protozoa
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Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their name comes from the Greek language, Greek wi ...

eukaryote
s, either free-living or
parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), ad ...

parasitic
, that feed on organic matter such as other
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
s or organic tissues and debris. Historically, protozoans were regarded as "one-celled animals", because they often possess
animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
-like behaviours, such as
motility Motility is the ability of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for ...

motility
and
predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...

predation
, and lack a
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
, as found in plants and many
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cel ...
. When first introduced by Georg Goldfuss (originally spelled Goldfuß) in 1818, the taxon Protozoa was erected as a
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
within the Animalia, with the word 'protozoa' meaning "first animals". In later classification schemes it was elevated to a variety of higher ranks, including
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
,
subkingdom In biology, kingdom (Latin: ''regnum'', plural ''regna'') is the second highest taxonomic rank, just below domain (biology), domain. Kingdoms are divided into smaller groups called Phylum, phyla. Traditionally, some textbooks from the United State ...
and
kingdom Kingdom may refer to: Monarchy * A type of monarchy * A realm ruled by: **A king, during the reign of a male monarch **A queen regnant, during the reign of a female monarch Taxonomy * Kingdom (biology), a category in biological taxonomy Arts an ...
, and sometimes included within Protoctista or Protista. The approach of classifying Protozoa within the context of Animalia was widespread in the 19th and early 20th century, but not universal. By the 1970s, it became usual to require that all taxa be
monophyletic 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...
(derived from a common ancestor that would also be regarded as protozoan), and holophyletic (containing all of the known descendants of that common ancestor). The taxon 'Protozoa' fails to meet these standards, and the practices of grouping protozoa with animals, and treating them as closely related, are no longer justifiable. The term continues to be used in a loose way to describe single-celled
protist A protist () is any eukaryotic organism (that is, an organism whose Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common descent, common ancestor (the last eukaryo ...
s (that is, eukaryotes that are not animals,
plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions o ...

plant
s, or
fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

fungi
) that feed by
heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following peri ...
y. Some examples of protozoa are ''
Amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape, primarily by extending and retracting pse ...
'', ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system ...

Paramecium
'', ''
Euglena ''Euglena'' is a genus of Unicellular organism, single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 800 species. Species of ''Eugl ...

Euglena
'' and ''
Trypanosoma ''Trypanosoma'' is a genus of Kinetoplastida, kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasite, parasitic flagellate protozoa. Trypanosoma is part of the phylum Sarcomastigophora. The name is derived from the ...

Trypanosoma
''. Despite awareness that the traditional taxonomic concept of "Protozoa" did not meet contemporary taxonomic standards, some authors have continued to use the name, while applying it to differing scopes of organisms. In a series of classifications by
Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford. His res ...
and collaborators since 1981, the taxon Protozoa was applied to a restricted circumscription of organisms, and ranked as a kingdom. A scheme presented by Ruggiero et al. in 2015, places eight not closely related phyla within Kingdom Protozoa:
Euglenozoa The euglenozoa are a large group of flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like Appendage, appendages called flagellum, flagella. The word ...
,
Amoebozoa Amoebozoa is a major Taxonomy (biology), taxonomic group containing about 2,400 described species of Amoeba, amoeboid protists, often possessing blunt, fingerlike, Pseudopod#Morphology, lobose pseudopods and tubular mitochondrial cristae. In most ...
, Metamonada,
Choanozoa Choanozoa is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineage (ev ...
''sensu'' Cavalier-Smith,
Loukozoa Loukozoa (+ Ancyromonads) (From Greek ''loukos'': groove) is a proposed taxon used in some classifications of eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. ...
,
Percolozoa The Percolozoa are a group of colourless, non-photosynthetic Excavata Excavata is a major supergroup of unicellular organisms belonging to the domain Eukaryota. It was first suggested by Simpson and Patterson in 1999 and introduced by Thoma ...
,
Microsporidia Microsporidia are a group of spore-forming unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indi ...
and
Sulcozoa Podiates (Thomas Cavalier-Smith, Cavalier-Smith, 2012, excl. Ancyromonadida) are a proposed clade containing the Amorphea (incl. Opisthokonta, Amoebozoa, apusomonads and breviates) and the organisms now assigned to the clade CRuMs. Ancyromonadida d ...
. Notably, this approach excludes several major groups of organisms traditionally placed among the protozoa, including the
ciliate The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a d ...

