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Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
s, either free-living or
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
, that feed on organic matter such as other
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s or organic tissues and debris. Historically, protozoans were regarded as "one-celled animals", because they often possess
animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
-like behaviours, such as
motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
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 ...
, and lack a
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
, as found in plants and many
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
. 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 differentl ...
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: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...
,
subkingdom In biology, a kingdom is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla. Traditionally, some textbooks from the United States and Canada used a system of six kingdoms (Animalia, Plant ...
and
kingdom Kingdom commonly refers to: * A monarchy ruled by a king or queen * Kingdom (biology), a category in biological taxonomy Kingdom may also refer to: Arts and media Television * ''Kingdom'' (British TV series), a 2007 British television drama s ...
, and sometimes included within Protoctista or
Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...
. 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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
(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 cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s (that is, eukaryotes that are not animals,
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s, or
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
) that feed by
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
y. Some examples of protozoa are ''
Amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
'', ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'', ''
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 200 species. Species of ''Eug ...
'' and ''
Trypanosoma ''Trypanosoma'' is a genus of kinetoplastids (class Trypanosomatidae), a monophyletic group of unicellular parasitic flagellate protozoa. Trypanosoma is part of the phylum Sarcomastigophora. The name is derived from the Greek ''trypano-'' (bore ...
''. 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, 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 research has led to discov ...
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 Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by three major clades, i.e., Kinetoplastea, Di ...
,
Amoebozoa Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae. In traditional and currently no longer supported classi ...
, Metamonada,
Choanozoa Choanozoa is a clade of opisthokont eukaryotes consisting of the choanoflagellates (Choanoflagellatea) and the animals (Animalia, Metazoa). The sister-group relationship between the choanoflagellates and animals has important implications for t ...
''sensu'' Cavalier-Smith,
Loukozoa Loukozoa (+ Ancyromonads) (From Greek ''loukos'': groove) is a proposed taxon used in some classifications of eukaryotes, consisting of the Metamonada and Malawimonadea. Ancyromonads are closely related to this group, as sister of the entire grou ...
,
Percolozoa The Percolozoa are a group of colourless, non-photosynthetic Excavata, including many that can transform between amoeboid, flagellate, and cyst stages. Characteristics Most Percolozoa are found as bacterivores in soil, fresh water and occasional ...
,
Microsporidia Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or prot ...
and Sulcozoa. Notably, this approach excludes several major groups of organisms traditionally placed among the protozoa, including the
ciliate The ciliates are a group of alveolates 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 ...
s,
dinoflagellate The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...
s,
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...
, 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 that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. T ...
ns, which were located in other groups such as Alveolata and
Stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...
, under the polyphyletic
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all protists whose plastids contain chlorophyll ''c'', such ...
. 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 organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
), but a
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
group or
evolutionary grade 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 strictly phylogenetic unit. Definition An evolutionary grade is a group of ...
, because it excludes some descendants of Protozoa, as used in this sense.


