Seaweed, or macroalgae, refers to several species of macroscopic, multicellular, marine algae. The term includes some types of Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) macroalgae. Seaweed species such as kelps provide essential nursery habitat for fisheries and other marine species and thus protect food sources; other species, such as planktonic algae, play a vital role in capturing carbon, producing up to 90% of Earth's oxygen. Understanding these roles offers principles for conservation and sustainable use. Mechanical dredging of kelp, for instance, destroys the resource and dependent fisheries.

Informal group of macroscopic marine algae
"Fucus serratus"
Fucus serratus
Scientific classification
Domain: Eukaryota
Seaweeds can be found in the following groups
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 the 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" lacks a formal definition. Generally it is one of several groups of multicellular algae: red, green and brown. They lack a common multicellular ancestor, forming a polyphyletic group. Some bluegreen algae (Cyanobacteria) are sometimes considered to be seaweed.[1][2]


Seaweed's appearance resembles non-arboreal terrestrial plants. Its anatomy includes:[3]

  • Thallus: algal body
    • Lamina or blade: flattened structure that is somewhat leaf-like
      • Sorus: spore cluster
      • pneumatocyst, air bladder: a flotation-assisting organ on the blade
      • Kelp, float: a flotation-assisting organ between the lamina and stipe
    • Stipe: stem-like structure, may be absent
    • Holdfast: basal structure providing attachment to a substrate
      • Haptera: finger-like extension of the holdfast that anchors to a benthic substrate

The stipe and blade are collectively known as the frond.


Seaweed covers this rocky seabed on the east coast of Australia

Two environmental requirements dominate seaweed ecology. These are seawater (or at least brackish water) and light sufficient to support photosynthesis. Another common requirement is an attachment point, although genera such as Sargassum and Gracilaria have species that float freely. Seaweed most commonly inhabits the littoral zone (nearshore waters) and, within that zone, on rocky shores more than on sand or shingle.

Seaweed occupies various ecological niches. At the surface, they are only wetted by the tops of sea spray, while some species may attach to a substrate several meters deep. In some areas, littoral seaweed colonies can extend miles out to sea.[citation needed] The deepest living seaweed are some species of red algae.

Others have adapted to live in tidal rock pools. In this habitat, seaweed must withstand rapidly changing temperature and salinity and occasional drying.[4]


As of 2018, the top 10 countries produced 96% of the global total of 2,165,675 metric tons.[5]

Seaweed production
Country Thousands metric tons
per year
China 699
France 617
United Kingdom 205
Japan 123
Chile 109
Philippines 96
North Korea 71
South Korea 67
Indonesia 47
Norway 41


Seaweed has a variety of uses, for which it is farmed[6] or foraged.[7]


Seaweed is consumed across the world, particularly in East Asia, e.g. Japan, China, Korea, Taiwan and Southeast Asia, e.g. Brunei, Singapore, Thailand, Burma, Cambodia, Vietnam, Indonesia, the Philippines, and Malaysia,[8] as well as in South Africa, Belize, Peru, Chile, the Canadian Maritimes, Scandinavia, South West England,[9] Ireland, Wales, Hawaii and California, and Scotland.

Gim (김, Korea), nori (海苔, Japan) and zicai (紫菜, China) are sheets of dried Porphyra used in soups, sushi or onigiri (rice balls). Chondrus crispus ('Irish moss' or carrageenan moss) is used in food additives, along with Kappaphycus and Gigartinoid seaweed. Porphyra is used in Wales to make laverbread (sometimes with oat flour). In northern Belize, seaweed is mixed with milk, nutmeg, cinnamon and vanilla to make "dulce" ("sweet").

Alginate, agar and carrageenan are gelatinous seaweed products collectively known as hydrocolloids or phycocolloids. Hydrocolloids are food additives.[10] 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, dairy items and baked goods.

