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. This is one of the few areas where a large biomass of brown algae may be found in tropical waters. Many brown algae such as members of the order Fucales are commonly found along rocky seashores. Some members of the division are used as food for humans.
Worldwide there are about 1500-2000 brown seaweed species.[1]
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, which closely resemble other heterokont cells. Genetic studies show their closest relatives to be the yellow-green algae.
Morphology
Browns are typically frondose organisms.verification needed The cells of most browns are connected by pores.[2] They bear a unilocular (one holed) sporangium.[2]
Evolutionary history
Phaeophyta evolved from the phaeothamniophyceae[2] between 150[3] & 200 million years ago.[4] Claims that earlier (Ediacaran) fossils are brown algae[5] have since been dismissed.[2] The linages of brown algae diverged in the following order, from oldest to youngest: Dictyotales; Sphacelariales; Cutleriales; Desmerestales; 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. Other algae 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 the brown algae. Miocene fossils of a soft-bodied brown macro algae, Julescrania, have been found well-preserved in Monterey Formation diatomites, but few other certain fossils, particularly of older specimens are known in the fossil record.[6][1]
Taxonomy
This is a list of the orders in the class Phaeophyceae:[7]
A few species, such as Botrydium stoloniferum, are placed incertae sedis, or of uncertain position, as to order in this classification scheme.
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 plant well know to most people. It produces sporangia from specialised microscopic structures, these divide meiotically (meiosis) before they are released. As they are haploid there are equal numbers of male and female spores.[1] With the exception of the Fucales all brown algae have a life cycle which consists of an alternation between morphologically haploid and diploid plants referred to as Monomorphic. A dimorphic life cycle consists of an alteration between dissimilar haploid and diploid plants. [8]
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 brackish water ecosystems, and four species survive in fresh water.[2] Most phaeophyceae are intertidal or upper littoral,[2] and they 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.[2]
Chemistry
Brown algae have a δ13C value between -21 and -13, in contrast with red algae (-27 and -32) and greens (in region of -2.5). This reflects their different metabolic pathways.[9]
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.[2]
References
- ^ a b Thomas,D.N. 2002. Seaweeds. The Natural History Museum, London. ISBN 0 56509175 1
- ^ a b c d e f g h Lee, R.E. (2008), Phycology, 4th edition, Cambridge University Press, ISBN 978-0521638838
- ^ 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", Origins of algae and their plastids: 187–219, <http://epic.awi.de/Publications/Med1997c.pdf>
- ^ 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
- ^ Loeblich, A. R. (1974), "Protistan Phylogeny as Indicated by the Fossil Record", Taxon 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 37: 574–585. Blackwell Publishing. doi:10.1046/j.1529-8817.2001.037001574.x.
- ^ Guiry, M. D. & G. M. Guiry (2006). "AlgaeBase version 4.2". National University of Ireland, Galway. Retrieved on 7 December 2006.
- ^ Fletcher,R.L. 1987. Seaweeds of the British Isles. Volume 3. Part 1. British Museum (Natural History), London. ISBN 0 565 00992 3
- ^ 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.
See also
External links
|