Science

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 clas ...
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. He was educated at Erlangen, where he graduated PhD in 1804 and became profes ...
(=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 differentl ...
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'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s but also some "lower"
multicellular A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
animals, such as
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...
s,
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
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 organisms that have bodies full of pores and channels allowing water to circulate through t ...
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- ...
,
bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...
and
polychaete worms Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are m ...
. The term ''Protozoa'' is formed from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
words (), meaning "first", and (), plural of (), meaning "animal". The use of Protozoa as a formal
taxon In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
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 In biology, cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are made up of Cell (biology), cells, that they are the basic structural/organizational unit of all organisms, and that all cell ...
pioneered by
Theodor Schwann Theodor Schwann (; 7 December 181011 January 1882) was a German physician and physiologist. His most significant contribution to biology is considered to be the extension of cell theory to animals. Other contributions include the discovery of ...
and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoa such as
ciliate The ciliates are a group of alveolates 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 ...
s and
amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
e consisted of single cells, similar to those from which the
multicellular A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such
unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
forms, to the exclusion of all
metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
(animals). At the same time, he raised the group to the level of a
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...
containing two broad classes of microorganisms:
Infusoria Infusoria are minute freshwater life forms including ciliates, euglenoids, protozoa, unicellular algae and small invertebrates. Some authors (e.g., Bütschli) used the term as a synonym for Ciliophora. In modern formal classifications, the term i ...
(mostly
ciliate The ciliates are a group of alveolates 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 ...
s) and
flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and their ...
s (flagellated
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s) and amoebae ( amoeboid organisms). The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist
Otto Bütschli Johann Adam Otto Bütschli (3 May 1848 – 2 February 1920) was a German zoologist and professor at the University of Heidelberg. He specialized in invertebrates and insect development. Many of the groups of protists were first recognized by him. ...
—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 200 species. Species of ''Eug ...
'' and ''
Dinobryon ''Dinobryon'' is a type of microscopic algae. It is one of the 22 genera in the family Dinobryaceae. ''Dinobryon'' are mixotrophs, capable of obtaining energy and carbon through photosynthesis and phagotrophy of bacteria. The genus comprises at ...
'' have
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s for
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
, like plants, but can also feed on organic matter and are
motile Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
, like animals. 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, 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 Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist and artist. He discovered, described and named thousands of new sp ...
also proposed a third kingdom of life, which he named
Protista A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...
. 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 A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
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 Herbert Faulkner Copeland (May 21, 1902 – October 15, 1968) was an American biologist who contributed to the theory of biological kingdoms. He grouped unicellular organisms into 2 large kingdoms: the Monera kingdom and the Protista kingdom. ...
, Robert H. Whittaker and
Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolutionary biologist, and was the primary modern proponent for the significance of symbiosis in evolution. Historian Jan Sapp has said that "Lynn Ma ...
, 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 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 δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...
s and
foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular Ectoplasm (cell biology), ectoplasm for catching food and ot ...
). This and similar concepts of Protozoa are of a
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
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. Some authors classify Protozoa as a subgroup of mostly motile
Protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s. Others class any unicellular eukaryotic microorganism as a Protist, and make no reference to 'Protozoa'. In 2005, members of the Society of
Protozoologist Protozoology is the study of protozoa, the "animal-like" (i.e., motile and heterotrophic) protists. The Protozoa are considered to be a subkingdom of Protista. They are free-living organisms that are found in almost every habitat. All humans have pr ...
s voted to change its name to the International Society of Protistologists.