ciliate
s,
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
s,
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class (biology), class of Amoeba, amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm ...
, and the parasitic
apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a sp ...
ns, which were located in other groups such as Alveolata and
Stramenopiles Stramenopile is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineage ( ...
, under the polyphyletic
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
. The Protozoa in this scheme do not form a monophyletic and holophyletic group (
clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell t ...

clade
), but a
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...
group or
evolutionary grade In alpha taxonomy, a grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade A clade (; from grc, , ''klados'', "branch"), also known ...
, because it excludes some descendants of Protozoa, as used in this sense.


History

The word "protozoa" ''(singular ''protozoon'')'' was coined in 1818 by
zoologist Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, embryology, evolution, Biological classi ...
Georg August Goldfuss Georg August Goldfuss (Goldfuß, 18 April 1782 – 2 October 1848) was a German palaeontologist, zoologist and botanist. Goldfuss was born at Thurnau near Bayreuth Bayreuth (, ; bar, Bareid) is a medium-sized town#Germany, town in n ...
(=Goldfuß), as the Greek equivalent of the German ', meaning "primitive, or original animals" (' ‘proto-’ + ' ‘animal’). Goldfuss created Protozoa as a
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
containing what he believed to be the simplest animals. From p. 1008: ''"Erste Klasse. Urthiere. Protozoa."'' (First class. Primordial animals. Protozoa.) ote: each column of each page of this journal is numbered; there are two columns per page./ref> Originally, the group included not only single-celled
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
s but also some "lower"
multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biolo ...
animals, such as
rotifer The rotifers (, from Latin ''wikt:rota#Latin, rota'' "wheel" and ''wikt:-fer#Latin, -fer'' "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, p ...

rotifer
s,
coral Corals are marine invertebrates Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vert ...

coral
s,
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
s,
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella-s ...

jellyfish
,
bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the ...

bryozoa
and . The term ''Protozoa'' is formed from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
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 the design and construction of microscopes and the emergence of a cell theory pioneered by Theodor Schwann and Matthias Jakob Schleiden, Matthias Schleiden, the anatomist and zoologist Karl Theodor Ernst von Siebold, C. T. von Siebold proposed that the bodies of protozoa such as
ciliate The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a d ...

ciliate
s and amoebae consisted of single cells, similar to those from which the
multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biolo ...
tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such Unicellular organism, unicellular forms, to the exclusion of all Animal, metazoa (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 protists and amoebae (Amoeba, amoeboid organisms). The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology". With its increasing visibility, the term 'protozoa' and the discipline of 'protozoology' came into wide use. As a phylum under Animalia, the Protozoa were firmly rooted in a simplistic "two-kingdom" concept 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 ''Euglena'' is a genus of Unicellular organism, single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 800 species. Species of ''Eugl ...

Euglena
'' and ''Dinobryon'' have chloroplasts for photosynthesis, like plants, but can also feed on organic matter and are Motility, motile, like animals. In 1860, John Hogg (biologist), 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, which he combined 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 Tissue (biology), tissues. Despite these proposals, Protozoa emerged as the preferred taxonomic placement for Heterotrophic nutrition, heterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, 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 Kingdom (biology), multi-kingdom systems were proposed, and the Kingdoms Protista and Protoctista became established in biology texts and curricula. While most taxonomists have abandoned Protozoa as a high-level group, Cavalier-Smith used the term with a different circumscription. In 2015, Protozoa ''sensu'' Cavalier-Smith excluded several major groups of organisms traditionally placed among the protozoa (such as ciliates,
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
s and
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class (biology), class of Amoeba, amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm ...
). This and similar concepts of Protozoa are of a
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...
group which does not include all organisms that descended from Protozoa. In this case, the most significant absences were of the animals and fungi. The continued use by some of the 'Protozoa' in its old sense highlights the uncertainty as to what is meant by the word 'Protozoa', the need for disambiguating statements (here, the term 'Protozoa' is used in the sense intended by Goldfuß), and the problems that arise when new meanings are given to familiar taxonomic terms. Since the protozoa can no longer be regarded as "primitive animals" the terms "protists", "Protista" or "Protoctista" are sometimes preferred - even though these too are paraphyletic groups that exclude most of the multicellular descendants of the protozoa. In 2005, members of the Society of Protozoologists voted to change its name to the International Society of Protistologists.


