Protozoa (also protozoan, plural protozoans) is an informal term for single-celled eukaryotes, either free-living or Parasitism, which feed on organic matter such as other or organic tissues and debris.
Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviors, such as motility and predation, and lack a cell wall, as found in plants and many algae. Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by .
In some systems of biological classification, Protozoa is a high-level Taxon. When first introduced in 1818, Protozoa was erected as a taxonomic class,
but in later classification schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom. The seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, , foraminifera, and the parasitic , all of which are classified under Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but a Paraphyly group or evolutionary grade, within which the members of Fungi, Animalia and Chromista are thought to have evolved.
The word "protozoa" (singular protozoon
) was coined in 1818 by zoologist
Georg August Goldfuss, as the Greek equivalent of the German Urthiere
, meaning "primitive, or original animals" ( ‘proto-’ +
‘animal’). Goldfuss created Protozoa as a class containing what he believed to be the simplest animals. Originally, the group included not only single-celled
but also some "lower" multicellular animals, such as
The term Protozoa
is formed from the Ancient Greek
words (), meaning "first", and (), plural of (), meaning "animal".
The use of Protozoa as a formal taxon
has been discouraged by some researchers, mainly because the term implies kinship with animals (Metazoa)
and promotes an arbitrary separation of "animal-like" from "plant-like" organisms.
In 1848, as a result of advancements in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoans such as and were made up of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all Animal (animals).
At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria (mostly ciliates and flagellated algae), and Rhizopoda (Amoeba). The definition of Protozoa as a phylum or sub-kingdom made up of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology" —and the term came into wide use.
As a phylum under Animalia, the Protozoa were firmly rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany.
Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have for photosynthesis, but can also feed on organic matter and are Motility. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms, or living beings, are animals or plants."
As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae (protophyta), which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum.
Six years later, Ernst Haeckel also proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in later work he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.
Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, however, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi.
A variety of multi-kingdom systems were proposed, and Kingdoms Protista and Protoctista became well established in biology texts and curricula.
While many taxonomists have abandoned Protozoa as a high-level group, Thomas Cavalier-Smith has retained it as a kingdom in the various classifications he has proposed. As of 2015, Cavalier-Smith's Protozoa excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, and foraminifera (all members of the SAR supergroup). In its current form, his kingdom Protozoa is a Paraphyly group which includes a common ancestor and most of its descendants, but excludes two important clades that branch within it: the animals and fungi.
Since the protozoa, as traditionally defined, can no longer be regarded as "primitive animals" the terms "protists", "Protista" or "Protoctista" are sometimes preferred. In 2005, members of the Society of voted to change its name to the International Society of .
Protozoa, as traditionally defined, range in size from as little as 1 micrometers
, or more.
Among the largest are the deep-sea–dwelling xenophyophores
, single-celled foraminifera whose shells can reach 20 cm in diameter.
|Plasmodium falciparum (malaria parasite, trophozoite phase)||1-2|
|Massisteria (free-living amoeboid)||2.3–3|
|Bodo saltans (free living Kinetoplastida flagellate)||5-8|
|Plasmodium falciparum (malaria parasite, gametocyte phase)||7-14|
|Trypanosoma cruzi (parasitic kinetoplastid, Chagas disease)||14-24|
|Entamoeba histolytica (parasitic )||15–60|
|Balantidium coli (parasitic ciliate)||50-100|
|Paramecium caudatum (free-living ciliate)||120-330|
|Amoeba proteus (free-living amoebozoan)||220–760|
|Noctiluca scintillans (free-living dinoflagellate)||700–2000|
|Syringammina fragilissima ( amoeboid)||up to|
Free-living protozoans are common and often abundant in fresh, brackish and salt water, as well as other moist environments, such as soils and mosses. Some species thrive in extreme environments such as hot springs
and hypersaline lakes and lagoons.
All protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by forming Microbial cyst
which enable them to remain dormant until conditions improve.
Parasitism and Symbiosis protozoa live on or within other organisms, including and , as well as plants and other single-celled organisms. Some are harmless or beneficial to their host organisms; others may be significant causes of diseases, such as babesia, malaria and toxoplasmosis.
Association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoans such as Trichonympha
inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex
into smaller, more easily digested molecules.
A wide range of protozoans live Commensalism
in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as Trichomonas
, and ciliated protozoa, such as Isotricha
The ciliate subclass Astomatia is made up entirely of mouthless symbionts adapted for life in the guts of annelid worms.
All protozoans are
, deriving nutrients from other organisms, either by ingesting them whole or consuming their organic remains and waste-products. Some protozoans take in food by phagocytosis
, engulfing organic particles with pseudopodia
do), or taking in food through a specialized mouth-like aperture called a cytostome
. Others take in food by osmotrophy
, absorbing dissolved nutrients through their
Parasitic protozoans use a wide variety of feeding strategies, and some may change methods of feeding in different phases of their life cycle. For instance, the malaria parasite Plasmodium feeds by pinocytosis during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cell.
