Protozoa
Protozoa (meaning “first animals”) are heterotrophic, single-celled or colo­nial eukaryotes. Individuals are microscopic and range in size from a few to hundreds of micrometers, depending on the species. Most protozoa are an­imal-like (heterotrophic) because their carbon and energy must be obtained by eating or absorbing organic compounds originating from other living organisms. As eukaryotes they have several organelles, including at least one nucleus that contains most of the cell’s deoxyribonucleic acid (DNA).
Beyond this broad description, it is difficult to define protozoa because they are so diverse and only distantly related to each other. While the term “protozoa” is commonly used, it has little basis in evolutionary history, or phylogeny, of these organisms. Taxonomic systems try to assign organisms to a monophyletic group, that is, one that includes an ancestor and all of its descendants. Plants, animals, and fungi are monophyletic groups; pro­tozoans are not. (The understanding of evolutionary relationships of uni­cellular eukaryotes is in a state of flux.) Further complicating a precise definition of protozoa is the close relationship between some protozoa and unicellular algae. Modern taxonomic treatments recognize these similari­ties and group protozoa, photosynthetic unicellular algae, and slime molds together as protists or protoctists. Whichever term one prefers, the classi­fication is not monophyletic. Despite the fact that protozoa are not a proper taxonomic name, it is a useful, functional term. Ecologists differentiate be­tween autotrophic and heterotrophic components of an ecosystem, and it is natural to separate the animal-like protozoa from the photosynthetic al­gae based on their nutritional mode. (However, Euglena, which can be in­duced to lose their chloroplast, illustrate why unicellular algae are included with protozoa.)

A photomicrograph of the protozoa Euglena.
As is appropriate for heterotrophic organisms that capture food, most protozoa are motile (able to move). The way they move is one of the im­portant characteristics historically used to divide them into major groups: amoebae, flagellates, and ciliates. Apicomplexa, formerly called Sporazoa, is a fourth group of generally obligate parasitic protozoa. Amoebae crawl along surfaces by extending a cytoplasm-filled pseudopod (false-foot) that bulges outward from any edge of the cell. Flagellates and ciliates use spe­cialized organelles, flagella and cilia, that differ primarily in length and num­ber, to propel the cells through water. Flagella are whiplike structures that usually occur one to a few per cell and have an undulating motion. Cilia are shorter and move in concert, like oars, with alternating power and recovery strokes. Sporozoa are either nonmotile or very slow.
Other organelles that are widely distributed among protozoa include food vacuoles, in which ingested particles are digested, and lysosomes that fuse with food vacuoles and supply digestive enzymes. Contractile vacuoles, common in freshwater protozoa, eliminate water that moves into the cells by osmosis. Extrusomes are associated with the membrane of many proto­zoa and contain material that can be ejected from the cell. Some extrusomes secrete an amorphous material that is involved in formation of a capsule or cyst, and others discharge a pointed projectile that may serve for protection or predation. The thousands of “trichocysts” distributed over the surface of the ciliate Paramecium are extrusomes that discharge rapidly in response to physical stimulation and are probably effective deterrents to some preda­tors. Ciliates are unique among protozoa in having two kinds of nuclei: the micronucleus, which is involved only in sexual reproduction; and the macronucleus, which is involved only in the production of messenger ri­bonucleic acid (RNA) for cell function.
Most protozoa reproduce most of the time by equal binary fission, in which a cell divides into two daughter cells after the chromosomes have been duplicated and distributed between them. This asexual mode of re­production leads to rapid population growth of a clone of genetically iden­tical cells. However, sex is widespread in protozoa and complicated life histories do exist. Sexuality is associated with environmental change and in­terrupts asexual reproduction; sex in protozoa usually marks the end of the existence of a genetically unique individual, when it becomes the gamete (reproductive cell) or gametes.
Protozoa are ubiquitous (found everywhere); they are present in all aquatic or moist environments, and their cysts can be found in even the most inhospitable parts of the biosphere. Most are free-living and eat bacteria, algae, or other protozoa. Protozoa are important components of aquatic and soil ecosystems, where they eat bacteria that are too small to be efficiently captured by most animals and are in turn eaten by other organisms. Bacterivorous protozoa also are abundant in activated sludge sewage treatment plants and, in fact, are necessary for their proper functioning. There are several protozoa of medical and economic importance. Examples include the fla­gellate Trypanosoma, which causes African sleeping sickness; the amoeba Entamoeba histolytica, which can attack the intestinal wall and cause amoebic dysentery, and the sporozoans of the Plasmodium species, which cause malaria.
Protozoa have many features linking them to the other kingdoms of life. Scientists widely believe that animals evolved from protozoan ancestors, probably colonial choanoflagellates. New tools and methods from molecu­lar biology are leading to a better understanding of the evolutionary rela­tionships to multicellular organisms and among protozoa.
References
Anderson, O. Roger, and Marvin Druger, eds. Explore the World Using Protozoa. Ar­lington, VA: National Science Teachers Association and the Society of Protozo­ologists, 1997.
Lee, John J., Seymour H. Hutner, and Eugene C. Bovee, eds. An Illustrated Guide to the Protozoa, 2nd ed. Lawrence, KS: Society of Protozoologists, 2001.
Patterson, David J. Free-Living Freshwater Protozoa: A Color Guide. New York: John Wiley & Sons, 1996.