protozoan

Protozoa are a broad and diverse group of mostly unicellular, eukaryotic organisms traditionally described as animal-like protists. They are usually microscopic, though some species reach large sizes, and they inhabit a wide range of environments including soil, freshwater, and marine systems, as well as the bodies of other organisms as parasites.

Most protozoans are motile at some life stage, using structures such as pseudopodia, cilia, or flagella. They obtain nutrients by engulfing particulate food (phagocytosis) or by absorption, and some contain symbiotic algae and photosynthesize.

Reproduction occurs mainly by asexual processes such as binary fission, budding, or schizogony, with sexual processes occurring in some groups. They display a variety of life cycles, including cyst formation to withstand adverse conditions.

Protozoa include several well-known groups: amoebae (pseudopod-forming), ciliates (ciliated), flagellates (flagellated), and apicomplexans (obligate parasites). Pathogenic examples include Plasmodium (malaria), Trypanosoma (sleeping sickness), Giardia (giardiasis), Entamoeba histolytica (amoebic dysentery), and Toxoplasma gondii. Not all protozoa are parasites; many are free-living and contribute to nutrient cycling and food webs.

Taxonomically, 'protozoa' is a historical term; modern classifications place organisms in various eukaryotic clades such as Amoebozoa, SAR, Excavata, and others. They are studied as model organisms in cell biology and ecology.