizontal, Vertical)

v t e Eukaryota Domain : Archaea · Bacteria · Eukaryota Bikonta AH/SAR AH Archaeplastida, or Plantae sensu lato Viridiplantae/Plantae sensu stricto · Rhodophyta · Glaucophyta Hacrobia, or non-SAR chromalveolata Haptophyta · Cryptophyta · Centroheliozoa SAR Halvaria Heterokont ("S") Ochrophyta · Bigyra · Pseudofungi Alveolata Ciliates · Myzozoa (Apicomplexa, Dinoflagellata) Rhizaria Cercozoa · Retaria (Foraminifera, Radiolaria) Excavata Discoba (Euglenozoa, Percolozoa, Loukozoa) · Metamonad · Malawimonas Apusozoa Apusomonadida (Apusomonas, Amastigomonas) · Ancyromonadida (Ancyromonas) · Hemimastigida (Hemimastix, Spironema, Stereonema) Unikonta Amoebozoa Lobosea · Conosa · Phalansterium · Breviata Opisthokonta Holozoa Mesomycetozoea Dermocystida · Ichthyophonida Filozoa Filasterea Capsaspora · Ministeria Choanoflagellate Codonosigidae · Salpingoecidae · Acanthoecidae Metazoa or "Animalia" Eumetazoa (Bilateria, Cnidaria, Ctenophora) · Mesozoa · Parazoa (Placozoa, Porifera) Holomycota Fungi Dikarya (Ascomycota, Basidiomycota) · Glomeromycota · Zygomycota · Blastocladiomycota · Chytridiomycota/Neocallimastigomycota · Microsporidia Nucleariidae Nuclearia · Micronuclearia · Rabdiophrys · Pinaciophora · Pompholyxophrys · Fonticula Unclassified Collodictyonidae Collodictyon triciliatum · Diphylleia rotans

v t e Microbiology: Protist Protozoa locomotion Flagellate (Flagellum) · Amoeboid (Pseudopod) · Ciliate (Cilium) Other structures/organelles Chromalveolate Ciliates: Macronucleus · Micronucleus Apicomplexans: Rhoptry · Apicoplast · Microneme Excavate Kinetoplastids: Kinetoplast · Glycosome Multiple groups Hydrogenosome M: PRO ambz, excv, chrm (strc) ambz, excv, chrm ambz, excv, chrm

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - protozo, urdyr

Nederlands (Dutch)
protozoön, oerdier

Français (French)
n. - protozoaire
adj. - du protozoaire

Deutsch (German)
n. - Protozoon, Urtierchen
adj. - Protozoen-

Ελληνική (Greek)
n. - (ζωολ.) πρωτόζωο
adj. - πρωτοζωικός

Italiano (Italian)
protozoo, protozoico

Português (Portuguese)
n., -
adj. - protozoário (Biol.)

Русский (Russian)
простейшее животное

Español (Spanish)
n. - protozoo, protozoario
adj. - relativo a los protozoarios o protozoos

Svenska (Swedish)
n. - urdjur, protozo
adj. - urdjurs-, protozoisk

中文（简体）(Chinese (Simplified))
原生动物

中文（繁體）(Chinese (Traditional))
n. - 原生動物

한국어 (Korean)
n. - 원생동물

日本語 (Japanese)
n. - 原生動物
adj. - 原生動物の

العربيه (Arabic)
‏(الاسم) حيوان ذو خليه واحدة (صفه) أحادي الخليه‏

עברית (Hebrew)
n. - ‮יצור חד-תאי או מורכב מסוג אחד של תאים, למשל אמבה, של יצור חד-תאי‬

If you are unable to view some languages clearly, click here.