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 uni-cellular eukaryotes is in a state of flux.) Further complicating a precise definition of protozoa is the close relationship between some protozoa and
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.
Robert W. Sanders
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.
Species in the genus Euglena are photosynthetic members of a diverse group of pigmented and colorless flagellates in the order Euglenida. Both protozoologists and botanists traditionally have studied them. The cylindrical shape of the body is maintained by a flexible pellicle composed of the cell membrane and a layer of protein strips. Euglena have large, bright green chloroplasts and two flagella that arise from within a pocket on the anterior end. Usually only one flagellum is long enough to emerge from the reservoir. Beside the reservoir is a small, red-pigmented spot, the stigma, which is associated with a light-sensing region. Many euglenids have visible rods or rings made of paramylum, which, like starch, is a glucose storage molecule.
Euglena swim with a gyrating motion using their emergent flagellum, which pull the organism forward like a propeller. When not swimming, Euglena often alternately contract and elongate the pellicle, causing a bulge to move from one end of the cell to the other in a characteristic "euglenoid motion." Euglena are most common in organically rich freshwater environments.
Most amoebae live on surfaces in moist soil or aquatic sediment. They are easily over-looked because they are small, predominantly transparent, and slow moving. Amoebae are characterized by the flow of granular cytoplasm into lobes of cell membrane (pseudopodia) that serve the dual functions of motility and food capture. These protozoa typically lack a fixed external anatomy and are flexible but can be categorized by the shape and number of the pseudopodia. Pseudopodia can occur as multiple rounded or needlelike projections or as a single advancing front. When a food particle is encountered, pseudopodia surround it with a membrane-enclosed sac that pinches off internally to form a food vacuole. In addition to food vacuoles, a contractile vacuole and a single large nucleus or many small nuclei can be distinguished. Some amoebae excrete tests, or shells. The marine foraminiferans harden their shells with calcium carbonate, and fossilized foram shells make up a large proportion of some marine sediments and terrestrial deposits (like the White Cliffs of Dover, England).