Dictionary > Chlorophyta

Chlorophyta

Chlorophyta is a taxonomic group (a phylum) comprised of green algae that live in marine habitats. Some of them are found in freshwater and on land. Some species have even become adapted to thriving in extreme environments, such as deserts, arctic regions, and hypersaline habitats. The predominant pigment is chlorophyll (particularly, a and b). Carotenoids are also present but relatively few. Their carbohydrate reserve is in the form of starch. They have flagella on the apical portion and they use them for movement.

Etymology

The term Chlorophyta came from Ancient Greek khlōrós, meaning “green” and‎ –phyta, meaning “plant”. Synonyms: Prasinophyta; Chlorophycota; Chlorophytina; Chlorophyllophyceae; Isokontae; Stephanokontae.

Classification

Chlorophyta is a taxonomic group that initially belongs to the Kingdom Plantae. In this regard, it may refer to a division within the Kingdom Plantae comprised of all green algal species. Later though, the green algae were split into two phyla: Chlorophyta (chlorophytes) and Charophyta (charophytes). The taxonomists classify those green algal species living predominantly in marine water as chlorophytes (i.e. belonging to Chlorophyta) whereas those green algal species thriving mainly in freshwater are classified as charophytes (i.e. belonging to Charophyta). Nevertheless, the habitat of the chlorophytes is wide-ranging. Apart from marine water, they may also be found in freshwater. Others may live as well on land (e.g. Trentepohliales). Some chlorophytes have become adapted into thriving in extreme habitats, such as deserts, hypersaline environment, and arctic regions.
In the first method of taxonomic classification, Chlorophyta includes about 7,000 known species of green algae. In the second sense of taxonomic classification, the Chlorophyta consists of only about 4,300 species, whereas the others initially regarded as chlorophytes are classified as charophytes. According to Hoek, Mann and Jahns system(1), an older taxonomic classification of algae, Chlorophyta is a division of Kingdom Plantae that included all green algae grouped into classes, Prasinophyceae, Chlorophyceae, Ulvophyceae, Cladophorophyceae, Bryopsidophyceae, Dasycladophyceae, Trentepohliophyceae, Pleurastrophyceae, Klebsormidiophyceae, Zygnematophyceae, and Charophyceae. In newer classification such as that of Leliaert et al.(2), the charophytes are separated from the chlorophytes and belong (together with the embryophytes) to Streptophyta of Viridiplantae. It should be noted, however, that the taxonomic classification of organisms is bound to change as further studies of the species would lead to newer system of classification, such as that in The NCBI taxonomy database.(3)

Chlorophytes vs. charophytes

Both the chlorophytes and the charophytes are green algae. Their predominant pigments are chlorophylls a and b. They store their carbohydrate reserves as starch. Their cell wall is chiefly comprised of cellulose. The charophytes, though, are the algal group presumed to have given rise to the embryophytes. They are more closely related and share common features that the chlorophytes lack. For instance, the charophytes and the embryophytes have enzymes such as class I aldolase, Cu/Zn superoxide dismutase, glycolate oxidase, and flagellar peroxidase that are absent in chlorophytes. The charophytes use phragmoplasts during cell division.

General characteristics

Common chlorophytes: (A) Chlamydomonas sp., (B) Volvox sp., (C) Pandorina sp., (D) Hydrodictyon, (E) Microspora sp., (F) Entoderma sp., (G) Coleochaete sp., (H) Oedogonium sp., (K) Struvea sp., and (L) Caulerpa sp.

The chlorophytes comprise most of the green algae. They are greenish in colour due to the abundance of chlorophyll pigments in the thylakoids. Accessory pigments, such as beta-carotene and xanthophylls, are present albeit few. Some species are single-celled whereas others are multicellular and macroscopic (e.g. Ulva sp.). Some of them form colonies, e.g. Volvox sp.. They have apical flagella that are used for locomotion. Reproduction may be asexual or sexual. Sexual reproduction in chlorophytes often involves the exchange of nuclei via the conjugation tubes. Sexual reproduction involving two identical gametes is referred to as isogamy. Oogamy, on the contrary, involves two non-identical gametes: one is small and motile whereas the other is large and non-motile. There is an alternation of generation such that the gametophyte and the sporophyte phases alternate. The gametophyte phase is the haploid phase and the sporophyte phase is the diploid phase. When both gametophyte and sporophyte phases involve multicellular forms of the species then it is described as diplobiontic. When only the gametophyte generation is multicellular, then it is described as haplobiontic.

Sub-groups

Below is the list of the taxonomic classes belonging to the phylum Chlorophyta(4):

  • Chlorodendrophyceae
  • Chlorophyceae
  • Chlorophyta classis incertae sedis
  • Chloropicophyceae
  • Mamiellophyceae
  • Nephrophyceae
  • Palmophyllophyceae
  • Pedinophyceae
  • Picocystophyceae
  • Pyramimonadophyceae
  • Trebouxiophyceae
  • Ulvophyceae

Evolution and phylogeny

Cladogram showing the phylogeny of Chlorophyta.

According to the endosymbiotic theory, the early chlorophytes might have come from the early eukaryotes that engulfed photosynthetic prokaryotes. Through time, the prokaryote became incorporated inside the eukaryote that it eventually became an organelle, which now is referred to as plastid. This event is presumed as well to have led to the rise of other clades of autotrophs, i.e. the red algae and the glaucophytes.(5) The chlorophytes eventually came about and in turn evolved, giving rise to most of the green algal species known today.

Biological importance

The chlorophytes, because of their photosynthetic activity, made them one of the most important producers in the ecosystem. They are a major source of starch and oxygen as a byproduct of photosynthesis. They serve as food for many heterotrophs. Many of them form symbiotic relationship with other groups of organisms. For example, they form lichens together with certain fungi. Some ciliates, cnidarians, and molluscs form symbiosis with the chlorophytes. Not all chlorophytes, though, are exclusively photosynthetic. Some of them are heterotrophic themselves. For example, the green alga Prototheca sp. can become pathogenic, causing protothecosis (a disease) in humans and animals. (6)

See also

References

  1. Hoek, C. van den, Mann, D.G. and Jahns, H.M. (1995). Algae An Introduction to Phycology. Cambridge: Cambridge University Press.
  2. Leliaert, F., Smith, D.R., Moreau, H., Herron, M.D., Verbruggen, H., Delwiche, C.F. & De Clerck, O. (2012). “Phylogeny and molecular evolution of the green algae” (PDF). Critical Reviews in Plant Sciences 31: 1–46.
  3. The NCBI taxonomy database. Retrieved from http://www.ncbi.nlm.nih.gov/taxonomy.
  4. Guiry, M. D. (2011). “AlgaeBase : Chlorophyta”. World-wide electronic publication, National University of Ireland, Galway. Retrieved from https://www.algaebase.org/browse/taxonomy/?id=97241&-session=abv4:AC1F03651d6c70FF99yHFE542B1A
  5. De Clerck, O., Bogaert, K. A., & Leliaert, F. (2012). “Diversity and Evolution of Algae”. Genomic Insights into the Biology of Algae. Advances in Botanical Research. 64. pp. 55–86
  6. Tartar, A., Boucias, D. G., Adams, B. J., & Becnel, J. J. (January 2002). “Phylogenetic analysis identifies the invertebrate pathogen Helicosporidium sp. as a green alga (Chlorophyta)”. International Journal of Systematic and Evolutionary Microbiology. 52 (Pt 1): 273–9. doi:10.1099/00207713-52-1-273.

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