Medicine and herbsEdit

Seaweed-covered rocks in the United Kingdom
Seaweed on rocks in Long Island

Alginates are used in wound dressings (see alginate dressing), and dental moulds. In microbiology, agar is used as a culture medium. Carrageenans, alginates and agaroses, with other macroalgal polysaccharides, have biomedicine applications. Delisea pulchra may interfere with bacterial colonization.[11] Sulfated saccharides from red and green algae inhibit some DNA and RNA-enveloped viruses.[12]

Seaweed extract is used in some diet pills.[13] Other seaweed pills exploit the same effect as gastric banding, expanding in the stomach to make the stomach feel more full.[14][15]


Algae's strong photosynthesis creates a large affinity for nutrients; this allows the seaweed to be used to remove undesired nutrients from water (as for instance in dead zones). Seaweed also generates oxygen, which benefits hypoxic (=oxygen-poor) dead zones.[16] Nutrients such as ammonia, ammonium nitrate, nitrite, phosphate, iron, copper, as well as CO2 are rapidly consumed by growing seaweed. Reefs and lakes are naturally filtered this way (seaweed is consumed by fish and invertebrates), and this filtering process is duplicated in artificial seaweed filters such as algae scrubbers. China could remove its entire phosphorus effluent by increasing seaweed production by 150%.[17]

Modern floating algae scrubber/cultivator on a reef pond

Seaweed (macroalgae), as opposed to phytoplankton (microalgae), is used almost universally for filtration purposes because of the need to be able to easily remove (harvest) the algae from the water, which then removes the nutrients. Microalgae require more processing to separate from the water than macroalgae do; macroalgae is simply pulled out.

Marine species of Cladophora, Ulva (sea lettuce) and Chaetomorpha are preferred for filtration. Freshwater filtration applications commonly involve species such as Spirogyra.

Climate changeEdit

"Ocean afforestation” is a proposal for farming seaweed for carbon removal. After harvesting the seaweed decomposes into biogas, (60% methane and 40% carbon dioxide) in an anaerobic digester. The methane can be used as a biofuel, while the carbon dioxide can be stored to keep it from the atmosphere. Seaweed grows quickly and takes no space on land. Afforesting 9% of the ocean could sequester 53 billion tons of carbon dioxide annually (annual emissions are about 40 billion tons).[17]

The approach requires efficient techniques for growing and harvesting, efficient gas separation, and carbon capture and storage. The Advanced Research Projects Agency for Energy has a $22 million program called Macroalgae Research Inspiring Novel Energy Resources (MARINER) supporting innovations that could aid a seaweed industry.<, ref name=":1" />

Other usesEdit

Other seaweed may be used as fertilizer, compost for landscaping, or to combat beach erosion through burial in beach dunes.[18]

Seaweed is under consideration as a potential source of bioethanol.[19][20]

Seaweed is lifted out of the top of algae scrubber/cultivator, to be discarded or used as food, fertilizer, or skin care

Alginates 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. Seaweed is an ingredient in toothpaste, cosmetics and paints. Seaweed is used for the production of bio yarn (a textile).[21]

Several of these resources can be obtained from seaweed through biorefining.

Seaweed collecting is the process of collecting, drying and pressing seaweed. It was a popular pastime in the Victorian era and remains a hobby today. In some emerging countries, Seaweed is harvested daily to support communities.

Women in Tanzania grow "Mwani" (seaweed in Swahili). The farms are made up of little sticks in neat rows in the warm, shallow water. Once they harvest the seaweed, it is used for many purposes: food, cosmetics, fabric, etc.

Seaweed is sometimes used to build roofs on houses on Læsø in Denmark[22]

Seaweeds are used as animal feeds. They have long been grazed by sheep, horses and cattle in Northern Europe. They are valued for fish production.[23] Adding seaweed to livestock feed can substantially reduce methane emissions from cattle.[24]

Health risksEdit

Rotting seaweed is a potent source of hydrogen sulfide, a highly toxic gas, and has been implicated in some incidents of apparent hydrogen-sulphide poisoning.[25] It can cause vomiting and diarrhea.