Characteristics


Reproduction

Reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
in Protozoa can be sexual or asexual. Most Protozoa reproduce asexually through
binary fission Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that t ...
. Many parasitic Protozoa reproduce both asexually and 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 Isogamy is a form of sexual reproduction that involves gametes of the same morphology (indistinguishable in shape and size), found in most unicellular eukaryotes. Because both gametes look alike, they generally cannot be classified as male or fe ...
and
anisogamy Different forms of anisogamy: A) anisogamy of motile cells, B) egg_cell.html"_;"title="oogamy_(egg_cell">oogamy_(egg_cell_and_sperm_cell),_C)_anisogamy_of_non-motile_cells_(egg_cell_and_spermatia)..html" ;"title="egg_cell_and_sperm_cell.html" ;" ...
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
micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
to several
millimetre 330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter. The millimetre (American and British English spelling differences#-re, -er, ...
s, or more. Among the largest are the deep-sea–dwelling
xenophyophores Xenophyophorea is a clade of foraminiferans. Members of this class are multinucleate unicellular organisms found on the ocean floor throughout the world's oceans, at depths of . They are a kind of foraminiferan that extract minerals from their s ...
, 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 resting cysts that enable them to remain dormant until conditions improve.
Parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
and
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
protozoa live on or within other organisms, including
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s and
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s, 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 ''Babesia'', also called ''Nuttallia'', is an apicomplexan parasite that infects red blood cells and is transmitted by ticks. Originally discovered by the Romanian bacteriologist Victor Babeș in 1888, over 100 species of ''Babesia'' have since ...
,
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...
and
toxoplasmosis Toxoplasmosis is a parasitic disease caused by ''Toxoplasma gondii'', an apicomplexan. Infections with toxoplasmosis are associated with a variety of neuropsychiatric and behavioral conditions. Occasionally, people may have a few weeks or months ...
. Association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoa such as ''
Trichonympha ''Trichonympha'' is a genus of single-celled, anaerobic parabasalids of the order Hypermastigia that is found exclusively in the hindgut of lower termites and wood roaches. ''Trichonympha''’s bell shape and thousands of flagella make it an ea ...
'' and ''
Pyrsonympha ''Pyrsonympha'' is a genus of 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 Thomas Cavalier-Smith in 2002 as a for ...
'' inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
s into smaller, more easily digested molecules. A wide range of protozoa live commensally in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as ''
Trichomonas ''Trichomonas'' is a genus of anaerobic excavate parasites of vertebrates. It was first discovered by Alfred François Donné in 1836 when he found these parasites in the pus of a patient suffering from vaginitis, an inflammation of the vagina. ...
'', 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 (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
ic, deriving nutrients from other organisms, either by ingesting them whole by
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
or taking up dissolved organic matter or micro-particles (
osmotrophy Osmotrophy is a feeding mechanism involving the movement of dissolved organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic microorganisms use osmotrophy to derive some of their energy. ...
).
Phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
may involve engulfing organic particles with pseudopodia (as
amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
e do), taking in food through a specialized mouth-like aperture called a
cytostome A cytostome (from ''cyto-'', cell and ''stome-'', mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuole ...
, or using stiffened ingestion organellesFenchel, 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 ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a blood-feeding insect host which then injects parasites into a vert ...
'' feeds by
pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell me ...
during its immature
trophozoite A trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria-causing ''Plasmodium falciparum'' and those of the '' Giardia'' group. (The complement of the trophozo ...
stage of life (ring phase), but develops a dedicated feeding
organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
(cytostome) as it matures within a host's red blood cell. Protozoa may also live as
mixotroph A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...
s, combining a heterotrophic diet with some form of
autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...
y. 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 Kleptoplasty or kleptoplastidy is a symbiotic phenomenon whereby plastids, notably chloroplasts from algae, are sequestered by host organisms. The word is derived from ''Kleptes'' (κλέπτης) which is Greek for thief. The alga is eaten norma ...
, stealing
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliate ''
Mesodinium rubrum ''Mesodinium rubrum'' (or ''Myrionecta rubra'') is a species of ciliates. It constitutes a plankton community and is found throughout the year, most abundantly in spring and fall, in coastal areas. Although discovered in 1908, its scientific im ...
'' retains functioning
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosy ...
s 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 ''Dinophysis'' is a genus of dinoflagellatesAlgaeBase''Dinophysis'' Ehrenberg, 1839/ref> common in tropical, temperate, coastal and oceanic waters.Hallegraeff, G.M., Lucas, I.A.N. 1988: The marine dinoflagellate genus Dinophysis (Dinophyceae): ph ...
'', 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 An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be rep ...
environments and
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
, occupying a range of
trophic level The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it i ...
s. The group includes
flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and their ...
s (which move with the help of undulating and beating
flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...
).
Ciliate The ciliates are a group of alveolates 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 ...
s (which move by using hair-like structures called
cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
) and
amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
e (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 (
sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...
). 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. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
, but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoa, such as the ciliates and
euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by three major clades, i.e., Kinetoplastea, Di ...
ns, 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 alveolates 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 ...
s and
Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. T ...
, the pellicle includes a layer of closely packed vesicles called alveoli. In
euglenid Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typical ...
s, the pellicle is formed from
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the
euglenoids Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typica ...
and the ciliate ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
''. In some protozoa, the pellicle hosts
epibiotic An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An epi ...
bacteria that adhere to the surface by their fimbriae (attachment pili).


