Protozoa may be considered as (mostly) microscopic, essentially single‐celled, eukaryotic organisms whose food is obtained by eating other organisms or their products. Some contain algal endosymbionts, or enslave plastids from their prey, and thus gain additional nutritional benefit from photosynthesis. Protozoa are no longer recognised as a taxonomic group and representatives can be found in all but one of the eukaryote supergroups. Protozoa play important roles in the ecology of aquatic habitats and soils, for example, by controlling bacterial populations and releasing nutrients for use by other organisms. As parasites, protozoa have very profound effects on humans, both directly and through their effects on domesticated animals. Some protozoa have shells or skeletal structures that can fossilise and certain types, particularly the foraminifera, are used in biostratigraphy and in locating oil deposits.

Key Concepts:

  • Protozoa are single‐celled eukaryotes that obtain their food by eating other organisms or their products.

  • Protozoa are not a taxonomic group but rather a diverse assemblage of distantly related lineages, with representatives in all but one of the eukaryote supergroups.

  • The identification of protozoa is based mainly on morphology as revealed by light microscopy.

  • The main functional groups of protozoa are the amoebae, flagellates, ciliates (all of which are characterised by their organelles of locomotion and food capture) and sporozoa (obligate parasites).

  • Protozoa are found in almost every habitat where there is sufficient water for them to move and feed, and may survive adverse environmental conditions by the production of cysts or spores.

  • Protozoa play key roles in nutrient cycling and soil fertility, and are the main predators of bacteria in many environments.

  • A wide range of human and animal diseases are caused by protozoa.

  • Fossil protozoa are used to help locate oil deposits and to reconstruct past climate change.

Keywords: amoebae; ciliates; cilia; flagellates; flagella; microfossils; parasites; pseudopodia

Figure 1.

Antony van Leeuwenhoek's draftsman's figures (not drawn to a single scale) of eukaryotic protists: (a) Anthophysa (chrysomonad); (b) Volvox (chlorophyte); (c) Coleps (prostome ciliate); (d) Cepedea (opalinid); (e) Nyctotheroides (heterotrich ciliate); (f) Vorticella (solitary peritrich ciliate); (g) Cothurnia (loricate peritirch); (h) Carchesium (colonial peritrich); (i) Elphidium (formaminiferan). Reproduced from Corliss JO . © Elsevier.

Figure 2.

The form of pseudopodia in three types of amoeboid protozoa. (a) The testate amoeba Difflugia (approximately 250 µm long) has broadly lobed pseudopodia; these emerge from a shell into which sand grains are incorporated. (b) The heliozoan Actinophrys (cell body 40 µm diameter) has axopodia supported by bundles of microtubular fibres. (c) The foraminiferan Elphidium (shell up to 500 µm across) has fine reticulopodia forming a network.

Figure 3.

The ciliate Tetrahymena has rows of simple cilia and a group of three membranelles and one membrane around the cell mouth (cell approximately 50 µm long).

Figure 4.

Examples of flagellate protozoa. (a) The kinetoplastid flagellate Bodo is approximately 10 µm long. (b) The collar flagellate Codosiga (cell 15 µm long) is attached by a stalk and has a single flagellum emerging from the centre of the collar. (c) The hypermastigote flagellate Trichonympha (approximately 200 µm long) has numerous flagella and ingests wood fragments at the posterior end.

Figure 5.

Spores and infective cells of parasitic protozoa and other microeukaryotic groups formerly classified as protozoa. (a) A spore of an apicomplexan (approximately 20 µm long) containing several sporozoites; these emerge as gliding cells (b) with an apical complex, seen here at the upper end. (c) A microsporidian spore (approximately 4 µm long) contains a laminated structure and an inverted and coiled polar thread surrounding the infective cell; on germination of the spore the tubular polar thread everts and penetrates a host cell so that when the infective amoeboid cell passes through the tube (d) it enters the cytoplasm of the host cell. (e) A myxozoan spore (approximately 20 µm long) with a multicellular spore case containing two polar capsules and a single infective cell; the latter emerges as an amoeboid cell (f) in the gut of the host.



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Further Reading

Anderson OR and Druger M (1997) Explore the World Using Protozoa. Arlington, Virginia: NSTA.

Coombs GH , Vickerman K , Sleigh MA and Warren A (eds) (1998) Evolutionary Relationships Among Protozoa. Dordrecht: Kluwer.

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Hausmann K , Hülsmann N and Radek R (2003) Protistology, 3rd edn. Stuttgart, Germany: E. Schweizerbartsche Buchhandlung.

Kreier JP and Baker JR (eds) (1991–1995) Parasitic Protozoa, 2nd edn. San Diego: Academic Press.

Lee JJ , Leedale GF and Bradbury P (eds) (2002) An Illustrated Guide to the Protozoa, 2nd edn. Lawrence, Kansas: Society of Protozoologists.

Margulis L , Corliss JO , Melkonian M and Chapman DJ (eds) (1990) Handbook of Protoctista. Boston: Jones and Bartlett.

Patterson DJ and Hedley S (1992) Free‐Living Freshwater Protozoa: A Colour Guide. London: Wolfe Publishing Ltd.

Sleigh MA (1989) Protozoa and Other Protists. Cambridge: Cambridge University Press.

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Warren, Alan(Jan 2014) Protozoa. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001924.pub3]