Protozoa
Protozoa (in Greek proto = first and zoa = animals) is a diverse group of single-celled, microscopic or near-microscopic eukaryotes (organisms whose cells have nuclei) that commonly show characteristics usually associated with animals, most notably mobility and heterotrophy. Among well-known protozoa are amoebas, ciliates, paramecia, and dinoflagellates. Protozoans may live freely or be parasites, and may live as single cells or in simple colonies without any differentiation into tissues. While most are microscopic, some grow large enough to be seen unaided.
Protozoa span several phyla, which are generally placed as part of the kingdom Protista, rather than the kingdom Animalia. Protista (or Protoctista) comprises those eukaryotes that are not animals, plants, or fungi, and is not a natural (monophyletic group), but rather the "left-overs" from other eukaryotic kingdoms. In some schemes, Protozoa is considered a sub-kingdom of Protista. There are also classifications in which the plant-like algae and fungus-like water molds and slime molds are not considered part of Protista and the remaining forms then may be grouped as a kingdom labeled Protista. Often, protozoa is just an informal term for unicellular, heterotrophic protists.
The name protozoa ("first animals") is misleading, since they are not animals (with the possible exception of the Myxozoa).
Overview
Most protozoans are too small to be seen with the naked eye—most are around 0.01–0.05 mm—but can easily be found under a microscope. However, forms that are up to 0.5 mm are still fairly common.
Protozoa are ubiquitous throughout aqueous environments and the soil, and play an important role in their ecology. Protozoa occupy a range of trophic levels. As predators upon unicellular or filamentous algae, bacteria, and microfungi, protozoa play a role both as herbivores and as consumers in the decomposer link of the food chain. Protozoa also play a vital role in controlling bacteria population and biomass. As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important.
Protozoa such as the malaria parasites, trypanosomes, and leishmania are also important as parasites and symbionts of multicellular animals.
Protozoan have traditionally been divided on the basis of their means of locomotion, although this is no longer believed to represent genuine relationships:
- Flagellates
- Amoeboids
- Sporozoans
- Apicomplexa
- Myxozoa
- Microsporidia
- Ciliates
The phyla into which protozoa fall vary with the taxonomic scheme. Three phyla that are commonly recognized are Sarcodina (those moving by cytoplasmic projection, or pseudopodia; the amoebas); Ciliophora (move by cilia; and Sporozoa (immobile and parasitic protozoans). Some other names used in different schemes for phyla include Zoomastigina (move by flagella), Mastigophora (flagellates), Sacromastigophora, Apicomplexa, Microspora, Myxozoa (spore-producing forms), and Opalinida (immobile and parasitic) as phyla, while others
Encystment - Under certain adverse conditions, some protozoa produce a protective capsule called a cyst. A cyst permits the organism to survive when food, moisture, or oxygen is lacking, when temperatures are not suitable, or when toxic chemicals are present. A cyst also enables a parasitic species to survive outside its host in order to get a new host. This is important because parasitic protozoa sometimes have to go through the excretory system of one host in order to get a new host. The cysts form in members of the phylum Apicomplexa is called an oocyst. It is a reproductive structure in which new cells are produced sexually.
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