Life cycle

Some protozoa have two-phase life cycles, alternating between proliferative stages (e.g.,
trophozoite A trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria-causing ''Plasmodium falciparum'' and those of the '' Giardia'' group. (The complement of the trophozo ...
s) and resting
cysts A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubble) ...
. 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
trophozoite A trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria-causing ''Plasmodium falciparum'' and those of the '' Giardia'' group. (The complement of the trophozo ...
s (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 Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that t ...
or multiple fission. Many protozoa also exchange genetic material by sexual means (typically, through
conjugation Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the chang ...
), but this is generally decoupled from the process of reproduction, and does not immediately result in increased population. Thus, sexuality can be optional. Although meiotic sex is widespread among present day
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
s, it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Owing to recent advances in
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
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.) 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 taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae. In traditional and currently no longer supported classi ...
'', ''
Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus '' Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. ...
'', ''
Leishmania ''Leishmania'' is a parasitic protozoan, a single-celled organism of the genus '' Leishmania'' that are responsible for the disease leishmaniasis. They are spread by sandflies of the genus ''Phlebotomus'' in the Old World, and of the genus '' ...
'', ''
Plasmodium falciparum biology ''Plasmodium falciparum'' is a unicellular protozoan parasite of humans, and the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mosquito and causes the di ...
'', ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'', ''
Toxoplasma gondii ''Toxoplasma gondii'' () is an obligate intracellular parasitic protozoan (specifically an apicomplexan) that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but felids, such as d ...
'', ''
Trichomonas vaginalis ''Trichomonas vaginalis'' is an anaerobic, flagellated protozoan parasite and the causative agent of a sexually transmitted disease called trichomoniasis. It is the most common pathogenic protozoan that infects humans in industrialized countries ...
'' and ''
Trypanosoma brucei ''Trypanosoma brucei'' is a species of parasitic Kinetoplastida, kinetoplastid belonging to the genus ''Trypanosoma'' that is present in sub-Saharan Africa. Unlike other protozoan parasites that normally infect blood and tissue cells, it is exclus ...
''.


