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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 organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s, either free-living or
parasitic Parasitism is a Symbiosis, close 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), adapted structurally to this w ...

parasitic
, that feed on organic matter such as other
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
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 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 ...

animal
-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 (motility), sessility, the ...

motility
and
predation Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

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 convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
. 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 abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken fr ...
,
subkingdom In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
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 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 grou ...
(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 Eukaryotes () are 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 c ...
s (that is, eukaryotes that are not animals,
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s, or
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
) that feed by
heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...
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 A unicellular organism, also known as a single-celled organism, is an organism ...
'', ''
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 Ocean, marine environments ...

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 Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including th ...

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, Royal Society, FRS, Royal Society of Canada, FRSC, Natural Environment Research Council, NERC Professorial Fellow (21 October 1942 - 19 March 2021), was a Professor of Evolutionary Biology in the Department of Zoolog ...
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 organism In biology Biology is the natural science that studies life and living org ...
,
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 Thom ...
,
Microsporidia Microsporidia are a group of spore )'', growing on a thinning, thinned hybrid black poplar ''(populus, Populus x canadensis)''. The last stage of the moss#Life cycle, moss lifecycle is shown, where the sporophytes are visible before dispersi ...
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 protozoan 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 c ...

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 Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
, 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 (), 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 (evolution), lineal descendants - ...
, 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 organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
), but a
paraphyletic In taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only ...
group or
evolutionary grade In , a grade is a taxon united by a level of morphological or physiological Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of s ...
, 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 Biology is the natural science that studies life and living organisms, including their anatomy, physical st ...
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 Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
s but also some "lower"
multicellular A multicellular organism is an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular ...
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, ...

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 ver ...

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 In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
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 cells ...
pioneered by
Theodor Schwann Theodor Schwann (; 7 December 181011 January 1882) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see ...
and
Matthias Schleiden Matthias Jakob Schleiden (; 1804–1881) was a German botanist Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific m ...

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 protozoan 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 c ...

ciliate
s and
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 A unicellular organism, also known as a single-celled organism, is an organism ...

amoeba
e consisted of single cells, similar to those from which the
multicellular A multicellular organism is an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular ...
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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemica ...
forms, to the exclusion of all
metazoa Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Bioch ...

metazoa
(animals). At the same time, he raised the group to the level of a
phylum In biology, a phylum (; plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken fr ...
containing two broad classes of microorganisms:
Infusoria Infusoria is a collective term for minute aquatic creatures such as ciliate The ciliates are a group of protozoan Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eu ...
(mostly ciliates) and flagellated protists and amoebae (). The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist —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
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s for
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
, 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 (motility), sessility, the ...

motile
, 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 Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that stud ...

Ernst Haeckel
also proposed a third kingdom of life, which he named
Protista A protist () is any eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...
. 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 (; from Ancient Greek "other" and "nutrition") 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 prim ...
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 kingdom (biology), kingdoms. he grouped unicellular organisms into 2 large kingdoms: the Monera kingdom and the Pro ...
, Robert H. Whittaker and
Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic ...

Lynn Margulis
, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi. A variety of multi-kingdom systems were proposed, and 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 Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
). This and similar concepts of Protozoa are of a
paraphyletic In taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only ...
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
Protozoologist Protozoology is the study of protozoa, the "animal-like" (i.e., Motility, 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 huma ...
s voted to change its name to the International Society of Protistologists.


Characteristics


Reproduction

Reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism () is any organic, life, living system that functions as an indivi ...

Reproduction
in Protozoa can be sexual or asexual. Most Protozoa reproduce asexually through
binary fission Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics Digital electronics is a field of electronics The field of electronics is a branch of physics and electrical engineeri ...

binary fission
. 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 Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism ...

isogamy
and
anisogamy Anisogamy (also called heterogamyHeterogamy is a term applied to a variety of distinct phenomena in different scientific domains. Usually having to do with some kind of difference, "hetero", in reproduction, "gamy". See below for more specific sen ...

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
micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental orga ...

micrometre
to several
millimetre 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its deriveds scales. The Microwave are in-between 1 meter to 1 millimeter. The millimetre (American and British English spelling differences#-re, ...
s, 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 resting cysts that enable them to remain dormant until conditions improve.
Parasitic Parasitism is a Symbiosis, close 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), adapted structurally to this w ...

Parasitic
and
symbiotic Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...

symbiotic
protozoa live on or within other organisms, including
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s and
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
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, over 100 species of ''Babesia'' have since been ide ...
,
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign fo ...

malaria
and
toxoplasmosis Toxoplasmosis is a parasitic disease A parasitic disease, also known as parasitosis, is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplicati ...
. 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
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosacc ...

sugar
s into smaller, more easily digested molecules. A wide range of protozoa live in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as ''
Trichomonas ''Trichomonas'' is a genus of anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also re ...