Characteristics


Reproduction

Reproduction in Protozoa can be sexual or asexual. Most Protozoa reproduce Asexual reproduction, asexually through binary fission. Many parasitic Protozoa reproduce both asexually and Sexual reproduction, sexually. However, sexual reproduction is rare among free-living protozoa and it usually occurs when food is scarce or the environment changes drastically. Both isogamy and anisogamy occur in Protozoa with anisogamy being the more common form of sexual reproduction.


Size

Protozoa, as traditionally defined, range in size from as little as 1 micrometers, micrometre to several millimetres, or more. Among the largest are the deep-sea–dwelling xenophyophores, single-celled foraminifera whose shells can reach 20 cm in diameter.


Habitat

Free-living protozoa 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, resting cysts that enable them to remain dormant until conditions improve. Parasitism, Parasitic and Symbiosis, symbiotic protozoa live on or within other organisms, including vertebrates and invertebrates, 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 protozoa such as ''Trichonympha'' and ''Pyrsonympha'' inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex sugars into smaller, more easily digested molecules. A wide range of protozoa live Commensalism, commensally in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as ''Trichomonas'', and ciliated protozoa, such as ''Isotricha'' and ''Entodinium''. The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms.


Feeding

All protozoa are
heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following peri ...
ic, deriving nutrients from other organisms, either by ingesting them whole by phagocytosis or taking up dissolved organic matter or micro-particles (osmotrophy). Phagocytosis may involve engulfing organic particles with pseudopodia (as amoebae do), taking in food through a specialized mouth-like aperture called a cytostome, or using stiffened ingestion organelles Fenchel, T. 1987. Ecology of protozoan: The biology of free-living phagotrophic protists. Springer-Verlag, Berlin. Parasitic protozoa 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 Apicomplexan life cycle, 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 mixotrophs, combining a heterotrophic diet with some form of autotrophy. Some protozoa form close associations with symbiotic photosynthetic algae (zoochlorellae), which live and grow within the membranes of the larger cell and provide nutrients to the host. The algae are not digested, but reproduce and are distributed between division products. The organism may benefit at times by deriving some of its nutrients from the algal endosymbionts or by surviving anoxic conditions because of the oxygen produced by algal photosynthesis. Some protozoans practice kleptoplasty, stealing chloroplasts 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 plastids from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. The symbionts 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.


Motility

Organisms traditionally classified as protozoa are abundant in aqueous environments and soil, occupying a range of trophic levels. The group includes flagellates (which move with the help of undulating and beating flagella). Ciliates (which move by using hair-like structures called cilia) and amoebae (which move by the use of temporary extensions of cytoplasm called pseudopodia). Many protozoa, such as the agents of amoebic meningitis, use both pseudopodia and flagella. Some protozoa attach to the substrate or form cysts so they do not move around (Sessility (zoology), sessile). Most sessile protozoa are able to move around at some stage in the life cycle, such as after cell division. The term 'theront' has been used for actively motile phases, as opposed to 'trophont' or 'trophozoite' that refers to feeding stages.


Walls, Pellicles, scales, and skeletons

Unlike plants, fungi and most types of algae, most protozoa do not have a rigid external
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
, but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoa, such as the ciliates and euglenozoans, the outer membrane of the cell is supported by a cytoskeletal infrastructure, which may be referred to as a "pellicle". The pellicle gives shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. In
ciliate The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a d ...

ciliate
s and Apicomplexa, the pellicle includes a layer of closely packed vesicles called alveoli. In euglenids, the pellicle 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 '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system ...

Paramecium
''. In some protozoa, the pellicle hosts Epibiont, epibiotic bacteria that adhere to the surface by their Pilus#fimbriae, fimbriae (attachment pili).


