Protozoa is a diverse group of single-celled, eukaryotic (have membrane-bound organelles and a nucleus) organisms that are either free-living or parasitic. They lack a cell wall, unlike plants and fungi, which allows them to have flexible shapes, and, for many, independent movement. In addition to an active feeding stage, many also have a dormant stage that allows survival in harsh conditions as well as transmission between hosts.
Several tens of thousands of species of protozoa are known to exist, with many more still undiscovered. One of the largest categories is ciliates, which includes parameciums and which are distinguished by the presence of numerous hair-like structures covering their surfaces which assist in feeding, serve in sensory roles, and enable swimming or crawling. Another is amoebae, which feature the use of pseudopods (temporary projections of the cell) to move and feed. A third group, flagellates, which includes euglenas, is characterized by the use of one or more flagella (a microscopic, whip-like structure) to move.
Protozoa play essential roles in ecosystems as consumers, regulators, and links in food webs. They help maintain ecological balances, especially in aquatic and soil environments, by feeding on bacteria, algae and other microorganisms, thereby releasing nutrients, including nitrogen and phosphorus, back into the environment and thus supporting plant growth. And they are an integral part of the food chain, supporting larger organisms that humans consume, especially some forms of seafood.
Although protozoa are neither essential nor normal components of the human body, some ultimately support human health and the quality of life by participating in symbiotic relationships that sustain other species upon which humans depend. And others indirectly benefit humans through helping to maintain environmental sanitation and ecological balances. However, parasitic protozoa can also be harmful, causing diseases such as malaria, sleeping sickness, amoebic dysentery and giardiasis.
Protozoa also participate in industrial processes, such as wastewater treatment, by consuming bacteria and organic debris, thereby improving water clarity and quality. In addition, they are useful as bioindicators because they respond rapidly to environmental changes such as pollution and ecosystem stability.
Some protozoa species are threatened with extinction as part of the current wave of mass extinctions that is resulting from habitat loss, climate change, pollution, etc. Particularly endangered are those that linked closely to vulnerable host species or sensitive ecosystems. Although around 300 were listed as endangered worldwide as of 2024, the actual number is likely far higher because only a small fraction of the total number of species have been examined. The ultimate effects of such extinctions remain unknown.