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Chromalveolata was a eukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes. It was a refinement of the kingdom Chromista, first proposed by Thomas Cavalier-Smith in 1981. Chromalveolata was proposed to represent the organisms descended from a single secondary endosymbiosis involving a red alga and a bikont. The in these organisms are those that contain chlorophyll c.
However, the monophyly of the Chromalveolata has been rejected. Thus, two papers published in 2008 have phylogenetic trees in which the chromalveolates are split up, and recent studies continue to support this view.
In 2005, in a classification reflecting the consensus at the time, the Chromalveolata were regarded as one of the six major of eukaryotes. Although not given a formal taxonomic status in this classification, elsewhere the group had been treated as a kingdom. The Chromalveolata were divided into four major subgroups:
Other groups that may be included within, or related to, chromalveolates, are:
Though several groups, such as the and the , have lost the ability to photosynthesize, most are . All photosynthetic chromalveolates use 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 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. In 2012, consensus emerged that the group is not monophyletic. The four original subgroups fall into at least two categories: one comprises the Stramenopiles and the Alveolata, to which the Rhizaria are now usually added to form the SAR supergroup; the other comprises the Cryptophyta and the Haptophyta. A 2010 paper splits the Cryptophyta and Haptophyta; the former are a sister group to the SAR group, the latter cluster with the Archaeplastida (plants in the broad sense). The are closely related to the cryptophytes and the Telonemia and may be related to the haptophytes.
A variety of names have been used for different combinations of the groups formerly thought to make up the Chromalveolata.
The only common chromalveolate features are these:
Since this is such a diverse group, it is difficult to summarize shared chromalveolate characteristics.
Some of these organisms can be very harmful. produce , which can devastate fish populations and intoxicate oyster harvests. are some of the most successful specific parasites to animals (including the genus Plasmodium, the malaria parasites). Water molds cause several plant diseases - it was the water mold Phytophthora infestans that caused the Irish potato blight that led to the Great Irish Famine.
However, many others are vital members of our ecosystem. are one of the major photosynthetic producers, and as such produce much of the oxygen that we breathe, and also take in much of the carbon dioxide from the atmosphere. Brown algae, most specifically , 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.
Groups and classification
Historically, many chromalveolates were considered , 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 (proposed in 1969) took prevalence over the animal–plant dichotomy, most of what we now call chromalveolates were put into the kingdom Protista, but the and slime nets were put into the kingdom Fungi, while the brown algae stayed in the plant kingdom. These various organisms were later grouped together and given the name Chromalveolata by Cavalier-Smith. He believed them to be a monophyletic group, but this is not the case.Harper, J. T., Waanders, E. & Keeling, P. J. 2005. On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes. Int. J. System. Evol. Microbiol., 55, 487-496.
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 Cryptophyta, and the two different flagella of the Heterokontophyta. However, none of these features are present in all of the groups.
Ecological role
Many chromalveolates affect our ecosystem in enormous ways.
Chromalveolata viruses
Like other organisms, chromalveolata have viruses. In the case of Gephyrocapsa huxleyi (a common algal bloom haptophyte), a virus believed to be specific to it causes mass death and the end of the bloom.
See also
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