Chromalveolate

Chromalveolata is a eukaryote supergroup first proposed by Thomas Cavalier-Smith as a refinement of his kingdom Chromista, which was first proposed in 1981. It is now currently regarded as one of six major clades of eukaryotes, and may be considered a “kingdom”, though it is not given any formal taxonomical classification. It comprises a line descending from a bikont which performed secondary endosymbiosis with a red alga, and it includes these four major groups:

• Heterokontophyta
• Haptophyta
• Cryptophyta
• Alveolata

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.

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Evolutionary relationship

Chromalveolata is part of the bikont clade, which also comprises the Archaeplastida, the Rhizaria, the Excavata, and some smaller, unresolved groups such as the Apusozoa and the Centrohelida. As bikonts, they all descend from a heterotrophic eukaryote with two flagella. It is also thought that the Chromalveolata share a closer relationship with the Archaeplastida than with the other groups, in a clade some call Corticata.

  Historically, many chromalveolates were considered plants, with their cell walls, photosynthetic ability, and in some cases their morphological resemblance to the 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. Then, after Woese published his three-domain system, because of the paraphyly of the kingdom Monera, taxonomists turned their attention to the eukaryote domain, and the inherent paraphyly of Protista. After much debate, which continues to this day, Chromalveolata emerged as a monophyletic group.

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

  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.  

However, many chromalveolates are vital members of our ecosystem. Diatoms are one of the major photosynthetic producers, and produce much of the oxygen we breathe, and also take in much of the carbon dioxide that is thought to be a cause of global warming. 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 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.

However, certain factors are threatening chromalveolates. Ocean pollution is reducing the numbers of diatoms and brown algae, and certain brown algae are also being overharvested. We must conserve these organisms if they are to continue to provide their invaluable ecological services in the future.

References

• 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. [1]

• Patron, N. J., Rogers, M. B. and Keeling, P. J. 2004. Gene replacement of fructose-1,6-bisphophate aldolase supports the hypothesis of a single photosynthetic ancestor of chromalveolates. Eukaryot. Cell. 3, 1169-1174. [2]

• Harper, J. T. & Keeling, P. J. 2003. Nucleus-encoded, plastid-targeted glyceraldehyde-3-phosphate dehydrogenase (GAPDH) indicates a single origin for chromalveolate plastids. Mol. Biol. Evol. 20, 1730-1735.

• Archibald, J. M. & Keeling, P. J. 2002. Recycled plastids: a "green movement" in eukaryotic evolution. Trends Genet., 18, 577-584.

• Fast, N. M., Kissinger, J. C., Roos, D. S., & Keeling, P. J. 2001. Nuclear-encoded, plastid-targeted genes suggest a single common origin for apicomplexan and dinoflagellate plastids. Mol. Biol. Evol., 18, 418-426.

• Cavalier-Smith, T. 1999. Principles of protein and lipid targeting in secondary symbiogenesis: Euglenoid, dinoflagellate, and sporozoan plastid origins and the eukaryote family tree. J. Eukaryot. Microbiol. 46:347–66.