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Archaeplastida

Archaeplastida

Overview
The Archaeplastida are a major group of eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s, comprising the red and green algae and the land plant
Embryophyte
The land plants or embryophytes, more formally Embryophyta or Metaphyta, are the most familiar group of plants. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees,...

s, together with a small group called the glaucophyte
Glaucophyte
The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and green algae plus land plants , they form the Archaeplastida...

s. The plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...

s (chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s) of all of these organisms are surrounded by two membranes, suggesting they developed directly from endosymbiotic cyanobacteria. In all other groups, plastids are surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae.
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Encyclopedia
The Archaeplastida are a major group of eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s, comprising the red and green algae and the land plant
Embryophyte
The land plants or embryophytes, more formally Embryophyta or Metaphyta, are the most familiar group of plants. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees,...

s, together with a small group called the glaucophyte
Glaucophyte
The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and green algae plus land plants , they form the Archaeplastida...

s. The plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...

s (chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s) of all of these organisms are surrounded by two membranes, suggesting they developed directly from endosymbiotic cyanobacteria. In all other groups, plastids are surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae.

Although many studies have suggested that the Archaeplastida form a monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 group, a 2009 paper argues that they are in fact paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...

. The enrichment of novel red algal genes in a recent study demonstrates a strong signal for Plantae (Archaeplastida) monophyly and an equally strong signal of gene sharing history between the red/green algae and other lineages, thus shedding lights on the complexity of eukaryote evolution.

The cells of the Archaeplastida typically lack centriole
Centriole
A Centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though it is absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...

s and have mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...

 with flat crista
Crista
Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...

e. There is usually a cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

 including cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

, and food is stored in the form of starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

. However, these characters are also shared with other eukaryotes. The main evidence the Archaeplastida form a monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 group comes from genetic studies, which indicate that their plastids probably had a single origin. This evidence is disputed.

The archaeplastidans fall into two main evolutionary lines. The red algae are pigmented with chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

 a
Chlorophyll a
Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary...

and phycobiliprotein
Phycobiliprotein
Phycobiliproteins are water-soluble proteins present in cyanobacteria and certain algae that capture light energy, which is then passed on to chlorophylls during photosynthesis. Phycobiliproteins are formed of a complex between proteins and covalently bound phycobilins that act as chromophores...

s, like most cyanobacteria. The green algae and land plants – together known as Viridiplantae
Viridiplantae
Viridiplantae are a clade comprising the green algae and land plants.In some classification systems they have been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional Plant Kingdom to include the green algae...

 (Latin for "green plants") or Chloroplastida – are pigmented with chlorophylls a and b
Chlorophyll b
Chlorophyll b is a form of chlorophyll. Chlorophyll b helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll a in polar solvents because of its carbonyl group. Its color is yellow, and it primarily absorbs blue light....

, but lack phycobiliproteins. The glaucophytes have typical cyanobacterial pigments, and are unusual in retaining a cell wall within their plastids (called cyanelles).

Taxonomy


The consensus in 2005, when the group consisting of the glaucophytes, red and green algae and land plants was named 'Archaeoplastida', was that it was a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

, i.e. was monophyletic. Many studies published since this date have provided evidence which is in agreement. On the other hand, other studies have suggested that the group is paraphyletic. To date, the situation appears unresolved, but a strong signal for Plantae (Archaeplastida) monophyly has been demonstrated in a recent study (with an enrichment of red algal genes). The assumption made here is that Archaeplastida is a valid clade.
Various names have been given to the group. Some authors have simply referred to the group as plants or Plantae.
However, the name Plantae is ambiguous, since it has also been applied to less inclusive clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s, such as Viridiplantae
Viridiplantae
Viridiplantae are a clade comprising the green algae and land plants.In some classification systems they have been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional Plant Kingdom to include the green algae...

 and embryophyte
Embryophyte
The land plants or embryophytes, more formally Embryophyta or Metaphyta, are the most familiar group of plants. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees,...

s. To distinguish, the larger group is sometimes known as Plantae sensu lato ("plants in the broad sense").

To avoid ambiguity, other names have been proposed. Primoplantae, which appeared in 2004, seems to be the first new name suggested for this group.

Another name that has been applied to this node is Plastida, defined as the clade sharing "plastids of primary (direct prokaryote) origin [as] in Magnolia virginiana Linnaeus 1753".

The name Archaeplastida was proposed in 2005 by a large international group of authors (Adl et al.) who aimed to produce a classification for the eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s which took into account morphology, biochemistry and phylogenetics, and which had "some stability in the near term." They rejected the use of formal taxonomic ranks in favour of a hierarchical arrangement where the clade names do not signify rank. Thus the phylum name 'Glaucophyta' and the class name 'Rhodophyceae' appear at the same level in their classification. The divisions proposed for the Archaeplastida are shown below in both tabular and diagrammatic form.

