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Click on image to go to genus |
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The brown algae known earlier as the Phaeophyceae and more recently as the Fucophyceae (Christensen 1978) are mainly marine and benthic (nearly 2,000 species), with at most 12, and more likely only 7 genera recognized worldwide to inhabit freshwater benthos (Wilce 1966, Wehr 2003) with 265 genera currently recognized (Dawes and Mathieson 2008). They share many subcellular properties with the golden-browns (Chrysophyceae) and diatoms (Bacillariophyceae) such as chloroplast thylakoids stacked in triplicate, two unequal flagella (“heterokonts”, although absent from diatoms), major pigments (Chl a, c1, c2, β-carotene and several xanthins), and laminarin (storage reserve). No fucophyceaens are planktonic throughout their life cycle, with only gametes suspended. All other stages are benthic. Freshwater species are mainly epiliths (Wehr 2003). All known freshwater species have a filamentous (either uniseriate or multiseriate) or in one case (Heribaudiella fluviatilis) crustose morphology, while none become parenchymatous (tissue structure) typical of larger marine browns. Wehr (2003) agrees with earlier investigators that many freshwater habitats have been poorly studied, and that the micro-browns may have been overlooked because they are small, and often encrusted with marl (precipitated CaCO3 in elevated pH such as in the micro-environment around an actively photosynthesizing cell). The browns are also grouped with other classes such as goldens and diatoms that have in common two different types of flagella in the motile stages, into the "stramenopiles" ("straw hairs") and/or "heterokonts" (differing poles i.e. flagella). Included are both photosynthetic and heterotrophic forms.
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Evolutionary Origin: |
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The brown algae arose from a secondary endosymbiotic event when a unicellular red algal cell was engulfed by an
‘ancestral’ protest (Reyes-Prieto et al. 2007). Subsequently the
development of an extracellular matrix of polysaccharides enabled multicellularity through cell to cell adherence. Such development was independent of multicellularity evolved in other classes of protists (Rhodophyceae, Chlorophyceae, Charophyceae),
fungi and animals. Inheritance of
cellulose production was inherited from the unicellular red algal symbiont,
while that of alginate was by horizontal gene transfer from one of the Actinobacteria (Michel et al. 2010).
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| Commercial use of kelp | ||||
Macroscopic brown algae are called ‘kelp’, many of which are harvested for their mineral (iodine and NaCO3) and organic ‘alginate’ content, as well as directly for food. Iodine has been used to treat goiters, NaCO3 (soda ash) has been used in the production of glass and soap, and alginate is used in food products as a thickener in ice cream, jelly, tooth paste and salad dressing. Alginate is also used in dentristy to make impressions of teeth. Traditionally kelp has been harvested by scraping it from rocks. More recently aquaculture on long lines has been found to be economic because of its rapid growth (Flavin et al. 2013).
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References: |
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Christensen, T. 1978. Annotations to a textbook of phycology. Botanisk Tidsskrift 73:65-70. Dawes, C.J. and A.C. Mathieson 2008. The Seaweeds of Florida. University Press of Florida (592 pp). Flavin, K., N. Flavin and W.J. Flahive 2013. Kelp Farming Manual. Ocean Approved, LLC, Portland ME 04103
USA. [online]
Wehr, J.D. 2003. Brown Algae. In: Freshwater Algae of North America. J.D. Wehr and R.G. Sheath (Eds). Chapter 22. Wilce, R.T. 1966. Pleurocladia lacustris in Arctic America. J. Phycol.
2:57-66.
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