Protozoan Ecology


Protozoan ecology is the interaction in space and time of protozoa (flagellates, amoeboid organisms and ciliates) with other living organisms and the physical environment. It encompasses, among other factors, sources of food and energy used to sustain life, food web dynamics, the role of protozoa in maintaining fertility of ecosystems and adaptive strategies for survival in varying habitats. Among the physical environmental (abiotic) factors, temperature, salinity, nutrients and levels of pollutants, when present, are major sources of influence. A complex set of interactions exists among microbial biota, including the role of nutrient regeneration during protozoan predation on bacteria resulting in release of nutrients due to digestion and disruption of the bacteria during feeding. These nutrients, in turn, are reutilised by algae during photosynthesis. This process of cyclical nutrient uptake by algae and subsequent release by protozoan predation is known as ‘the microbial loop’. The use of molecular genetic analyses has substantially improved our identification of protozoan species and their role in ecology including the microbial loop.

Key Concepts

  • Characteristics of the physical environment (including temperature, available nutrients and salinity) set the boundary conditions for the abundance and diversity of protozoa within a given habitat.
  • Algae and other photosynthetic organisms are primary producers that fix carbon into organic compounds through photosynthesis and thus provide food sources for all other organisms, including protozoa, within a food web.
  • Heterotrophic (nonphotosynthetic) microorganisms, particularly the protozoa, prey on algae and other microbes, including bacteria, and are consumers within food webs.
  • A complex set of interactions exist in microbial communities, including predator–prey relationships, that determine the flow of matter and energy through the food webs.
  • Competition within and among species partially accounts for the abundance and diversity of species in a microbial community.
  • The stability of microbial communities depends in part on the diversity of species and available resources needed to support life.
  • Protozoa, as predators and also as prey for larger organisms, are often key members of microbial communities and sustain the flow of energy and matter through food webs, including the regeneration of nutrients through their significant role in the microbial loop.

Keywords: abiotic factors; biotic factors; ecosystem dynamics; food webs; microbial loop; molecular genetics; predator–prey relations; soil fertility

Figure 1. Protozoa: (a) flagellate, Polytoma uvella (15 μm); (b) naked amoeba, Metachaos discoides (400 μm) and (c) ciliate, Phacodinium metchnikoffi (100 μm). (From Lee JJ, Hutner SH and Bovee EC (eds) (1985) Illustrated Guide to the Protozoa. Lawrence, KS: Society of Protozoologists © 1985 International Society of Protistologists.)
Figure 2. The effect of temperature on the population density of Tetrahymena pyriformis. Growth is best at 25 °C, with a minimum temperature of 10 °C. (Adapted from Sleigh M (1989) Protozoa and Other Protists, p. 260. London: Edward Arnold.)
Figure 3. A photosynthesis‐based protozoan food web. 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).
Figure 4. A terrestrial protozoan, bacterial‐based food chain showing predator–prey relations. (Adapted from Griffiths BS (1994) Soil nutrient flow. In: Darbyshire JF (ed.) Soil Protozoa, p. 80. Wallingford, UK: CAB International.)


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Further Reading

Anderson OR (1983) Radiolaria. Springer: New York.

Archibald JM, Simpson A and Slamovits C (eds) (2017) Handbook of the Protists, 2nd edn. Springer International Publishing AG: Cham, Switzerland.

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Anderson, O Roger(Dec 2020) Protozoan Ecology. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0029234]