Trichomonas
'', and ciliated protozoa, such as '' Isotricha'' and ''''. 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 Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...
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 Endocytosis is a cellular process i ...

phagocytosis
or taking up dissolved organic matter or micro-particles (
osmotrophy Osmotrophy is a feeding mechanism involving the movement of dissolved Organic compound, organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic Microorganism, microorganisms use osmotrophy t ...
).
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 Endocytosis is a cellular process i ...

Phagocytosis
may involve engulfing organic particles with
pseudopodia A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryote, eukaryotic cell membrane that is developed in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of m ...
(as
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 A unicellular organism, also known as a single-celled organism, is an organism ...

amoeba
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 Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving ri ...
, 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 ''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 Hematophagy, blood-feeding insect host (biology), host which then inj ...

Plasmodium
'' feeds by
pinocytosisImage:Pinocytosis.svg, 250px, Pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell ...

pinocytosis
during its immature
trophozoiteA trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria cau ...
stage of life (ring phase), but develops a dedicated feeding
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
(cytostome) as it matures within a host's red blood cell. Protozoa may also live as
mixotrophA 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 com ...
s, combining a heterotrophic diet with some form of
autotroph An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
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 plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other euka ...
, stealing
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
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 imp ...

Mesodinium rubrum
'' retains functioning
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ' ...
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'', 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 Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute an ...
environments and
soil Soil is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, comp ...

soil
, occupying a range of
trophic level The trophic level of an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular int ...
s. The group includes
flagellate A flagellate is a cell or organism with one or more whip File:Cat o' nine.JPG, upright=1.35, A leather cat o' nine tails pictured with a U.S dollar bill for size comparison. '' reins, featuring a quirt at the end of the ''romal'' A whip is ...
s (which move with the help of undulating and beating
flagella A flagellum (; ) is a hairlike appendage that protrudes from a wide range of microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and ...

flagella
).
Ciliate The ciliates are a group of protozoan 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 c ...

Ciliate
s (which move by using hair-like structures called
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

cilia
) and
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 A unicellular organism, also known as a single-celled organism, is an organism ...

amoeba
e (which move by the use of temporary extensions of cytoplasm called
pseudopodia A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryote, eukaryotic cell membrane that is developed in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of m ...
). 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). 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
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 ...
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 protozoan 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 c ...

ciliate
s and
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 ...
, 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 flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is ...
s, the pellicle is formed from
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

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, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and Ocean, marine environments ...

Paramecium
''. In some protozoa, the pellicle hosts epibiotic 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.,
trophozoiteA trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria cau ...
s) and resting
cysts A cyst is a closed sac, having a distinct envelope and division Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multiplication *Division algorithm, a method for computing the result of mathematical division ...
. 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
trophozoiteA trophozoite (G. ''trope'', nourishment + ''zoon'', animal) is the activated, feeding stage in the life cycle of certain protozoa such as malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria cau ...
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 In mathematics and digital electronics Digital electronics is a field of electronics The field of electronics is a branch of physics and electrical engineeri ...

binary fission
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 change ...

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 meiotic sex is widespread among present day
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
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 Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

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 .) 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 ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like ...

Giardia lamblia
'', ''
Leishmania ''Leishmania'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viru ...
'', '' Plasmodium falciparum biology'', ''
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 Ocean, marine environments ...

Paramecium
'', ''
Toxoplasma gondii ''Toxoplasma gondii'' () is an obligate intracellular parasitic protozoan Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organisms ...

Toxoplasma gondii
'', ''
Trichomonas vaginalis ''Trichomonas vaginalis'' is an Anaerobic organism, anaerobic, flagellated protozoan parasite and the causative agent of trichomoniasis. It is the most common pathogenic protozoan infection of humans in industrialized countries. Infection rates i ...

Trichomonas vaginalis
'' and ''
Trypanosoma brucei ''Trypanosoma brucei'' is a species of parasitic kinetoplastid Kinetoplastida (or Kinetoplastea, as a class) is a group of flagellated A flagellum (; plural: flagella) is a lash-like appendage that protrudes from the soma (biology), cell ...

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: *
Flagellate A flagellate is a cell or organism with one or more whip File:Cat o' nine.JPG, upright=1.35, A leather cat o' nine tails pictured with a U.S dollar bill for size comparison. '' reins, featuring a quirt at the end of the ''romal'' A whip is ...
s, or
Mastigophora 's ''Kunstformen der Natur, Artforms of Nature'', 1904 (''Giardia lamblia'') (''Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like appendages called flagellum, flagella. The word ''flagellate'' also describes a partic ...
(motile cells equipped with whiplike organelles of locomotion, e.g., ''
Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like ...

Giardia lamblia
'') * (cells that move by extending
pseudopodia A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryote, eukaryotic cell membrane that is developed in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of m ...
or
lamellipodia The lamellipodium (plural lamellipodia) (from Latin ', "thin sheet"; ''pod'', "foot") is a cytoskeleton, cytoskeletal actin, protein actin projection on the leading edge of the cell (biology), cell. It contains a quasi-two-dimensional actin mesh ...
, e.g., ''
Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic organism, 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 mill ...

