Protozoa are a diverse group of unicellular eukaryotic organisms, many of which are motile. Historically, protozoa were defined as unicellular protists with animal-like behaviour, such as movement or i.e., motility. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behavior, e.g. photosynthesis. The term protozoan has become highly problematic due to the introduction of modern ultrastructural, biochemical, and genetic techniques. Today, protozoan are usually single-celled and heterotrophic eukaryotes containing non-filamentous structures that belong to any of the major lineages of protists. They are restricted to moist or aquatic habitats (i.e., they are obligate aquatic organisms). Many protozoan species are symbionts, some are parasites, and some are predators of faeces bacteria and algae. There are an estimated 30,000 protozoan species. 
Following the Greek root of the name, the singular form is protozoon //(protos=first, zoon=animal). Its use has, however, partially been replaced by the word protozoan, which was originally only used as an adjective. In the same manner the plural form protozoans is sometimes being used instead of protozoa.
In general, protozoa are referred to as animal-like protists because of movement (motility). However, both protozoa and protists are paraphyletic groups (not including all genetic relatives of the group). For example, Entamoeba is more closely related to humans than to Euglena. "Protozoa" is considered an outdated classification in more formal contexts. However, the term is still used in children's education.
While there is no exact definition for the term protozoa, it is often referred to as a unicellular heterotrophic protist, such as the amoeba and ciliates. The term algae is used for microorganisms that photosynthesize. However, distinction between protozoa and algae is often vague. For example, the alga Dinobryon has chloroplasts for photosynthesis, but it can also feed on organic matter and is motile.
Protozoa commonly range in length between 10 to 52 micrometers, but can grow as large as 1 mm. They are easily seen with a microscope. The largest protozoa are known as deep-sea dwelling xenophyophores, which can grow up to 20 cm in diameter. They were formerly considered to be part of the protista family. Protozoa exist throughout aqueous environments and soil, occupying a range of trophic levels. They are eukaryotic unicellular and aquatic organisms that can be flagellates (motile with flagella), ciliates (motile with cilia), and amoebas (motile by means of pseudopodia). Flagellates are the most numerous soil protozoa.
Motility and digestion
Tulodens are 2 of the slow-moving form of protozoa. They move around with whip-like tails called flagella (5-10µm long), hair-like structures called cilia (20-30 µm long), or foot-like structures called pseudopodia (2µm thick by 20 µm). Others do not move at all. Protozoa may absorb food via their cell membranes, some, e.g., amoebas, surround food and engulf it, and yet others have openings or "mouth pores" into which they sweep food,and that engulfing of food is said to be phagocytosis. All protozoa digest their food in stomach-like compartments called vacuoles.
The pellicle is a thin layer supporting the cell membrane in various protozoa, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more hydrodynamic. They vary from flexible and elastic to rigid. Although somewhat stiff, the pellicle is also flexible and allows the protist to fit into tighter spaces. In ciliates and Apicomplexa, it is formed from closely packed vesicles called alveoli. In euglenids, it is formed from protein strips arranged spirally along the length of the body. Examples of protists with a pellicle are the euglenoids and the paramecium, a ciliate. In some protozoa, the pellicle consists of many bacteria that adhere to the surface by their fimbriae or "attachment pili". Thus, attachment pili allow the organisms to remain in the broth, from which they take nutrients, while they congregate near air, where the oxygen concentration is greatest.
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. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi. Protozoa are both herbivores and consumers in the decomposer link of the food chain. They also control bacteria populations and biomass to some extent. On average, Protozoa eat ~ 100 to 1,000 bacteria per hour. Protozoa such as the malaria parasites (Plasmodium spp.), trypanosomes and leishmania, are also important disease causing agents in humans. Protozoa can stimulate OM decomposition, digest cellulose in rumen of cows and termite guts, and play a role in nutrient mobilization.
Some protozoa have life stages alternating between proliferative stages (e.g., trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. Being a cyst enables parasitic species to survive outside of a host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites (Greek, tropho = to nourish), they actively feed. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation. Protozoa can reproduce by binary fission or multiple fission. Some protozoa reproduce sexually, some asexually, while some use a combination, (e.g., Coccidia). An individual protozoan is hermaphroditic.
Protozoa were previously often grouped in the kingdom of Protista, together with the plant-like algae and fungus-like slime molds. As a result of 21st-century systematics, protozoa, along with ciliates, mastigophorans, and apicomplexans, are arranged as animal-like protists. Protozoa are unicellular organisms and are often called the animal-like protists because they subsist entirely on other organisms for food. Most protozoa can move about on their own. Amoebas, paramecia, and trypanosomes are all examples of animal-like protists.
The classification of protozoa has been and remains a problematic area of taxonomy. Where they are available, DNA sequences are used as the basis for classification but for the majority of described protozoa such material is not available. They have been and still are mostly on the basis of their morphology and for the parasitic species their hosts. Protozoa have been divided traditionally on the basis of their means of locomotion.
- Flagellates (e.g., Giardia lamblia)
- Amoeboids (e.g., Entamoeba histolytica)
- Sporozoans (e.g., Plasmodium knowlesi)
- Ciliates (e.g., Balantidium coli)
As a phylum the Protozoa had been divided into four subphyla reflecting the means of locomotion:
- Subphylum Sarcomastigophora
- Subphylum Sporozoa (includes apicomplexans)
- Class Microsporidea
- Subphylum Ciliophora (includes ciliates)
These systems are no longer considered to be valid. For an example of a system of classification of protozoa, see Kudo system.
Some protozoa are human parasites, causing diseases. Examples of human diseases caused by protozoa:
- Chagas disease
- Sleeping Sickness
- Acanthamoeba Keratitis
- Primary Amoebic Meningoencephalitis
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