protozoan

Any of a large group of single-celled, usually microscopic, eukaryotic organisms, such as amoebas, ciliates, flagellates, and sporozoans.

[From New Latin Protozōa, former subkingdom name : PROTO- + -zōa, pl. of -zōon, -zoon.]

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.

(click to enlarge)
Representative protozoans. The zooflagellate Trypanosoma brucei is the causative agent of … (credit: © Merriam-Webster Inc.)

Any of a group of small (usually microscopic) single-celled protists. 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 microns 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 (see eukaryote). They show wide variation in modes of movement, nutrition, and reproduction. Various classification systems exist to group the protozoans. The major phyla include Sarcomastigophora (flagellated forms and forms possessing cytoplasmic extensions called pseudopodia), Ciliophora (ciliated forms), and Apicomplexa, Microspora, and Myxozoa (spore-producing forms). Apicomplexa and Microspora are sometimes included in the single phylum Sporozoa. Commonly known protozoans include dinoflagellates, amoebas, and paramecia (see paramecium).

For more information on protozoan, visit Britannica.com.

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.

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

Protozoa or Cornelius protozoans (from Greek proton proton "first" and ζῷα zoa "animals"; singular protozoon; (the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes.^[1]

While there is no exact definition of the term "protozoan", most scientists use the word to refer to a unicellular heterotrophic protist, such as an amoeba or a ciliate. The term algae is used for microorganisms that photosynthesize. However, the distinction between protozoa and algae is often vague. For example, the alga Dinobryon has chloroplasts for photosynthesis, but it can also feed on organic matter and is motile. Many people refer to them as animal-like protists.

Protozoa are paraphyletic. Though they have sometimes been described as a subkingdom or phylum, they do not constitute a formal taxon in modern classification systems.

Contents 1 Characteristics 2 Classification 2.1 Sub-groups 3 References

Characteristics

Protozoa usually range from 10–50 μm, but can grow up to 1 mm, and are easily seen under a microscope. They move around with whip-like tails called flagella. They fall under the protista family. Over 30,000 different types have been found. Protozoa exist throughout aqueous environments and soil, occupying a range of trophic levels. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi. Protozoa play a role as both herbivores and consumers in the decomposer link of the food chain. Protozoa also play a vital role in controlling bacteria populations and biomass. 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.^[2]

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. Protozoa such as the malaria parasites (Plasmodium spp.), trypanosomes and leishmania are also important as parasites and symbionts of multicellular animals.

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 and 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 the 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 and grow. The process by which the protozoa takes its cyst form is called encystation, while the process of transforming back into trophozoite is called excystation.

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

Another name for protozoa is Acrita (R. Owen, 1861). They can cause malaria or amoebic dysentery.

Classification

Protozoa were previously often grouped in the kingdom of Protista, together with the plant-like algae and fungus-like slime molds and animal-like protozoa. In the 21st-century systematics, protozoa, along with ciliates, mastigophorans, and apicomplexans, are arranged as animal-like protists. However, protozoa are not Metazoa (with the possible exception of the enigmatic Myxozoa).^{[citation needed]}

Sub-groups

Protozoa have traditionally been divided on the basis of their means of locomotion, although this character is no longer believed to represent genuine relationships:

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

References

1. ^ Protozoa at Dorland's Medical Dictionary
2. ^ [1] Protozoa, defined at Microbe World. 2006 American Society for chemistry. Retrieved June 15, 2008.

v • d • e Eukaryota classification Domain : Archaea - Bacteria - Eukaryota Bikonta AH/SAR AH Archaeplastida, or Plantae sensu lato Viridiplantae/Plantae sensu stricto · Rhodophyta  · Glaucocystophyceae Hacrobia, or non-SAR chromalveolata Haptophyta · Cryptophyta · Centroheliozoa SAR SA Heterokont ("S") Ochrophyta · Bigyra · Pseudofungi Alveolata Ciliates · Myzozoa (Apicomplexa, Dinoflagellata) Rhizaria Cercozoa · Retaria (Foraminifera, Radiolaria) Excavata Discoba (Euglenozoa, Percolozoa) · Metamonad · Malawimonas Apusozoa Apusomonadida (Apusomonas) · Ancyromonadida (Ancyromonas) · Hemimastigida (Hemimastix, Spironema, Stereonema) Unikonta Amoebozoa Lobosea · Conosa · Phalansterium Opisthokonta Holozoa Mesomycetozoea Dermocystida · Ichthyophonida Filozoa Filasterea Capsaspora · Ministeria Choanoflagellatea Codonosigidae Metazoa or "Animalia" Eumetazoa (Bilateria, Cnidaria, Ctenophora) · Mesozoa · Parazoa (Placozoa, Porifera) Fungi Dikarya (Ascomycota, Basidiomycota) · Glomeromycota · Zygomycota · Blastocladiomycota  · Chytridiomycota/Neocallimastigomycota  · Microsporidia sister: Nucleariidae (Nuclearia, Micronuclearia)

v • d • 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 Subdomains/Superkingdoms Bikont: Chromalveolate · Excavate · Rhizaria · Archaeplastida Unikont: Opisthokont · Amoebozoa

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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

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