Protozoa Definition, Occurrence, Ecology

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Protozoa Definition

The term protozoa comes from the Greek word protos and Zoon, which means “first animal”. Protozoa are microscopic single-celled, eukaryotic protists that lack cell walls. The size of protozoa varies from 5 to 250 micrometers. The detailed study of protozoa is known as protozoology.

They also form colonies. The individual cells of the protozoan colony are linked by a cytoplasmic thread or are embedded within a common matrix. 

It is estimated that there are more than 65,00 species of protozoa which are distributed among 7 named phyla. Among them, about 50% of species are in fossil form while among the remaining 50%, about 22,000 are free-living and 10,000 are parasitic, which causes disease in humans. There are hundreds of thousands of species that are yet to be discovered.

Occurrence of Protozoa

  • Protozoa can grow in every moist environment, mainly they are found in sea, soil, and freshwater.
  • There are also present free-living protozoa, which can be found in the polar regions and at very high altitudes.
  • The parasitic group of fungi is mainly found in association with animal groups.
  • Protozoa also live in a dry environment by the formation of a resistant cyst or dormant stage. For example, soil amoeba Naegleria forms a resistant cyst in dry weather, becomes nake amoeba in moist soil, and flagellates when flooded with water.

Factors influence the Protozoan distribution

There are different factors which are influence the distribution of protozoa such as;


  • There are present some photosynthetic protozoa, which contain chromatophores, that’s why they need sunlight to continue their photosynthesis.
  • Some protozoa are fed on photosynthetic organisms, they require sunlight but indirectly.
  • There are other protozoa that avoid sunlight and thrive in a dark environment where sunlight is absent.


There are several species of protozoa which live in water based on chemical constituents such as;

  • Some protozoa thrive in water where the oxygen concentration is high but the concentration of organic matter is low, such as water of mountain springs, brooks or ponds.
  • Some protozoa also thrive in mineral-rich water.
  • Some of them also grow in water, where active oxidation and degradation of organic matter occur.
  • Protozoa can grow in a water-containing a lower concentration of oxygen.
  • Some of them also found, thrive in saltwater and freshwater.

What is holozoic protozoa? Holozoic protozoa are a type of protozoa that feed on a variety of organisms that are widely distributed; those that are more selective and feed only a few species are limited in their distribution.

Hydrogen-Ion concentration

  • The majority of protozoa lives in pH between 6.0 to 8.0, because this range is considered as the optimum pH for the growth of protozoa.
  • Some protozoa can also tolerate a wide range of pH, for example, pH 3.2 tp 8.7.


  • The optimum temperature for protozoa is between 16-25 degrees centigrade and maximum temperature is between 36 to 40 degrees centigrade.
  • Some groups of protozoa are found in warm water ( 30 to 56) of hot springs.
  • Protozoa is also found in high altitudes where temperatures are very low. The red snow is an example of protozoa. It is a hematochrome-bearing flagellate.

Ecology of Protozoa

Based on ecology of protozoa can be divided into two groups such as; Free-living protozoa and symbiotic protozoa, which live on or in other organisms. 

Free-Living Protozoa

The free-living protozoa can be found in different environments. Various factors affect the distribution and numbers of free-living protozoa such as light, nutrient, pH, etc. 

Symbiotic Protozoa

Those protozoa are live in association with other host or other organisms are known as symbiotic protozoa. The symbiotic protozoa are different tyeps such as;


In this symbiotic relationship, the host is neither injured nor benefited but the commensal is benefited. Commensalism is two types;

  1. Ectocommensalism: Protozoa remain attached to the host cell. Example: Ciliates & Suctorians lives on molluscs, arthropods, fishes and frogs, etc.
  2. Endocommensalism: The Protozoa live within the host’s body. Example: Trichomonas, Giardia, etc., live within the alimentary canal of man. Entamoeba coli live in alimentary canal of frogs.


In this symbiotic relationship, both the host cell and protozoa are benefited. An example of Mutualism relationship is; certain flagellates live in the guts of termites and digest the woody material eaten by the termite to a glycogenous substance that can be used by the host cell. If deprived of these flagellates, the termite dies; if the flagellates are removed from the termite gut, they too pet.


In this type of relationship, one organism the parasite lives at the expense of the other. The parasite feeds on the host cells or cell fragments by pseudopodia or cytostome (an opening for ingestion of food;), or enters the host tissues and cells, living upon the cytoplasm and even the nuclei. 

As a result, the host may develop pathological conditions. The Sporozoa are strictly parasitic and are among the most important of the disease-producing protozoa.


There are some parasitic protozoa that parasitize other protozoan or metazoan (animals whose bodies consist of many cells) parasites, they are known as Hyperparasitism.

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