Amoeba

Abstract

Amoebae are single‐celled eukaryotic microorganisms that move by cytoplasmic streaming and possess characteristic surface projections of the cell known as pseudopodia. They are categorised among the heterotrophic protozoa and are grouped with photosynthetic protozoa and algae in an eclectic group, the protists. Modern molecular genetic research has begun to clarify the evolutionary relationships among the amoeboid protists leading to a more natural classification system. The focus here is on the naked amoebae (formerly known as gymnamoebae) that are included in the taxonomic group Amoebozoa. They are important as members of microbial communities where they are significant links in the foodwebs, promote purification of water in the natural environment and waste water treatment plants and contribute to the fertility of aquatic and terrestrial environments. Only a few are known to be human pathogens or carriers of pathogens, including the pathogenic bacterium causing Legionnaires disease.

Key Concepts:

  • Amoebae, though considered to be primitive unicellular organisms, have a highly complex cellular organisation and exhibit diverse behaviour.

  • Amoeboid protists, a group of pseudopod‐bearing unicellular organisms, are an eclectic group of phylogenetically diverse organisms that have adapted a similar amoeboid morphology through convergent evolution.

  • Naked amoebae lack a substantial surface coat or layer that distinguishes them from the testate amoebae, enclosed by an external test or tectum with one or more openings where the pseudopodia emerge.

  • Amoebae prey on a wide variety of other unicellular organisms, including bacteria and protists, though the smaller amoebae are almost exclusively bacterial predators.

  • Free‐living amoebae are significant members of aquatic and terrestrial communities where they serve as links at the base of the food web, promote remineralisation of nutrients through predation on bacteria and help to clarify waste water supply by removing bacteria and promoting the degradation of suspended waste particles.

Keywords: amoebae; amoebo‐encephalitis; endosymbiosis; medical protozoology; nutrient remineralisation

Figure 1.

Amoeba proteus (c. 400 μm) with lobe‐like pseudopodia (Ps) containing hyaloplasm (Hy) appearing as a clear region and endoplasm (En) with granular cytoplasm. The nucleus (N) typically occurs near the centre of the cell and is surrounded by food vacuoles (V) and a contractile vacuole (CV) that is more posterior and expels excess water. The uroid (Ur) is the most posterior part of the cell and is a knob‐like protrusion from the surface of the cell. Adapted from Lee et al..

Figure 2.

Light micrograph of a living Amoeba proteus, that has become broadly expanded compared to Figure , during locomotion on a glass slide. Courtesy of Dr Andrew Rogerson.

Figure 3.

A living Vannella sp. showing its flattened, broadly fan‐shaped morphology. Courtesy of Dr Andrew Rogerson.

Figure 4.

A living Acanthamoeba sp. with characteristic peripheral, finely tapered, sometimes furcate, peripheral pseudopodia. Courtesy of Dr Andrew Rogerson.

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References

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Further Reading

Anderson OR (1987) Comparative Protozoology: Ecology, Physiology, Life History. Heidelberg: Springer.

Darbyshire JF (ed.) (1994) Soil Protozoa. Wallingford, UK: CAB International.

Kwaik AK, Gao L‐Y, Stone BJ, Venkataraman C and Harb OS (1998) Invasion of protozoa by Legionella pneumophila and its role in bacterial ecology and pathogenesis. Applied and Environmental Microbiology 64: 3127–3133.

Laybourn‐Parry J (1992) Protozoan Plankton Ecology. London: Chapman and Hall.

Pawlowski J and Burki F (2009) Untangling the phylogeny of amoeboid protists. Journal of Eukaryotic Microbiology 56: 16–25.

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How to Cite close
Anderson, O Roger, and Rogerson, Andrew(Jan 2011) Amoeba. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001961.pub2]