Photo of seaweed with small swollen areas at the end of each frond
Ascophyllum nodosum exposed to the sun in Nova Scotia, Canada
Photo of detached seaweed frond lying on sand
Dead Man's Fingers (Codium fragile) off Massachusetts coast in the United States
Photo of seaweed with the tip floating at the surface
The top of a kelp forest in Otago, New Zealand

Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae.[1] The term includes some members of the red, brown and green algae. Seaweeds can also be classified by use (as food, medicine, fertilizer, industrial, etc.).



A seaweed is in the kingdom Algaeil and may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups are not thought to have a common multicellular ancestor, the seaweeds are a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered as seaweeds — "seaweed" is a colloquial term and lacks a formal definition.


Seaweeds' appearance somewhat resembles non-arboreal terrestrial plants.

  • thallus: the algal body
    • lamina: a flattened structure that is somewhat leaf-like
    • stipe: a stem-like structure, may be absent
    • holdfast: specialized basal structure providing attachment to a surface, often a rock or another alga.
    • haptera: finger-like extensions of holdfast anchoring to benthic substrate

The stipe and blade are collectively known as the frond.


Two specific environmental requirements dominate seaweed ecology. These are the presence of seawater (or at least brackish water) and the presence of light sufficient to drive photosynthesis. Another common requirement is a firm attachment point. As a result, seaweeds most commonly inhabit the littoral zone and within that zone more frequently on rocky shores than on sand or shingle. Seaweeds occupy a wide range of ecological niches. The highest elevation is only wetted by the tops of sea spray, the lowest is several meters deep. In some areas, littoral seaweeds can extend several miles out to sea. The limiting factor in such cases is sunlight availability. The deepest living seaweeds are some species of red algae.

A number of species such as Sargassum have adapted to a fully planktonic niche and are free-floating, depending on gas-filled sacs to maintain an acceptable depth.

Others have adapted to live in tidal rock pools. In this habitat seaweeds must withstand rapidly changing temperature and salinity and even occasional drying.[2]


Photo of near-shore ocean, divided into rectangles, most containing a yards-long, narrow boat
Small plots being used to farm seaweed in Indonesia, with each rectangle belonging to a different family

Seaweed has a variety of purposes, for which it is farmed[3] or foraged from the wild.[4]

At the beginning of 2011, Indonesia produced 3 millions tonnes of seaweed and surpassed Philippines as the world's largest seaweed producer. By 2012 the production will hit 10 million tonnes.[5]


Seaweeds are consumed by coastal people, particularly in East Asia, e.g., Japan, China, Korea, Taiwan, Thailand, Cambodia, and Vietnam, but also in Indonesia, Belize, Peru, Chile the Canadian Maritimes, Scandinavia, South West England,[6] Ireland, Wales, California, Philippines, and Scotland.

In Asia, Nori(海苔)(in Japan), Zicai (紫菜) (in China), and Gim (김) (in Korea) are sheets of dried Porphyra used in soups or to wrap sushi. Chondrus crispus (commonly known as Irish Moss or carrageenan moss) is another red alga used in producing various food additives, along with Kappaphycus and various gigartinoid seaweeds. Porphyra is a red alga used in Wales to make laver. Laverbread, made from oats and the laver, is a popular dish there. Affectionately called "Dulce" in northern Belize, seaweeds are mixed with milk, nutmeg, cinnamon, and vanilla to make a common beverage.

Seaweeds are also harvested or cultivated for the extraction of alginate, agar and carrageenan, gelatinous substances collectively known as hydrocolloids or phycocolloids. Hydrocolloids have attained commercial significance as food additives.[7] The food industry exploits their gelling, water-retention, emulsifying and other physical properties. Agar is used in foods such as confectionery, meat and poultry products, desserts and beverages and moulded foods. Carrageenan is used in salad dressings and sauces, dietetic foods, and as a preservative in meat and fish products, dairy items and baked goods.


Photo of rocks covered by dried plant matter
Seaweed-covered rocks in the United Kingdom
Photo of a rock jetty covered with seaweed
Seaweed on rocks in Long Island

Alginates are used in wound dressings, and production of dental moulds. In microbiology research, agar is extensively used as culture medium.[citation needed]

Seaweed is a source of iodine,[8] necessary for thyroid function and to prevent goitre.

Seaweeds may have curative properties for tuberculosis, arthritis, colds and influenza, worm infestations and even tumors.[9][dubious ]

Seaweed extract is used in some diet pills.[10][11][12] Other seaweed pills exploit the same effect as gastric banding, expanding in the stomach to make the body feel more full.[13][14]

Other uses

Other seaweeds may be used as fertilizer.[citation needed] Seaweed is currently under consideration as a potential source of bioethanol.[15][16] Seaweed is an ingredient in toothpaste, cosmetics and paints.[3]

Alginates enjoy many of the same uses as carrageenan, and are used in industrial products such as paper coatings, adhesives, dyes, gels, explosives and in processes such as paper sizing, textile printing, hydro-mulching and drilling.

Health risks

The high iodine content of seaweed can produce iodine toxicity if large amounts of seaweed are consumed.

Rotting seaweed is a potent source of hydrogen sulfide, a highly toxic gas, and has been implicated in some incidents of apparent hydrogen-sulfide poisoning.[17] It can cause vomiting and diarrhea.

See also

Claudea elegans tetrasporangia
Seaweed used to thatch houses on Læsø, Denmark
Seaweed genera


  1. ^ Smith, G.M. 1944. Marine Algae of the Monterey Peninsula, California. Stanford Univ., 2nd Edition.
  2. ^ Lewis, J.R. 1964. The Ecology of Rocky Shores. The English Universities Press Ltd.
  3. ^ a b "Seaweed farmers get better prices if united". Sun.Star. 2008-06-19. Retrieved 2008-07-16. 
  4. ^ "Springtime's foraging treats". Life and Health, The Guardian. 2007-01-06.,,1981372,00.html. Retrieved 2008-07-16. 
  5. ^
  6. ^
  7. ^ Round F.E. 1962 The Biology of the Algae. Edward Arnold Ltd.
  8. ^ Iodine in Seaweed
  9. ^
  10. ^
  11. ^ Maeda, H; Hosokawa, M; Sashima, T; Funayama, K; Miyashita, K (Jul 2005). "Fucoxanthin from edible seaweed, Undaria pinnatifida, shows antiobesity effect through UCP1 expression in white adipose tissues". Biochemical and biophysical research communications 332 (2): 392–7. doi:10.1016/j.bbrc.2005.05.002. PMID 15896707. 
  12. ^
  13. ^,2933,476766,00.html?sPage=fnc/health/nutrition
  14. ^
  15. ^ Ireland Taps New Energy Source : Discovery News : Discovery Channel
  16. ^ Seaweed Biofuels: Production of Biogas and Bioethanol from Brown Macroalgae
  17. ^ "Algues vertes: la famille du chauffeur décédé porte plainte contre X" AFP, retrieved 2010-04-22 (in French)

External links

  • Michael Guiry's Seaweed Site, information on all aspects of algae, seaweeds and marine algal biology
  • AlgaeBase, a searchable taxonomic, image, and utilization database of freshwater, marine and terrestrial algae, including seaweed.
  • SeaweedAfrica, information on seaweed utilisation for the African continent.
  • Seaweed Malaysia Site, information on seaweed nutrition, facts and information for human health.

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