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: *
Flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and their ...
s, or
Mastigophora A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and th ...
(motile cells equipped with whiplike organelles of locomotion, e.g., ''
Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus '' Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. ...
'') * Amoebae or Sarcodina (cells that move by extending pseudopodia or
lamellipodia The lamellipodium (plural lamellipodia) (from Latin ''lamella'', related to ', "thin sheet", and the Greek radical ''pod-'', "foot") is a cytoskeletal protein actin projection on the leading edge of the cell. It contains a quasi-two-dimensional ...
, e.g., ''
Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 million people worldwid ...
'') *
Sporozoa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. The ...
, or Apicomplexa or Sporozoans (parasitic, spore-producing cells, whose adult form lacks organs of motility, e.g., ''
Plasmodium knowlesi ''Plasmodium knowlesi'' is a parasite that causes malaria in humans and other primates. It is found throughout Southeast Asia, and is the most common cause of human malaria in Malaysia. Like other ''Plasmodium'' species, ''P. knowlesi'' has a li ...
'') **
Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. T ...
(now in Alveolata) **
Microsporidia Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or prot ...
(now in
Fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
) **
Ascetosporea The Ascetosporea are a group of eukaryotes that are parasites of animals, especially marine invertebrates. The two groups, the haplosporids and paramyxids, are not particularly similar morphologically, but consistently group together on molecula ...
(now in
Rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foramini ...
) **
Myxosporidia Myxosporea is a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid but sometimes a bryozoan) ...
(now in
Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...
) *
Ciliate The ciliates are a group of alveolates 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 ...
s, or Ciliophora (cells equipped with large numbers of cilia used for movement and feeding, e.g. '' Balantidium coli'') With the emergence of
molecular phylogenetics Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
, once a few extraneous members (such as ''Stephanopogon'' or protociliates and
opalinids The opalines are a small group of peculiar heterokonts, currently assigned to the family Opalinidae, in the order Slopalinida. Their name is derived from the opalescent appearance of these microscopic organisms when illuminated with full sunli ...
) were removed. The Mastigophora, Sarcodina, and Sporozoa were
polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...
groups. The similarities of appearance and ways of life by which these groups were defined had emerged independently in their members by
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
. In most systems of
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
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 levels The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it i ...
, but also they solubilize the nutrients within microbial biomass, allowing stimulation of microbial growth. As consumers, protozoa prey upon
unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
or
filamentous algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
, microfungi, and micro-carrion. In the context of older ecological models of the
micro- ''Micro'' (Greek letter μ ( U+03BC) or the legacy symbol µ (U+00B5)) is a unit prefix in the metric system denoting a factor of 10−6 (one millionth). Confirmed in 1960, the prefix comes from the Greek ('), meaning "small". The symbol for th ...
and
meiofauna Meiobenthos, also called meiofauna, are small benthic invertebrates that live in both marine and fresh water environments. The term ''meiofauna'' loosely defines a group of organisms by their size, larger than microfauna but smaller than macrof ...
, protozoa may be a food source for
microinvertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate s ...
s. 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. Schewiakoff). In the 1930s,
Lourens Baas Becking Lourens Gerhard Marinus Baas Becking (4 January 1895 in Deventer – 6 January 1963 in Canberra, Australia) was a Dutch botanist and microbiologist. He is known for the Baas Becking hypothesis, which he originally formulated as ''"Everything ...
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
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s are
human parasite Human parasites include various protozoa and worms. Human parasites are divided into endoparasites, which cause infection inside the body, and ectoparasites, which cause infection superficially within the skin. The cysts and eggs of endoparasit ...
s, causing diseases such as
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...
(by ''
Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a blood-feeding insect host which then injects parasites into a vert ...
''),
amoebiasis Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba ''Entamoeba histolytica''. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, loss of weight, colonic ulce ...
,
giardiasis Giardiasis is a parasitic disease caused by ''Giardia duodenalis'' (also known as ''G. lamblia'' and ''G. intestinalis''). Infected individuals who experience symptoms (about 10% have no symptoms) may have diarrhea, abdominal pain, and weight los ...
,
toxoplasmosis Toxoplasmosis is a parasitic disease caused by ''Toxoplasma gondii'', an apicomplexan. Infections with toxoplasmosis are associated with a variety of neuropsychiatric and behavioral conditions. Occasionally, people may have a few weeks or months ...
,
cryptosporidiosis Cryptosporidiosis, sometimes informally called crypto, is a parasitic disease caused by ''Cryptosporidium'', a genus of protozoan parasites in the phylum Apicomplexa. It affects the distal small intestine and can affect the respiratory tract in ...
,
trichomoniasis Trichomoniasis (trich) is an infectious disease caused by the parasite ''Trichomonas vaginalis''. About 70% of affected people do not have symptoms when infected. When symptoms occur, they typically begin 5 to 28 days after exposure. Symptoms ca ...
,
Chagas disease Chagas disease, also known as American trypanosomiasis, is a tropical parasitic disease caused by ''Trypanosoma cruzi''. It is spread mostly by insects in the subfamily ''Triatominae'', known as "kissing bugs". The symptoms change over the cou ...
,
leishmaniasis Leishmaniasis is a wide array of clinical manifestations caused by parasites of the trypanosome genus ''Leishmania''. It is generally spread through the bite of phlebotomine sandflies, ''Phlebotomus'' and ''Lutzomyia'', and occurs most freq ...
,
African trypanosomiasis African trypanosomiasis, also known as African sleeping sickness or simply sleeping sickness, is an insect-borne parasitic infection of humans and other animals. It is caused by the species ''Trypanosoma brucei''. Humans are infected by two typ ...
(sleeping sickness), ''Acanthamoeba'' keratitis, and
primary amoebic meningoencephalitis Naegleriasis (also known as primary amoebic meningoencephalitis; PAM) is an almost invariably fatal infection of the brain by the free-living unicellular eukaryote ''Naegleria fowleri''. Symptoms are meningitis-like and include headache, fever ...
(naegleriasis). Protozoa include the agents of the most significant entrenched infectious diseases, particularly
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...
, and, historically, sleeping sickness. The protozoon '' Ophryocystis elektroscirrha'' is a parasite of
butterfly Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...
e, passed from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. Infection creates a
culling In biology, culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is the process of removing or segregating animals from a breeding stock based on a specific tr ...
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:


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. ''
Protozoology Protozoology is the study of protozoa, the "animal-like" (i.e., motile and heterotrophic) protists. The Protozoa are considered to be a subkingdom of Protista. They are free-living organisms that are found in almost every habitat. All humans have pr ...
''. 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 tc. 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