Life cycle

Some protozoa have two-phase life cycles, alternating between proliferative stages (e.g., trophozoites) and resting Microbial cyst, cysts. As cysts, some protozoa can survive harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen. Encysting 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 excystment. Protozoa mostly reproduce asexually by binary fission or multiple fission. Many protozoa also exchange genetic material by sexual means (typically, through Isogamy#Conjugation, conjugation), but this is generally decoupled from the process of reproduction, and does not immediately result in increased population. As such, sexuality can be optional. Although Origin and function of meiosis, meiotic sex is widespread among present day
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their name comes from the Greek language, Greek wi ...

eukaryote
s, it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Owing to recent advances in gene detection and other techniques, evidence has been found for some form of meiotic sex in an increasing number of protozoa of lineages that diverged early in eukaryotic evolution. (See Eukaryote#Reproduction, eukaryote reproduction.) Such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles ''
Amoebozoa Amoebozoa is a major Taxonomy (biology), taxonomic group containing about 2,400 described species of Amoeba, amoeboid protists, often possessing blunt, fingerlike, Pseudopod#Morphology, lobose pseudopods and tubular mitochondrial cristae. In most ...
'', ''Giardia lamblia'', ''Leishmania'', ''Plasmodium falciparum biology'', ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system ...

Paramecium
'', ''Toxoplasma gondii'', ''Trichomonas vaginalis'' and ''Trypanosoma brucei''.


Classification

Historically, 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: * Flagellates, or Flagellate, Mastigophora (motile cells equipped with whiplike organelles of locomotion, e.g., ''Giardia lamblia'') * Amoeba, Amoebae or Sarcodina (cells that move by extending pseudopodia or Lamellipodium, lamellipodia, e.g., ''Entamoeba histolytica'') * Sporozoa, or Sporozoans (parasitic, spore-producing cells, whose adult form lacks organs of motility, e.g., ''Plasmodium knowlesi'') ** Apicomplexa (now in Alveolata) **
Microsporidia Microsporidia are a group of spore-forming unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indi ...
(now in Fungi) ** Ascetosporea (now in Rhizaria) ** Myxosporidia (now in Cnidaria) * Ciliates, or Ciliophora (cells equipped with large numbers of cilia used for movement and feeding, 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
monophyletic 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...
clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell t ...

clade
, once a few extraneous members (such as ''Stephanopogon'' or protociliates and opalinids were removed. The Mastigophora, Sarcodina, and Sporozoa were Polyphyly, polyphyletic groups. The similarities of appearance and ways of life by which these groups were defined had emerged independently in their members by convergent evolution. In most systems of
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their name comes from the Greek language, Greek wi ...

eukaryote
classification, such as one published by the International Society of Protistologists, members of the old phylum Protozoa have been distributed among a variety of supergroups.


Ecology

Free-living protozoa are found in almost all ecosystems that contain, at least some of the time, free water. They have a critical role in the mobilization of nutrients in natural ecosystems. Their role is best conceived within the context of the microbial food web in which they include the most important bacterivores. In part, they facilitate the transfer of bacterial and algal production to successive Trophic pyramid, trophic levels, but also they solubilize the nutrients within microbial biomass, allowing stimulation of microbial growth. As consumers, protozoa prey upon unicellular or filamentous algae, bacteria, microfungi, and micro-carrion. In the context of older ecological models of the Fauna (animals)#Microfauna, micro- and Fauna (animals)#Meiofauna, meiofauna, protozoa may be a food source for microinvertebrates. That most species of free-living protozoa have been found in similar habitats in all parts of the globe is an observation that dates back to the 19th Century (e.g. Shewiakoff, Schewiakoff). In the 1930's, Lourens Baas Becking asserted "Everything is everywhere, but the environment selects". This has been restated and explained, especially by Tom Fenchel and Bland Findlay and methodically explored and affirmed at least in respect of morphospecies of free-living flagellates. The widespread distribution of microbial is explained by the ready dispersal of physically small organisms. While Baas Becking's hypothesis is not universally accepted, the natural microbial world is undersampled, and this will favour conclusions of endemism.


