Brown algae

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Brown algae
Temporal range: 150–0Ma12
Giant kelp (Macrocystis pyrifera)
Scientific classification
Domain: Eukaryota
Kingdom: Chromalveolata
Division: Heterokontophyta
Class: Phaeophyceae
Kjellman, 18913
Orders

see Classification.
Synonyms

Fucophyceae
Melanophyceae
Phaeophyta

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 kelp of the order Laminariales, 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, commonly grow along rocky seashores. Some members of the class, such as kelp, 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 flagellated spores and gametes that closely resemble cells of other heterokonts. Genetic studies show their closest relatives to be the yellow-green algae.

Contents

Morphology

between 1501 and 200 million years ago.2 In many ways, the evolution of the brown algae parallels that of the green algae and red algae,6 as all three groups possess complex multicellular species with an alternation of generations. Analysis of 5S rRNA sequences reveals much smaller evolutionary distances among genera of the brown algae than among genera of red or green algae,27 which suggests that the brown algae have diversified much more recently than the other two groups.

Fossils

The occurrence of Phaeophyceae as fossils is rare due to their generally soft-bodied nature,8 and scientists continue to debate the identification of some finds.9 Part of the problem with identification lies in the convergent evolution of morphologies between many brown and red algae.10 Most fossils of soft-tissue algae preserve only a flattened outline, without the microscopic features that permit the major groups of multicellular algae to be reliably distinguished. Among the brown algae, only species of the genus Padina deposit significant quantities of minerals in or around their cell walls.11 Other algal groups, such as the red algae and green algae, have a number of calcareous members. Because of this, they are more likely to leave evidence in the fossil record than the soft bodies of most brown algae and more often can be precisely classified.12

Fossils comparable in morphology to brown algae are known from strata as old as the Upper Ordovician,13 but the taxonomic affinity of these impression fossils is far from certain.14 Claims that earlier Ediacaran fossils are brown algae15 have since been dismissed.16 While many carbonaceous fossils have been described from the Precambrian, they are typically preserved as flattened outlines or fragments measuring only millimeters long.17 Because these fossils lack features diagnostic for identification at even the highest level, they are assigned to fossil form taxa according to their shape and other gross morphological features.18 A number of Devonian fossils termed fucoids, from their resemblance in outline to species in the genus Fucus, have proven to be inorganic rather than true fossils.8 The Devonian megafossil Prototaxites, which consists of masses of filaments grouped into trunk-like axes, has been considered a possible brown alga.19 However, modern research favors reinterpretation of this fossil as a terrestrial fungus or fungal-like organism.20 Likewise, the fossil Protosalvinia was once considered a possible brown alga, but is now thought to be an early land plant.21

A number of Paleozoic fossils have been tentatively classified with the brown algae, although most have also been compared to known red algae species. Phascolophyllaphycus possesses numerous elongate, inflated blades attached to a stipe. It is the most abundant of algal fossils found in a collection made from Carboniferous strata in Illinois.22 Each hollow blade bears up to eight pneumatocysts at its base, and the stipes appear to have been hollow and inflated as well. This combination of characteristics is similar to certain modern genera in the order Laminariales (kelps). Several fossils of Drydenia and a single specimen of Hungerfordia from the Upper Devonian of New York have also been compared to both brown and red algae.10 Fossils of Drydenia consist of an elliptical blade attached to a branching filamentous holdfast, not unlike some species of Laminaria, Porphyra, or Gigartina. The single known specimen of Hungerfordia branches dichotomously into lobes and resembles genera like Chondrus and Fucus10 or Dictyota.23

The earliest known fossils that can be assigned reliably to the Phaeophyceae come from Miocene diatomite deposits of the Monterey Formation in California.4 Several soft-bodied brown macroalgae, such as Julescraneia, have been found.24

Classification

Further information: List of brown algal genera

This is a list of the orders in the class Phaeophyceae:25

Life cycle

Sexual reproduction may be isogamous, oogamous, or anisogamous. Union of gametes may take place in water or within the oogonium (oogamous species). 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, producing sporangia from specialised microscopic structures, which divide meiotically before they are released. As they are haploid there are equal numbers of male and female spores.26 With the exception of the Fucales, all brown algae have a life cycle with an alternation between haploid and diploid forms.

Ecology

Saccharina latissima on a beach.

