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  1. Alan Warren

Published Online: 15 JAN 2014

DOI: 10.1002/9780470015902.a0001924.pub3



How to Cite

Warren, A. 2014. Protozoa. eLS. .

Author Information

  1. Natural History Museum, London, UK

  1. Based in part on the previous version of this eLS article ‘Protozoa’ (2005) by Michael A Sleigh.

Publication History

  1. Published Online: 15 JAN 2014


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.


  • amoebae;
  • ciliates;
  • cilia;
  • flagellates;
  • flagella;
  • microfossils;
  • parasites;
  • pseudopodia