Chromalveolate

Chromalveolate
225px
Clockwise from top-left: a haptophyte, some diatoms, a water mold, a cryptomonad, and Macrocystis, a phaeophyte
Scientific classification
Domain:	Eukarya
(unranked)	Bikonta
(unranked):	Corticata
Kingdom:	Chromalveolata*
Phyla
Heterokontophyta Haptophyta Cryptophyta Alveolata (Superphylum) Ciliophora Apicomplexa Dinoflagellata

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 alga that became the progenitor of chlorophyll c containing plastids. In a major classification produced in 2005, Chromalveolata was regarded as one of the six major groups within the eukaryotes. However this has been increasingly challenged. Thus two papers published in 2008 have phylogenetic trees in which the chromalveolates are split up.

Groups and classification

Historically, many chromalveolates were considered plants, because of their cell walls, photosynthetic ability, and in some cases their morphological resemblance to the land plants (Embryophyta). However, when the five-kingdom system took prevalence over the animal-plant dichotomy, most chromalveolates were put into the kingdom Protista, with the water molds and slime nets put into the kingdom Fungi, and the brown algae staying in the plant kingdom.

In 2005, in a classification reflecting the consensus at the time, the Chromalveolata were regarded as one of the six major clades of eukaryotes. Although not given a formal taxonomic status in this classification, elsewhere the group has been treated as a Kingdom. The Chromalveolata were divided into four major subgroups:

• Cryptophyta
• Haptophyta
• Stramenopiles (or Heterokontophyta)
• Alveolata

Other groups which may be included within, or related to, chromalveolates, are:

• Telonemia

Though several groups, such as the ciliates and the water molds, have lost the ability to photosynthesize, most are autotrophic. All photosynthetic chromalveolates use chlorophylls a and c, and many use accessory pigments. Chromalveolates share similar glyceraldehyde 3-phosphate dehydrogenase proteins.

However, as early as 2005, doubts were being expressed as to whether the Chromalveolata was monophyletic, and a review in 2006 noted the lack of evidence for several of the supposed six major eukaryote groups, including the Chromalveolata.. As of 2010 there seems to be an emerging consensus that the group is not monophyletic. The four original subgroups fall into two categories, one comprising the Cryptophyta and the Haptophyta, the other the Stramenopiles and the Alveolata.

Halvaria

Main article: Halvaria

Analyses in 2007 and 2008 agree that the Stramenopiles and the Alveolata are related, forming a reduced chromalveolate clade.

Hacrobia

Main article: Hacrobia

The Haptophyta and Cryptophyta are related in these analyses, forming a clade which has been called 'Hacrobia'.

Position of SAR and Hacrobia

The Hacrobia appear to be more closely related to the Archaeplastida (plants in the very broad sense), being a sister group in one analysis, and actually nested inside this group in another. (Earlier, Cavalier-Smith had suggested a clade called Corticata for the grouping of all the chromalveolates and the Archaeplastida.) The SAR supergroup and the Archaeplastida/Hacrobia combination are grouped in the bikonts, which all appear to descend from a heterotrophic eukaryote with two flagella.

Morphology

Chromalveolates, unlike other groups with multicellular representatives, do not have very many common morphological characteristics. Each major subgroup has certain unique features, including the alveoli of the Alveolata, the haptonema of the Haptophyta, the ejectisome of the Crytophyta, and the two different flagella of the Heterokontophyta. However, none of these features are present in all of the groups.

The only common chromalveolate features are these:

• The shared origin of chloroplasts, as mentioned above
• Presence of cellulose in most cell walls

Since this is such a diverse group, it is difficult to summarize shared chromalveolate characteristics.

Ecological role

File:Symptom potato late blight.jpg

A potato plant infected with Phytophthora infestans.

Many chromalveolates affect our ecosystem in enormous ways. Some of these organisms can be very harmful. Dinoflagellates produce red tides which can devastate fish populations and intoxicate oyster harvests. Apicomplexans are some of the most successful specific parasites to animals. Water molds cause several plant diseases. In fact, it was a water mold, Phytophthora infestans, that caused the Irish potato famine.

File:Kelp forest.jpg

A Californian kelp forest.

However, many chromalveolates are vital members of our ecosystem. Diatoms are one of the major photosynthetic producers, and as such produce much of the oxygen we breathe, and also take in much of the carbon dioxide from the atmosphere. Brown algae, most specifically kelps, create underwater "forest" habitats for many marine creatures, and provide a large portion of the diet of coastal communities.

Chromalveolates also provide many products that we use. The algin in brown algae is used as a food thickener, most famously in ice cream. The siliceous shells of diatoms have many uses, such as in reflective paint, in toothpaste, or as a filter, in what is known as diatomaceous earth.

External links

• Tree of Life Eukaryotes

ar:أسناخ صبغية zh:囊泡藻界