Protozoa are unicellular, heterotrophic eukaryotes that have been studied for more than 300 years, at first as microscopic curiosities, later as organisms causing disease and more recently as important components of ecosystems. In addition to being of intrinsic interest in their own right, protozoa are important in a number of fields of study, including: (1) as model organisms for investigations of cell biology, physiology and biochemistry; (2) ecological processes, such as predation and competition, and how these might affect the evolution of life history traits; (3) nutrient cycling, food webs and soil fertility; (4) reconstructing past climate change and (5) locating oil deposits. In addition, protozoa cause a number of important diseases of humans and animals, including malaria, sleeping sickness, Chagas disease, amoebic dysentery, coccidiosis, leishmaniasis and toxoplasmosis. Consequently, protozoology – the study of protozoa – is a discipline with a substantial following throughout the world.

Key Concepts:

  • Protozoa have been studied by microscopists for more than 300 years.

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

  • Many fundamental processes in cell biology and biochemistry were first discovered in protozoa.

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

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

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

Keywords: cell biology; cryptic diversity; ecology; light microscopy; parasitology; systematics; ultrastructure

Figure 1.

A small sample from the variety of free‐living protozoan species, drawn to scale next to a pinhead. Virtually all species fall within the size range 0.002–2 mm. Reproduced with permission from Finlay . © AAAS.

Figure 2.

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) and (i) Elphidium (formaminiferan). Reproduced from Corliss . © Elsevier.

Figure 3.

Cross‐sectional diagram of a 9+2 axoneme (viewed from the proximal end towards the distal tip). Actual position of inner dynein arms and nexin links are not precisely drawn. The central pair singlet microtubules (1–2) possess a complex array of elements that are highly species specific. Reproduced from Linck RW (2009) Cilia and flagella. In: eLS. Chichester, UK: John Wiley & Sons, Ltd. doi:10.1002/9780470015902.a0001258.pub2.

Figure 4.

A photosynthesis‐based protozoan food web depicting the microbial loop. Green arrows show dissolved nutrient flow and orange and blue arrows indicate prey and predator relations. Feeding activity of protozoa mineralises nutrients that are available for primary production (recursive arrows). Reproduced from Anderson OR (2010) Protozoan ecology. In: eLS. Chichester, UK: John Wiley & Sons, Ltd. doi: 10.1002/9780470015902.a0001929.pub2.



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Further Reading

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Warren, A(Dec 2013) Protozoology. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001923.pub3]