Chromista
Encyclopedia
The Chromista are a eukaryotic
supergroup, probably polyphyletic, which may be treated as a separate kingdom
or included among the Protist
a. They include all algae whose chloroplast
s contain chlorophyll
s a and c, as well as various colorless forms that are closely related to them. These are surrounded by four membranes, and are believed to have been acquired from some red alga.
It has been described as consisting of three different groups:
In 2010, Thomas Cavalier-Smith
indicated his desire to move Alveolata, Rhizaria
and Heliozoa
into Chromista.
in 1981; the earlier names chromophyte and chromobiont correspond to roughly the same group. Molecular trees have had some difficulty resolving relationships between the different groups. All three may share a common ancestor with the alveolate
s (see chromalveolate
s), but there is evidence that suggests that the haptophytes and cryptomonads do not belong together with the heterokonts.
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...
supergroup, probably polyphyletic, which may be treated as a separate kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
or included among the Protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
a. They include all algae whose 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 contain 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...
s a and c, as well as various colorless forms that are closely related to them. These are surrounded by four membranes, and are believed to have been acquired from some red alga.
Groups
Chromista has been defined in different ways at different times.It has been described as consisting of three different groups:
- HeterokontHeterokontThe 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...
s or stramenopiles: brown algae, diatomDiatomDiatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s, water mouldWater mouldOömycota or oömycetes form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms . They are filamentous, microscopic, absorptive organisms that reproduce both sexually and asexually...
s, etc. - HaptophyteHaptophyteThe 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 - CryptomonadCryptomonadThe cryptomonads are a group of algae, most of which have plastids. They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10-50 μm in size and flattened in shape, with an anterior groove or pocket...
s
In 2010, Thomas Cavalier-Smith
Thomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...
indicated his desire to move Alveolata, Rhizaria
Rhizaria
The Rhizaria are a species-rich supergroup of unicellular eukaryotes. This supergroup was proposed by Cavalier-Smith in 2002. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods...
and Heliozoa
Heliozoa
Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...
into Chromista.
History and controversy
The name Chromista was first introduced by Cavalier-SmithThomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...
in 1981; the earlier names chromophyte and chromobiont correspond to roughly the same group. Molecular trees have had some difficulty resolving relationships between the different groups. All three may share a common ancestor with the alveolate
Alveolate
The alveolates are a major line of protists.-Phyla:There are four phyla, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:...
s (see chromalveolate
Chromalveolate
Chromalveolata is a eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first put forward in 1981. Chromalveolata was proposed to represent the result of a single secondary endosymbiosis between a line descending from a bikont and a red...
s), but there is evidence that suggests that the haptophytes and cryptomonads do not belong together with the heterokonts.