Disease

A number of protozoan pathogens are human parasites, causing diseases such as malaria (by ''Plasmodium''), amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, African trypanosomiasis (sleeping sickness), Acanthamoeba keratitis, ''Acanthamoeba'' keratitis, and primary amoebic meningoencephalitis (naegleriasis). Protozoa include the agents of the most significant entrenched infectious diseases, particularly malaria, and, historically, sleeping sickness. The protozoon ''Ophryocystis elektroscirrha'' is a parasite of butterfly larvae, passed from Vertical transmission, female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to Eclosion, eclose, 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. List of protozoan diseases in humans: List of protozoan diseases in humans:


References


Bibliography

; General * Dogiel, V. A., revised by J.I. Poljanskij and E. M. Chejsin. ''General Protozoology'', 2nd ed., Oxford University Press, 1965. * Hausmann, K., N. Hulsmann. ''Protozoology''. Thieme Verlag; New York, 1996. * Kudo, R.R. ''iarchive:protozoology1954kudo, Protozoology''. Springfield, Illinois: C.C. Thomas, 1954; 4th ed. * Manwell, R.D. ''Introduction to Protozoology'', second revised edition, Dover Publications Inc., New York, 1968. * Roger Anderson, O. ''Comparative protozoology: ecology, physiology, life history''. Berlin [etc.]: Springer-Verlag, 1988. * Sleigh, M. ''The Biology of Protozoa''. E. Arnold: London, 1981. ; Identification * Jahn, T.L.- Bovee, E.C. & Jahn, F.F. ''How to Know the Protozoa''. Wm. C. Brown Publishers, Div. of McGraw Hill, Dubuque, Iowa, 1979; 2nd ed. * Lee, J.J., Leedale, G.F. & Bradbury, P. ''An Illustrated Guide to the Protozoa''. Lawrence, Kansas, U.S.A: Society of Protozoologists, 2000; 2nd ed. * Patterson, D.J. ''Free-Living Freshwater Protozoa. A Colour Guide''. Manson Publishing; London, 1996. * Patterson, D.J., M.A. Burford. ''A Guide to the Protozoa of Marine Aquaculture Ponds''. CSIRO Publishing, 2001. ; Morphology * Harrison, F.W., Corliss, J.O. (ed.). 1991. ''Microscopic Anatomy of Invertebrates'', vol. 1, Protozoa. New York: Wiley-Liss, 512 pp. * Pitelka, D. R. 1963
''Electron-Microscopic Structure of Protozoa''
Pergamon Press, Oxford. ; Physiology and biochemistry * Nisbet, B. 1984. ''Nutrition and feeding strategies in Protozoa.'' Croom Helm Publ., London, 280 pp. * Coombs, G.H. & North, M. 1991. ''Biochemical protozoology''. Taylor & Francis, London, Washington. * Laybourn-Parry J. 1984. ''A Functional Biology of Free-Living Protozoa''. Berkeley, California: University of California Press. * Levandowski, M., S.H. Hutner (eds). 1979. ''Biochemistry and physiology of protozoa''. Volumes 1, 2, and 3. Academic Press: New York, NY; 2nd ed. * Sukhareva-Buell, N.N. 2003. ''Biologically active substances of protozoa''. Dordrecht: Kluwer. ; Ecology * Capriulo, G.M. (ed.). 1990. ''Ecology of Marine Protozoa.'' Oxford Univ. Press, New York. * Darbyshire, J.F. (ed.). 1994. ''Soil Protozoa.'' CAB International: Wallingford, U.K. 2009 pp. * Laybourn-Parry, J. 1992. ''Protozoan plankton ecology.'' Chapman & Hall, New York. 213 pp. * Fenchel, T. 1987. ''Ecology of protozoan: The biology of free-living phagotrophic protists.'' Springer-Verlag, Berlin. 197 pp. ; Parasitology * Kreier, J.P. (ed.). 1991–1995. ''Parasitic Protozoa'', 2nd ed. 10 vols (1-3 coedited by Baker, J.R.). Academic Press, San Diego, California

; Methods * Lee, J. J., & Soldo, A. T. (1992). ''Protocols in protozoology''. Kansas, USA: Society of Protozoologists, Lawrence


External links

* {{Authority control 1670s in science Obsolete eukaryote taxa Paraphyletic groups Microscopic discoveries by Antonie van Leeuwenhoek