Protozoa (also protozoan, plural protozoans) is an informal term for single-celled Eukaryote organisms, either free-living or parasitic, which feed on organic matter such as other Microorganism or organic tissues and debris.
In some systems of biological classification, Protozoa is a high-level Taxon. When first introduced in 1818, Protozoa was erected as a taxonomic class , but in later classification schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom. The seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, Dinoflagellate, foraminifera, and the parasitic Apicomplexa, all of which are classified under Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but a Paraphyly group or evolutionary grade, within which the members of Fungi, Animalia and Chromista are thought to have evolved.
The use of Protozoa as a formal taxon has been discouraged by some recent researchers, mainly because the term, which is formed from the Ancient Greek πρῶτος prōtos "first" and ζῶον ζῶα zōa, plural of ζῶον zōon, "animal", implies kinship with animals (metazoa) and promotes an arbitrary separation of "animal-like" from "plant-like" organisms. Modern ultrastructural, biochemical, and genetic techniques have shown that protozoa, as traditionally defined, belong to widely divergent lineages, and can no longer be regarded as "primitive animals". Genetic techniques have conclusively shown that protozoa, as traditionally defined, belong to widely divergent lineages distributed across the eukaryotic tree of life, and can no longer be regarded as "primitive animals". For this reason, the terms "protists", "Protista" or "Protoctista" are sometimes preferred for the high-level classification of Eukaryote Microorganism. In 2005, members of the Society of Protozoologist voted to change the name of that organization to the International Society of Protistologist.
In 1848, as a result of advancements in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C.T. von Siebold proposed that the bodies of protozoans such as Ciliate and Amoeba were made up of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all Animal (animals). At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria (mostly ciliates and flagellated algae), and Rhizopoda (Amoeba). The definition of Protozoa as a phylum or sub-kingdom made up of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"—and the term came into wide use. As a phylum under Animalia, the Protozoa were firmly rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany.
Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have Chloroplast for photosynthesis, but can also feed on organic matter and are Motility. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms, or living beings, are animals or plants." As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae (protophyta), which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum.
Six years later, Ernst Haeckel also proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in later work he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.
Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, however, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi. A variety of multi-kingdom systems were proposed, and Kingdoms Protista and Protoctista became well established in biology texts and curricula.
While many taxonomists have abandoned Protozoa as a high-level group, Thomas Cavalier-Smith has retained it as a kingdom in the various classifications he has proposed. As of 2015, Cavalier-Smith's Protozoa excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, Dinoflagellate and foraminifera (all members of the SAR supergroup). In its current form, his kingdom Protozoa is a Paraphyly group which includes a common ancestor and most of its descendents, but excludes two important clades that branch within it: the animals and fungi.
|Plasmodium falciparum (malaria parasite, trophozoite phase)||1-2|
|Massisteria (free-living Cercozoa amoeboid)||2.3–3|
|Bodo saltans (free living Kinetoplastida flagellate)||5-8|
|Plasmodium falciparum (malaria parasite, gametocyte phase)||7-14|
|Trypanosoma cruzi (parasitic kinetoplastid, Chagas disease)||14-24|
|Entamoeba histolytica (parasitic Amoebozoa)||15–60|
|Balantidium coli (parasitic ciliate)||50-100|
|Paramecium caudatum (free-living ciliate)||120-330|
|Amoeba proteus (free-living amoebozoan)||220–760|
|Noctiluca scintillans (free-living dinoflagellate)||700–2000|
|Syringammina fragilissima (Foraminifera amoeboid)||up to|
Parasitism and Symbiosis protozoa live on or within other organisms, including Vertebrate and Invertebrate, as well as plants and other single-celled organisms. Some are harmless or beneficial to their host organisms; others may be significant causes of diseases, such as babesia, malaria and toxoplasmosis.Trichonympha and Pyrsonympha inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex Sugar into smaller, more easily-digested molecules. A wide range of protozoans live Commensalism in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as Trichomonas, and ciliated protozoa, such as Isotricha and Entodinium.
Parasitic protozoans use a wide variety of feeding strategies, and some may change methods of feeding in different phases of their life cycle. For instance, the malaria parasite Plasmodium feeds by pinocytosis during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cell.
Protozoa may also live as Mixotroph, supplementing a heterotrophic diet with some form of Autotroph. Some protozoa form close associations with symbiotic photosynthetic algae, which live and grow within the membranes of the larger cell and provide nutrients to the host. Others practice kleptoplasty, stealing Chloroplast from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliate Mesodinium rubrum retains functioning Plastid from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. These, in turn, may be passed along to dinoflagellates of the genus Dinophysis , which prey on Mesodinium rubrum but keep the enslaved plastids for themselves. Within Dinophysis, these plastids can continue to function for months.
All protozoans reproduce asexually by binary fission or multiple fission. Many protozoan species exchange genetic material by sexual means (typically, through conjugation); however, sexuality is generally decoupled from the process of reproduction, and does not immediately result in increased population.
Although meiotic sex is widespread among present day Eukaryote, it has, until recently, been unclear whether or not eukaryotes were sexual early in their evolution. Due to recent advances in gene detection and other techniques, evidence has been found for some form of meiotic sex in an increasing number of protozoans of ancient lineage that diverged early in eukaryotic evolution.Bernstein H, Bernstein C (2013). Bernstein C, Bernstein H eds. Evolutionary Origin and Adaptive Function of Meiosis'. Meiosis. InTech. (See eukaryote reproduction.) Thus, such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles Amoebozoa, Giardia lamblia, Leishmania, Plasmodium falciparum biology, Paramecium, Toxoplasma gondii, Trichomonas vaginalis and Trypanosoma brucei.