Protozoa (from the Greek words proton, meaning "first", and zoa, meaning "animals") is a subkingdom of microorganisms that are generally classified as unicellular eukaryotes. Protozoans play a key role in maintaining the balance of the ecosystem.
The word protozoan is originally an adjective and is used as a noun.[citation needed] While there is no exact definition for the term protozoan, most scientists use the word to refer to a unicellular heterotrophic protist, such as the amoeba and ciliate. The term algae is used for microorganisms that photosynthesize. However, the distinction between protozoa and algae is often vague. For example, the algae Dinobryon has chloroplasts for photosynthesis, but it can also feed on organic matter and is motile. Protozoans are generally referred to as animal-like protists.



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