Brown algae
| Brown algae Fossil range: 150–0 Ma[1][2] |
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| Giant kelp (Macrocystis pyrifera) | |
| Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Chromalveolata |
| Phylum: | Heterokontophyta |
| Class: | Phaeophyceae Kjellman, 1891[3] |
| Orders | |
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see Classification. |
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| Synonyms | |
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Fucophyceae |
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The Phaeophyceae or brown algae, (singular: alga) is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments both as food, and for the habitats they form. For instance Macrocystis, a member of the Laminariales or kelps, may reach 60 m in length, and forms prominent underwater forests. Another example is Sargassum, which creates unique habitats in the tropical waters of the Sargasso Sea. Many brown algae such as members of the order Fucales are commonly found along rocky seashores. Some members of the class are used as food for humans.
Worldwide there are about 1500-2000 species of brown algae.[4] Some species are of sufficient commercial importance, such as Ascophyllum nodosum, that they have become subjects of extensive research in their own right.[5]
Brown algae belong to a very large group, the Heterokontophyta, a eukaryotic group of organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among heterokonts in developing into multicellular forms with differentiated tissues, but they reproduce by means of flagellate spores and gametes, which closely resemble other heterokont cells. Genetic studies show their closest relatives to be the yellow-green algae.
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Morphology
Brown algae are filamentous, macroscopic or microscopic some polysiphonous. Some form crusts, cushions or are hollow and others grow to form large leathery fronds.[6]
Evolutionary history
Phaeophyta evolved from the phaeothamniophyceae[7] between 150[1] & 200 million years ago.[2] Claims that earlier (Ediacaran) fossils are brown algae[8] have since been dismissed.[7] The lineages of brown algae diverged in the following order, from oldest to youngest: Dictyotales; Sphacelariales; Cutleriales; Desmarestiales; Ectocarpales; Laminarales; Fucales. Their occurrence as fossils is rare due to their generally soft-bodied habit, and scientists continue to debate the identification of some finds. Only a few species of brown algae deposit significant quantities of minerals in or around their cell walls. Other algal groups, such as the red algae and green algae have a number of calcareous members, which are more likely to leave evidence in the fossil record than the soft bodies of most brown algae. Miocene fossils of a soft-bodied brown macro algae, Julescrania, have been found well-preserved in Monterey Formation diatomites, but few other dubiously assigned fossils, particularly of older specimens are known in the fossil record.[9][1]
Classification
This is a list of the orders in the class Phaeophyceae:[10]
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Life cycle
The life cycle shows great variability from one group to another. However the life cycle of Laminaria consists of the diploid generation, that is the large kelp well known to most people. It produces sporangia from specialised microscopic structures, these divide meiotically before they are released. As they are haploid there are equal numbers of male and female spores.[11] With the exception of the Fucales all brown algae have a life cycle which consists of an alternation between haploid and diploid forms.
Ecology
Brown algae have adapted to a wide variety of marine ecological niches including the tidal splash zone, rock pools, the whole intertidal zone and relatively deep near shore waters. They are an important constituent of some brackish water ecosystems, and four species are restricted to life in fresh water.[7] A large number of Phaeophyceae are intertidal or upper littoral,[7] and they are predominantly cool and cold water organisms that benefit from nutrients in up welling cold water currents and inflows from land; Sargassum being a prominent exception to this generalisation.
Brown algae growing in brackish waters are almost solely asexual.[7]
Chemistry
| Algal group | δ13C range[12] |
|---|---|
| HCO3-using red algae | −22.5‰ – −9.6‰ |
| CO2-using red algae | −34.5‰ – −29.9‰ |
| Brown algae | −20.8‰ – −10.5‰ |
| Green algae | −20.3‰ – −8.8‰ |
Brown algae have a δ13C value between −-20.8‰ – −10.5‰, in contrast with red algae and greens. This reflects their different metabolic pathways.[13]
They have Cellulose walls with alginic acid; fucoidin also important in amorphous section of cell walls. A few species (of Padina) calcify with aragonite needles.[7]
See also
- Diatom
- Golden algae
- Kelp
- List of brown algal genera
- Wrack (seaweed)
- Edible seaweed
References
- ↑ 1.0 1.1 Medlin, L. K.; Kooistra, W. H. F.; Potter, D.; Saunders, G. W.; Andersen, R. A. (1997). "Phylogenetic relationships of the 'golden algae'(haptophytes, heterokont chromophytes) and their plastids" (PDF). Origins of algae and their plastids: 187–219. http://epic.awi.de/Publications/Med1997c.pdf
- ↑ 2.0 2.1 Lim, B.L.A.K; Kawai, H.; Hori, H.; Osawa, S. (1986). "Molecular evolution of 5S ribosomal RNA from red and brown algae". Idengaku Zasshi 61 (2): 169–176
- ↑ Kjellman, F.R. (1891). "Phaeophyceae (Fucoideae)". In Engler, A. & Prantl, K. (eds.). Die natürlichen Pflanzenfamilien. 1 (2). Leipzig: Wilhelm Engelmann. pp. 176–192.
- ↑ Hoek, C. van den; D. G. Mann; H. M. Jahns (1995). Algae: An Introduction to Phycology. Cambridge: Cambridge University Press. pp. 166. ISBN 0-521-31687-1.
- ↑ T. L. Senn (1987). Seaweed and Plant Growth. Clemson, S.C.: T.L. Senn. p. 181 pp. ISBN 0-939241-01-3.
- ↑ Jones, W.E. 1962. A key to the genera of the British seaweeds. Field Studies.: 1 (4) 1 - 32
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Lee, R.E. (2008). Phycology, 4th edition. Cambridge University Press. ISBN 978-0521638838
- ↑ Loeblich, A. R. (1974). "Protistan Phylogeny as Indicated by the Fossil Record". Taxon (International Association for Plant Taxonomy (IAPT)) 23 (2/3): 277–290. doi:10.2307/1218707. http://links.jstor.org/sici?sici=0040-0262(197405)23%3A2%2F3%3C277%3APPAIBT%3E2.0.CO%3B2-8
- ↑ Coyer, J.A.; G.J. Smith, R.A. Anderson (2001). "Evolution of Macrocystis spp. (Phaeophyta) as determined by ITS1 and ITS2 sequences". Journal of Phycology (Blackwell Publishing) 37: 574–585. doi:10.1046/j.1529-8817.2001.037001574.x.
- ↑ Guiry, M. D. & G. M. Guiry (2009). "AlgaeBase". World-wide electronic publication: National University of Ireland, Galway. http://www.algaebase.org. Retrieved 18 August 2009.
- ↑ Thomas,D.N. 2002. Seaweeds The Natural History Museum, London. ISBN 0 56509175 1
- ↑ Maberly, S. C.; Raven, J. A.; Johnston, A. M. (1992), "Discrimination between 12C and 13C by marine plants", Oecologia 91: 481, doi:10.1007/BF00650320
- ↑ Fletcher, B. J.; Beerling, D. J.; Chaloner, W. G. (2004). "Stable carbon isotopes and the metabolism of the terrestrial Devonian organism Spongiophyton". Geobiology 2: 107. doi:10.1111/j.1472-4677.2004.00026.x
Further reading
Druehl, L.D. 1988. Cultivated edible kelp. in Algae and Human Affairs. Lembi, C.A. and Waaland, J.R. (Editors) 1988.ISBN 0 521 32115 8.
External links
- Monterey Bay Flora
- Phaeophyceae, National University of Ireland, Galway
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