Archaeplastida:
  • Glaucophyta Skuja 1954 (Glaucocystophyta Kies and Kremer 1986) – glaucophytes
Glaucophytes are a small group of freshwater single-celled algae. Their chloroplasts, called cyanelles, have a peptidoglycan
Peptidoglycan
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...

 layer, making them more similar to cyanobacteria than those of the remaining Archaeplastida.
  • Rhodophyceae Thuret 1855, emend. Rabenhorst 1863, emend. Adl et al. 2005 (Rhodophyta Wettstein 1901) – red algae
Red algae form one of the largest groups of algae. Most are seaweeds, being multicellular and marine. Their red colour comes from phycobiliprotein
Phycobiliprotein
Phycobiliproteins are water-soluble proteins present in cyanobacteria and certain algae that capture light energy, which is then passed on to chlorophylls during photosynthesis. Phycobiliproteins are formed of a complex between proteins and covalently bound phycobilins that act as chromophores...

s, used as accessory pigment
Accessory pigment
Accessory pigments are light-absorbing compounds, found in photosynthetic organisms, that work in conjunction with chlorophyll a. They include other forms of this pigment, such as chlorophyll b in green algal and higher plant antennae, while other algae may contain chlorophyll c or d...

s in light capture for photosynthesis.
  • Chloroplastida
    Viridiplantae
    Viridiplantae are a clade comprising the green algae and land plants.In some classification systems they have been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional Plant Kingdom to include the green algae...

     Adl et al. 2005 (Viridiplantae Cavalier-Smith 1981; Chlorobionta Jeffrey 1982, emend. Bremer 1985, emend. Lewis and McCourt 2004; Chlorobiota Kendrick and Crane 1997)
Chloroplastida is the term chosen by Adl et al. for the group made up of the green algae and land plants (embryophytes). Except where lost secondarily, all have chloroplasts without a peptidoglycan layer and lack phycobiliproteins.
  • Chlorophyta
    Chlorophyta
    Chlorophyta is a division of green algae, informally called chlorophytes. The name is used in two very different senses so that care is needed to determine the use by a particular author...

     Pascher 1914, emend. Lewis and Mc Court 2004 – green algae (part)
Adl et al. employ a narrow definition of the Chlorophyta; other sources include the Chlorodendrales and Prasinophytae, which may themselves be combined.
  • Ulvophyceae
    Ulvophyceae
    The Ulvophyceae or Ulvophytes are class of green algae, distinguished mainly on the basis of ultrastructural morphology. The sea lettuce, Ulva, belongs here. Other well-known members include Ulothrix and Acetabularia....

     Mattox and Stewart 1984
  • Trebouxiophyceae
    Trebouxiophyceae
    In taxonomy, the Trebouxiophyceae are a class of green algae, in the division Chlorophyta.-Links to scientific databases:...

     Friedl 1995 (Pleurastrophyceae Mattox et al. 1984; Microthamniales Melkonian 1990)
  • Chlorophyceae
    Chlorophyceae
    The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. For example the chlorophycean CW clade, and chlorophycean DO clade, are defined by the arrangement of their flagella. Members of the CW clade have flagella that are displaced...

     Christensen 1994
    • Chlorodendrales
      Chlorodendrales
      In taxonomy, the Chlorodendrales are an order of green algae, specifically the Prasinophyceae ....

       Fritsch 1917 – green algae (part)
    • Prasinophytae Cavalier-Smith 1998, emend. Lewis and McCourt 2004 – green algae (part)
    • Mesostigma
      Mesostigma
      Mesostigma viride is a species of freshwater green algae. It is now considered to be one of the earliest diverging members of the Streptophyta, one of the two lineages of green plants ....

      Lauterborn 1894, emend. McCourt in Adl et al. 2005 (Mesostigmata Turmel, Otis, and Lemieux 2002)
    • Charophyta
      Charophyta
      The Charophyta are a division of green algae, including the closest relatives of the embryophyte plants. In some groups, such as conjugating green algae, flagellate cells do not occur. The latter group does engage in sexual reproduction, and motility does not involve flagella, since they are...

       Karol et al. 2001, emend. Lewis and McCourt 2004 (Charophyceae Smith 1938, emend. Mattox and Stewart 1984) – green algae (part) and land plants
Charophyta sensu lato, as used by Adl et al., is a monophyletic group which is made up of some green algae, including the stoneworts (Charophyta sensu stricto), as well as the land plants (embryophytes).
  • Sub-divisions other than Streptophytina (below) were not given by Adl et al.
Other sources would include the green algal groups Chlorokybales, Klebsormidiales
Klebsormidiales
The Klebsormidiaceae are a family containing three genera of charophyte green alga forming multicellular, non-branching filaments. A fourth genus Chlorokybus is sometimes included as well, but this problematic and poorly known genus is sometimes placed in a separate class Chlorokybophyceae.The...

