Chromalveolata was aeukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes.[3] It was a refinement of thekingdomChromista, first proposed byThomas Cavalier-Smith in 1981. Chromalveolata was proposed to represent the organisms descended from a single secondaryendosymbiosis involving ared alga and abikont.[4] Theplastids in these organisms are those that containchlorophyll c.
However, themonophyly of the Chromalveolata has been rejected. Thus, two papers published in 2008 have phylogenetic trees in which the chromalveolates are split up,[5][6] and recent studies continue to support this view.[7][8]
Historically, many chromalveolates were consideredplants, because of their cell walls, photosynthetic ability, and in some cases their morphological resemblance to the land plants (Embryophyta). However, when thefive-kingdom system (proposed in 1969) took prevalence over the animal–plant dichotomy, most of what we now call chromalveolates were put into the kingdomProtista, but thewater molds andslime nets were put into the kingdomFungi, while thebrown 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 amonophyletic group, but this is not the case.[9]
In 2005, in a classification reflecting the consensus at the time, the Chromalveolata were regarded as one of the six majorclades of eukaryotes.[3] Although not given a formal taxonomic status in this classification, elsewhere the group had been treated as a kingdom.[citation needed] The Chromalveolata were divided into four major subgroups:
However, as early as 2005, doubts were being expressed as to whether Chromalveolata was monophyletic,[9] and a review in 2006 noted the lack of evidence for several of the supposed six major eukaryote groups, including the Chromalveolata.[12] 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 theRhizaria are now usually added to form theSAR group; the other comprises the Cryptophyta and the Haptophyta.[5][6] A 2010 paper splits the Cryptophyta and Haptophyta; the former are a sister group to the SAR group, the latter cluster with theArchaeplastida (plants in the broad sense). Thekatablepharids are closely related to the cryptophytes and thetelonemids andcentrohelids may be related to the haptophytes.[7]
A variety of names have been used for different combinations of the groups formerly thought to make up the Chromalveolata.
Halvaria Analyses in 2007 and 2008 agreed that the Stramenopiles and the Alveolata were related, forming a reduced chromalveolate clade, calledHalvaria.[5][6][13]
SAR group The Rhizaria, which were originally not considered to be chromalveolates, belong with the Stramenopiles and Alveolata in many analyses, forming theSAR group, i.e. Halvaria plus Rhizaria.[13][14]
Hacrobia The other two groups originally included in Chromalveolata, the Haptophyta and the Cryptophyta, were related in some analyses,[5][6] forming a clade which has been calledHacrobia. Alternatively, the Hacrobia appeared to be more closely related to theArchaeplastida (plants in the very broad sense), being a sister group in one analysis,[5] and actually nested inside this group in another.[6] (Earlier,Cavalier-Smith had suggested a clade calledCorticata for the grouping ofall the chromalveolates and the Archaeplastida.) More recently, as noted above, Hacrobia has been split, with the Haptophyta being sister to the SAR group and the Cryptophyta instead related to the Archaeplastida.[7]
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.
The only common chromalveolate features are these:
The shared origin of chloroplasts, as mentioned above
However, many others are vital members of our ecosystem.Diatoms are one of the major photosynthetic producers, and as such produce much of theoxygen that we breathe, and also take in much of thecarbon dioxide from the atmosphere.Brown algae, most specificallykelps, 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. Thealgin in brown algae is used as a food thickener, most famously inice cream. The siliceous shells of diatoms have many uses, such as in reflective paint, in toothpaste, or as a filter, in what is known asdiatomaceous earth.
Like other organisms, chromalveolata have viruses. In the case ofGephyrocapsa huxleyi (a commonalgal bloom haptophyte), a virus believed to be specific to it causes mass death and the end of the bloom.[15]
^abHarper, 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."Archived copy"(PDF). Archived fromthe original(PDF) on 2008-12-17. Retrieved2010-04-26.{{cite web}}: CS1 maint: archived copy as title (link)