Entamoeba histolytica
'') *
Sporozoa The Apicomplexa (also called Apicomplexia) are a large 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, grammatica ...
, 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 Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experie ...
'') **
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 ...
(now in Alveolata) **
Microsporidia Microsporidia are a group of spore )'', growing on a thinning, thinned hybrid black poplar ''(populus, Populus x canadensis)''. The last stage of the moss#Life cycle, moss lifecycle is shown, where the sporophytes are visible before dispersi ...
(now in
Fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

Fungi
) **
Ascetosporea The Ascetosporea are a group 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 ...
(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 (now in
Cnidaria Cnidaria () is 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 grammatical number, grammatical category of number. The plural of ...

Cnidaria
) *
Ciliate The ciliates are a group of protozoan 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 c ...

Ciliate
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 A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree (graph theor ...
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 grou ...
clade A clade (), 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 (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
, once a few extraneous members (such as ''Stephanopogon'' or protociliates and opalinids were removed. The Mastigophora, Sarcodina, and Sporozoa were
polyphyletic A polyphyletic group or assemblage is a set of organisms, or other evolving elements, that have been grouped together based on characteristics that do not imply that they share a common ancestor that is not also the common ancestor of many othe ...
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 Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; eit ...
. In most systems of
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

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 levels, 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemica ...
or filamentous algae,
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
,
microfungiMicrofungi or micromycetes are fungi—eukaryotic organisms such as Mold (fungus), molds, mildews and Rust (fungus), rusts—which has microscopic spore-producing structures. They exhibit tube tip-growth and have cell walls composed of chitin, a pol ...
, 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 A unit prefix is a specifier or mnemonic that is prepended to units of measurement A unit of measurement is a definite magnitude Magnitude may refer t ...
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 macrofaun ...
, 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. Schewiakoff). In the 1930's,
Lourens Baas Becking Lourens Gerhard Marinus Baas Becking (4 January 1895 in Deventer Deventer (; Sallaans dialect, Sallaans: ) is a List of cities in the Netherlands by province, city and Municipalities of the Netherlands, municipality in the Salland historical r ...
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 Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
s are human parasites, causing diseases such as
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign fo ...

malaria
(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 Hematophagy, blood-feeding insect host (biology), host which then inj ...

Plasmodium
''),
amoebiasis Amoebiasis or amoebic dysentery Dysentery () is a type of gastroenteritis Gastroenteritis, also known as infectious diarrhea and gastro, is inflammation of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, digest ...
,
giardiasis Giardiasis is a parasitic disease caused by ''Giardia duodenalis'' (also known as ''G. lamblia'' and ''G. intestinalis''). About 10% of those infected have no symptoms. Individuals who experience symptoms may have diarrhea, abdominal pain ...
,
toxoplasmosis Toxoplasmosis is a parasitic disease A parasitic disease, also known as parasitosis, is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multiplicati ...
,
cryptosporidiosis Cryptosporidiosis, sometimes informally called crypto, is a parasitic disease A parasitic disease, also known as parasitosis, is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, ...
,
trichomoniasis Trichomoniasis (trich) is an infectious disease caused by the parasite '' Trichomonas vaginalis''. About 70% of women and men do not have symptoms when infected. When symptoms do occur they typically begin 5 to 28 days after exposure. Symptoms ca ...
,
Chagas disease Chagas disease, also known as American trypanosomiasis, is a tropical disease, tropical parasitic disease caused by ''Trypanosoma cruzi''. It is spread mostly by insects known as ''Triatominae'', or "kissing bugs". The symptoms change over the ...
,
leishmaniasis Leishmaniasis is a wide array of clinical manifestations caused by parasites of the Trypanosoma, trypanosome genus ''Leishmania''. It is generally spread through the bite of Phlebotominae, phlebotomine Sandfly, sandflies, ''Phlebotomus'' and ...
, African trypanosomiasis (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 A unicellular organism, also known as a single-celled organism, is an organism that consi ...
(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 Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign fo ...

malaria
, and, historically, sleeping sickness. The protozoon ''
Ophryocystis elektroscirrha ''Ophryocystis elektroscirrha'' (sometimes abbreviated OE or ''O.e.'') is an obligate, neogregarine protozoan Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are org ...
'' is a parasite of
butterfly Butterflies are insect Insects (from Latin ') are pancrustacean Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three ...

butterfly
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
e, passed from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to , and have shortened lifespans, but parasite levels vary in populations. Infection creates a
culling In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
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. ''
ProtozoologyProtozoology is the study of protozoa, the "animal-like" (i.e., motility, motile and heterotrophic) protists. This term has become dated as understanding of the evolutionary relationships of the eukaryotes has improved. For example, the Society of ...
''. 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. TC, T.C., Tc, Tc, tc, tC, or .tc may refer to: Arts and entertainment Film and television * Theodore "T.C." Calvin, a character on the TV series ''Magnum, P.I. ''Magnum, P.I.'' is an American crime drama Crime films, in the broadest sens ...
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