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Protozoa

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

Protozoa are 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.

Terminology

Following the Greek root of the name, the singular form is protozoon /proʊtəˈzoʊ.ɒn/(protos=first, zoon=animal). Its use has, however, partially been replaced by the word protozoan, which was originally only used as an adjective.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 (motility). 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 "Protozoa" is considered an outdated classification in more formal contexts.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 is often referred to as 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 alga 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] referring to unicellular animals, with Metazoa referring to multicellular animals.

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,and that engulfing of food is said to be phagocytosis. All protozoa digest their food in stomach-like compartments called vacuoles.[5]

Pellicle

The pellicle is a thin layer supporting the cell membrane in various protozoa, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more hydrodynamic. They vary from flexible and elastic to rigid. Although somewhat stiff, the pellicle is also flexible and allows the protist to fit into tighter spaces. In ciliates and Apicomplexa, it is formed from closely packed vesicles called alveoli. In euglenids, it is formed from protein strips arranged spirally along the length of the body. Examples of protists with a pellicle are the euglenoids and the paramecium, a ciliate. In some protozoa, the pellicle consists of many bacteria that adhere to the surface by their fimbriae or "attachment pili".[6] Thus, attachment pili allow the organisms to remain in the broth, from which they take nutrients, while they congregate near air, where the oxygen concentration is greatest.

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 disease causing agents in humans.

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. 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 their morphology and for the parasitic species their hosts. Protozoa have been divided traditionally[citation needed] on the basis of their means of locomotion.

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

These systems are no longer considered to be valid. For an example of a system of classification of protozoa, see Kudo system.

Human disease

Main article: Protozoan infection

Some protozoa are human parasites, causing diseases. Examples of human diseases caused by protozoa:

See also

References

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  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. 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. 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. Retrieved 14 November 2010.
  5. ^ "Protozoa". MicrobeWorld. American Society for Chemistry. 2006. Archived from the original on 19 May 008. Retrieved 15 June 2008. {{cite web}}: Check date values in: |archivedate= (help)
  6. ^ https://archive.org/stream/protozoainbiolog00calk#page/1008/mode/2up
  7. ^ Honigberg, B. M. (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

Bibliography

General

  • Dogiel, V. A., revised by J. I. Poljanskij and E. M. Chejsin. General Protozoology, 2nd ed., Oxford University Press, 1965.
  • Hausmann, K., N. Hulsmann. Protozoology. Thieme Verlag; New York, 1996.
  • Jahn,T.L.- Bovee, E.C. & Jahn, F.F. How to Know the Protozoa. Wm. C. Brown Publishers, Div. of McGraw Hill, Dubuque, Iowa, 1979; 2nd ed.
  • Kudo, R.R. Protozoology. Springfield, Illinois: C.C. Thomas, 1954; 4th ed.
  • Lee, J.J., Leedale, G.F. & Bradbury, P. An Illustrated Guide to the Protozoa. Lawrence, Kansas, U.S.A: Society of Protozoologists, 2000; 2nd ed.
  • Manwell, R.D. Introduction to Protozoology, second revised edition, Dover Publications Inc., New York, 1968.
  • Patterson, D.J. Free-Living Freshwater Protozoa. A Colour Guide. Manson Publishing; London, 1996.
  • Patterson, D.J., M.A. Burford. A Guide to the Protozoa of Marine Aquaculture Ponds. CSIRO Publishing, 2001.
  • Roger Anderson, O. Comparative protozoology: ecology, physiology, life history. Berlin [etc.]: Springer-Verlag, 1988.
  • Sleigh, M., E. Arnold. The Biology of Protozoa. London, 1981.

Physiology

  • Levandowski, M., S.H. Hutner (eds). Biochemistry and physiology of protozoa. Volumes 1, 2, and 3. Academic Press: New York, NY, 1979; 2nd ed.
  • Laybourn-Parry J. A Functional Biology of Free-Living Protozoa. Berkley, California: University of California Press; 1984.

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