, Zygnematales
Zygnematales
The Zygnematales + νήμα , νήματος ), also called the Conjugales, are an order of green algae, comprising several thousand different species in genera such as the well-known Zygnema and Spirogyra. All the members of this group develop into unbranched filaments, one cell thick, which grow longer...

 and Coleochaetales
Coleochaetales
The Coleochaetales are an order of parenchymous charophyte algae, so they represents one of the most primitive multicellular relatives of land plants.They questionably include the fossil genus Parka....

.
  • Streptophytina
    Streptophytina
    Streptophytina is formally a subdivision or subphylum which contains two classes: Charophyceae, containing the Charales order ; and Embryophyceae, which contains the embryophytes .The term...

     Lewis and McCourt 2004 – stoneworts and land plants
  • Charales
    Charales
    Charales is an order of pondweeds, freshwater algae in the division Charophyta. They are green plants believed to be the closest relatives of the green land plants. Linnaeus established the genus Chara in 1753.-Description:...

     Lindley 1836 (Charophytae Engler 1887) – stoneworts
  • Plantae Haeckel 1866 (Cormophyta Endlicher 1836; Embryophyta Endlicher 1836, emend. Lewis and McCourt 2004) – land plants (embryophytes)

Morphology


All archaeplastidans have plastids (chloroplasts) that carry out photosynthesis and are believed to be derived from captured cyanobacteria. In glaucophytes, perhaps the most primitive members of the group, the chloroplast is called a cyanelle and shares several features with cyanobacteria, including a peptidoglycan cell wall, that are not retained in other members of the group. The resemblance of cyanelles to cyanobacteria supports the endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...

.

The cells of most archaeplastidans have walls, commonly but not always made of cellulose.

The Archaeplastida vary widely in the degree of their cell organization, from isolated cells to filaments to colonies to multi-celled organisms. The earliest were unicellular, and many groups remain so today. Multicellularity evolved separately in several groups, including red algae, ulvophyte green algae
Ulvophyceae
The Ulvophyceae or Ulvophytes are class of green algae, distinguished mainly on the basis of ultrastructural morphology. The sea lettuce, Ulva, belongs here. Other well-known members include Ulothrix and Acetabularia....

, and in the green algae that gave rise to stoneworts and land plants.

Endosymbiosis


Because the ancestral archaeplastidan is hypothesized to have acquired its chloroplasts directly by engulfing cyanobacteria, the event is known as a primary endosymbiosis (as reflected in the name chosen for the group 'Archaeplastida' i.e. 'ancient plastid'). Evidence for primary endosymbosis includes the presence of a double membrane around the chloroplasts; one membrane belonged to the bacterium, and the other to the eukaryote that captured it. Over time, many genes from the chloroplast have been transferred to the nucleus of the host cell. The presence of such genes in the nuclei of eukaryotes without chloroplasts suggests this transfer happened early in the evolution of the group.

Other eukaryotes with chloroplasts appear to have gained them by engulfing a single-celled archaeplastidan with its own bacterially-derived chloroplasts. Because these events involve endosymbiosis of cells that have their own endosymbionts, the process is called secondary endosymbiosis. The chloroplasts of such eukaryotes are typically surrounded by more than two membranes, reflecting a history of multiple engulfment. The chloroplasts of euglenid
Euglenid
Euglenoids are one of the best-known groups of flagellates, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Most euglenids are unicellular. Many euglenids have chloroplasts and produce energy through photosynthesis, but...

s and chlorarachniophyte
Chlorarachniophyte
Chlorarachniophytes are a small group of algae occasionally found in tropical oceans. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally they have the form of small amoebae, with branching cytoplasmic extensions that capture prey...

s appear to be captured green algae, whereas those of other photosynthetic eukaryotes, such as heterokont
Heterokont
The heterokonts or stramenopiles are a major line of eukaryotes currently containing more than 100,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton...

 algae, cryptophyte
Cryptophyte
Cryptophyte may refer to:*a plant which survives the unfavorable season underground or underwater in the Raunkiær plant life-form classification...

s, haptophyte
Haptophyte
The haptophytes, classified either as the Prymnesiophyta or Haptophyta, are a division of algae.The term "Haptophyceae" is sometimes used. This ending implies classification at a lower level...

s, and dinoflagellate
Dinoflagellate
The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...

s, appear to be captured red algae.

Fossil record


Perhaps the most ancient remains of Archaeplastida are microfossils from the Roper group in northern Australia. The structure of these single-celled fossils resembles that of modern green algae. They date to the Mesoproterozoic
Mesoproterozoic
The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma . The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them...

 Era, about 1500 to 1300 Ma (million years ago)
These fossils are consistent with a molecular clock
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...

 study that calculated that this clade diverged about 1500 Ma.
The oldest fossil that can be assigned to a specific modern group is the red alga Bangiomorpha, from 1200 Ma.

In the late Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...

 Era, algal fossils became more numerous and diverse. Eventually, in the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

Era, plants emerged onto land, and have continued to flourish up to the present.

External links