Protozoa may also live as , supplementing a heterotrophic diet with some form of . Some protozoa form close associations with symbiotic photosynthetic algae, which live and grow within the membranes of the larger cell and provide nutrients to the host. Others practice kleptoplasty, stealing from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliate Mesodinium rubrum retains functioning from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. These, in turn, may be passed along to dinoflagellates of the genus Dinophysis , which prey on Mesodinium rubrum but keep the enslaved plastids for themselves. Within Dinophysis, these plastids can continue to function for months.
Organisms traditionally classified as protozoa are abundant in aqueous
environments and soil
, occupying a range of
. The group includes
(which move with the help of whip-like structures called flagella
(which move by using hair-like structures called cilia
(which move by the use of foot-like structures called pseudopodia
). Some protozoa are sessile, and do not move at all.
Unlike plants, fungi and most types of algae, protozoans do not typically have a rigid cell wall
, but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoans, such as the ciliates and
, the cell is supported by a composite membranous envelope called the "pellicle." The pellicle gives some shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. In
, the pellicle is supported by closely packed vesicles called alveoli. In
, it is formed from protein
strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the euglenoids
and the ciliate Paramecium
. In some protozoa, the pellicle hosts Epibiont
bacteria that adhere to the surface by their fimbriae (attachment pili).
[ Protozoa in biological research]
Some protozoa have two-phase life cycles, alternating between proliferative stages (e.g.,
) and dormant Microbial cyst
. 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 periods 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
= 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.
All protozoans reproduce asexually by binary fission or multiple fission. Many protozoan species exchange genetic material by sexual means (typically, through conjugation); however, sexuality is generally decoupled from the process of reproduction, and does not immediately result in increased population.
Although meiotic sex is widespread among present day , it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Due to recent advances in gene detection and other techniques, evidence has been found for some form of meiotic sex in an increasing number of protozoans of ancient lineage that diverged early in eukaryotic evolution.
[Bernstein H, Bernstein C (2013). Bernstein C, Bernstein H eds. Evolutionary Origin and Adaptive Function of Meiosis'. Meiosis. InTech. ] (See eukaryote reproduction.) Thus, such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles Amoebozoa, Giardia lamblia, Leishmania, Plasmodium falciparum biology, Paramecium, Toxoplasma gondii, Trichomonas vaginalis and Trypanosoma brucei.
Historically, the Protozoa were classified as "unicellular animals", as distinct from the Protophyta, single-celled photosynthetic organisms (algae) which were considered primitive plants. Both groups were commonly given the rank of phylum, under the kingdom Protista.
In older systems of classification, the phylum Protozoa was commonly divided into several sub-groups, reflecting the means of locomotion:
Classification schemes differed, but throughout much of the 20th century the major groups of Protozoa included:
, or Flagellate (motile cells equipped with whiplike organelles of locomotion, e.g., Giardia lamblia)
Amoeba (cells that move by extending pseudopodia or Lamellipodium, e.g., Entamoeba histolytica)
Sporozoans, or Sporozoa (parasitic, spore-producing cells, whose adult form lacks organs of motility, e.g., Plasmodium knowlesi)
, or Ciliophora (cells equipped with large numbers of short hairlike organs of locomotion, e.g., Balantidium coli
With the emergence of molecular phylogenetics and tools enabling researchers to directly compare the DNA of different organisms, it became evident that, of the main sub-groups of Protozoa, only the ciliates (Ciliophora) formed a natural group, or Monophyly clade
(that is, a distinct lineage of organisms sharing common ancestry). The other classes or subphyla of Protozoa were all Polyphyly
groups made up of organisms that, despite similarities of appearance or way of life, were not necessarily closely related to one another. In the system of eukaryote
classification currently endorsed by the International Society of Protistologists, members of the old phylum Protozoa have been distributed among a variety of supergroups.
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 pyramid
is important. As predators, they prey upon unicellular
or filamentous algae, bacteria
, and microfungi
. Protozoan species include both
in the decomposer link of the food chain
. They also control bacteria populations and biomass
to some extent.
A number of protozoan
, causing diseases such as malaria
, cryptosporidiosis, trichomoniasis
, Chagas disease
, African trypanosomiasis (sleeping sickness), amoebic dysentery, acanthamoeba keratitis, and primary amoebic meningoencephalitis (naegleriasis).
The protozoan Ophryocystis elektroscirrha is a parasite of butterfly , passed from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to Eclosion, and have shortened lifespans, but parasite levels vary in populations. Infection creates a culling effect, whereby infected migrating animals are less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration.
This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.
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