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.16 A large number of Phaeophyceae are intertidal or upper littoral,16 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.16

Chemistry

Algal group δ13C range27
HCO3-using red algae −22.5‰ to −9.6‰
CO2-using red algae −34.5‰ to −29.9‰
Brown algae −20.8‰ to −10.5‰
Green algae −20.3‰ to −8.8‰

Brown algae have a δ13C value in the range of −20.8‰ to −10.5‰, in contrast with red algae and greens. This reflects their different metabolic pathways.28

They have cellulose walls with alginic acid and also contain the polysaccharide fucoidan in the amorphous sections of their cell walls. A few species (of Padina) calcify with aragonite needles.16

Brown algae produce a specific type of tannin called phlorotannins.

Importance and uses

The brown algae include a number of edible seaweeds.

See also

References

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  2. ^ a b c Lim, B.-L.; Kawai, H.; Hori, H.; Osawa, S. (1986). "Molecular evolution of 5S ribosomal RNA from red and brown algae". Japanese Journal of Genetics 61 (2): 169–176. doi:10.1266/jjg.61.169. 
  3. ^ Kjellman, F. R. (1891). "Phaeophyceae (Fucoideae)". In Engler, A.; Prantl, K. Die natürlichen Pflanzenfamilien 1 (2). Leipzig: Wilhelm Engelmann. pp. 176–192. 
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  8. ^ a b Arnold, C. A. (1947). An Introduction to Paleobotany. New York; London: McGraw-Hill. p. 48. ISBN 1-4067-1861-0. 
  9. ^ Coyer, J. A.; Smith, G. J.; Andersen, R. A. (2001). "Evolution of Macrocystis spp. (Phaeophyta) as determined by ITS1 and ITS2 sequences". Journal of Phycology 37 (4): 574–585. doi:10.1046/j.1529-8817.2001.037001574.x. 
  10. ^ a b c Fry, W. L.; Banks, H. P. (1955). "Three new genera of algae from the Upper Devonian of New York". Journal of Paleontology 29: 37–44. JSTOR 1300127. 
  11. ^ Prescott, G. W. (1968). The Algae: A Review. Boston: Houghton Mifflin Company. pp. 207–231, 371–372. ISBN 3-87429-244-4. 
  12. ^ Simpson, G. G. (1953). Life of the Past: An Introduction to Paleontology. New Haven: Yale University Press. pp. 158–159. 
  13. ^ Fry, W. L. (1983). "An algal flora from the Upper Ordovician of the Lake Winnipeg region, Manitoba, Canada". Review of Palaeobotany and Palynology 39 (3–4): 313–341. doi:10.1016/0034-6667(83)90018-0. 
  14. ^ Speer, B. R.; Waggoner, B. M. (2000). "Phaeophyta: Fossil Record". 
  15. ^ Loeblich, A. R. (1974). "Protistan Phylogeny as Indicated by the Fossil Record". Taxon 23 (2/3): 277–290. doi:10.2307/1218707. JSTOR 1218707. 
  16. ^ a b c d e Lee, R. E. (2008). Phycology (4th ed.). Cambridge University Press. ISBN 978-0-521-63883-8. 
  17. ^ Hofmann, H. J. (1985). "Precambrian Carbonaceous Megafossils". In D. F. Toomey & M. H. Nitecki. Paleoalgology: Contemporary Research and Applications. Berlin: Springer-Verlag. pp. 20–33. 
  18. ^ Hofmann, H. J. (1994). "Proterozoic carbonaceous compressions ("metaphytes" and "worms")". In Bengtson, S. Life on Earth. Nobel Symposium 84. New York: Columbia University Press. pp. 342–357. 
  19. ^ Bold, H. C.; Alexopoulos, C. J.; Delevoryas, T. (1987). Morphology of Plants and Fungi (5th ed.). New York: Harper & Row Publishers. pp. 112–131, 174–186. ISBN 0-06-040839-1. 
  20. ^ Hueber, F. M. (2001). "Rotted wood-alga-fungus: the history and life of Prototaxites Dawson 1859". Review of Palaeobotany and Palynology 116 (1): 123–158. doi:10.1016/S0034-6667(01)00058-6. 
  21. ^ Taylor, W. A.; Taylor, T. N. (1987). "Spore wall ultrastructure of Protosalvinia". American Journal of Botany 74 (3): 437–433. doi:10.2307/2443819. JSTOR 2443819. 
  22. ^ Leary, R. L. (1986). "Three new genera of fossil noncalcareous algae from Valmeyeran (Mississippian) strata of Illinois". American Journal of Botany 73 (3): 369–375. doi:10.2307/2444080. JSTOR 2444080. 
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External links

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Ochrophyta/
(photosynthetic)
Phaeophyceae
Fungus-like/
(nonphotosynthetic)