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Protozoa

(Kingdom)

Overview

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Protozoa or protozoans (from p??t?? proton "first" and ??a 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.

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.

Characteristics

Protozoa usually range from 10–50 µm, but can grow up to 1 mm, and are easily seen under a microscope. 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-likes 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

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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:

Photos

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Taxonomy

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The Kingdom Protozoa is a member of the Domain Eukaryota. Here is the complete "parentage" of Protozoa:

The Kingdom Protozoa is further organized into finer groupings including:

Phyla

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Acrasiomycota

Acrasidae is a family of which belongs to the protist group Percolozoa. The name acrasio- comes from the Greek Akrasia, meaning "acting against one's judgement." This group consists of cellular slime molds. [more]

Amoebozoa

The Amoebozoa are a major group of protozoa, including the majority that move by means of internal cytoplasmic flow. Their pseudopodia are characteristically blunt and finger-like, called lobopodia. Most are unicellular, and are common in soils and aquatic habitats, with some found as symbiotes of other organisms, including several pathogens. The Amoebozoa also include the slime moulds, multinucleate or multicellular forms that produce spores and are usually visible to the unaided eye. [more]

Apusozoa

[more]

Cercozoa

[more]

Choanozoa

Choanozoa (: (choanos) = "funnel" + ???? (zoon) = "animal") is the name of a phylum of protists that belongs to the line of opisthokonts. [more]

Ciliophora

Dinophyta

The dinoflagellates are a large group of protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth. About half of all dinoflagellates are photosynthetic, and these make up the largest group of eukaryotic algae aside from the diatoms. Being primary producers make them an important part of the aquatic food chain. Some species, called zooxanthellae, are endosymbionts of marine animals and protozoa, and play an important part in the biology of coral reefs. Other dinoflagellates are colorless predators on other protozoa, and a few forms are parasitic (see for example Oodinium, Pfiesteria). [more]

Euglenozoa

The Euglenozoa are a large group of protozoa. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. There are two main subgroups, the euglenids and kinetoplastids. Euglenozoa are unicellular, mostly around 15-40 µm in size, although some euglenids get up to 500 µm long. [more]

Foraminifera

The Foraminifera, ("Hole Bearers") or forams for short, are a large group of protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are made of calcium carbonate (CaCO3) or agglutinated sediment particles. About 275,000 species are recognized, both living and fossil.[citation needed] They are usually less than 1 mm in size, but some are much larger, and the largest recorded specimen reached 19 cm.[citation needed] [more]

Heliozoa

Heterokontophyta

[more]

Loukozoa

[more]

Metamonada

[more]

Myxomycota

Myxogastria is a grouping of . [more]

Myzozoa

[more]

Percolozoa

[more]

Plasmodiophoromycota

The Phytomyxea are a group of that are parasites of plants. A more common name for them is the plasmodiophorids, but this does not always include Phagomyxa. They typically develop within plant cells, causing the infected tissue to grow into a gall or scab. Important diseases caused by phytomyxeans include club root in cabbage and its relatives, and powdery scab in potatoes. These are caused by species of Plasmodiophora and Spongospora, respectively. [more]

Protozoa

Protozoa or protozoans (from p??t?? proton "first" and ??a zoa "animals"; singular protozoon; the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes. [more]

Radiozoa

[more]

Sarcomastigophora

The Sarcomastigophora belongs to the Protist kingdom and it includes many unicellular or colonial, autotrophic, or heterotrophic organisms. The two main subphyla are Mastigophora and Sarcodina. A third is Opalinata. [more]

At least 1,091 species and subspecies belong to the Phylum Sarcomastigophora.

More info about the Phylum Sarcomastigophora may be found here.

References

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

Sources

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Last Revised: September 22, 2009
2009/09/22 05:48:54