The so-called "stinging seaweed" Microcoleus lyngbyaceus is a filamentous cyanobacteria which contains toxins including lyngbyatoxin-a and debromoaplysiatoxin. Direct skin contact can cause seaweed dermatitis characterized by painful, burning lesions that last for days.[1][26]


Bacterial disease ice-ice infects Kappaphycus (red seaweed), turning its branches white. The disease caused heavy crop losses in the Philippines, Tanzania and Mozambique.[17]

Sea urchin barrens have replaced kelp forests in multiple areas. They are “almost immune to starvation”. Lifespans can exceed 50 years. When stressed by hunger, their jaws and teeth enlarge, and they form "fronts" and hunt for food collectively.[17]


Claudea elegans tetrasporangia

The following table lists a very few example genera of seaweed.

Genus Algae Phylum Remarks
Caulerpa   Green Submerged
Fucus   Brown In intertidal zones on rocky shores.
Gracilaria   Red Cultivated for food
Laminaria   Brown Also known as kelp, 8–30 m under water, cultivated for food.
Macrocystis   Brown Giant kelp, forming floating canopies.
Monostroma   Green
Porphyra   Red Intertidal zones in temperate climate. Cultivated for food.

See alsoEdit


  1. ^ a b c "Escharotic stomatitis caused by the "stinging seaweed" Microcoleus lyngbyaceus (formerly Lyngbya majuscula): case report and literature review"
  2. ^ a b James, William D.; Berger, Timothy G.; et al. (2006). Andrews' Diseases of the Skin: clinical Dermatology. Saunders Elsevier. ISBN 978-0-7216-2921-6.
  3. ^ "seaweed menu". Retrieved 2019-04-28.
  4. ^ Lewis, J. R. 1964. The Ecology of Rocky Shores. The English Universities Press Ltd.
  5. ^ "Volume of seaweed production ranked by country". Retrieved 2019-04-28.
  6. ^ "Seaweed farmers get better prices if united". Sun.Star. 2008-06-19. Archived from the original on 2008-09-09. Retrieved 2008-07-16.
  7. ^ "Springtime's foraging treats". The Guardian. London. 2007-01-06. Retrieved 2008-07-16.
  8. ^ Mohammad, Salma (4 Jan 2020). "Application of seaweed (Kappaphycus alvarezii) in Malaysian food products". International Food Research Journal. 26: 1677–1687.
  9. ^ "Devon Family Friendly – Tasty Seaweed Recipe – Honest!". BBC. 2005-05-25. Retrieved 2012-06-28.
  10. ^ Round F. E. 1962 The Biology of the Algae. Edward Arnold Ltd.
  11. ^ Francesca Cappitelli; Claudia Sorlini (2008). "Microorganisms attack synthetic polymers in items representing our cultural heritage". Applied and Environmental Microbiology. 74 (3): 564–569. doi:10.1128/AEM.01768-07. PMC 2227722. PMID 18065627.
  12. ^ Kazłowski B.; Chiu Y. H.; Kazłowska K.; Pan C. L.; Wu C. J. (August 2012). "Prevention of Japanese encephalitis virus infections by low-degree-polymerisation sulfated saccharides from Gracilaria sp. and Monostroma nitidum". Food Chem. 133 (3): 866–74. doi:10.1016/j.foodchem.2012.01.106.
  13. ^ Maeda, Hayato; Hosokawa, Masashi; Sashima, Tokutake; Funayama, Katsura; Miyashita, Kazuo (2005-07-01). "Fucoxanthin from edible seaweed, Undaria pinnatifida, shows antiobesity effect through UCP1 expression in white adipose tissues". Biochemical and Biophysical Research Communications. 332 (2): 392–397. doi:10.1016/j.bbrc.2005.05.002. ISSN 0006-291X. PMID 15896707.
  14. ^ "New Seaweed Pill Works Like Gastric Banding". Fox News.
  15. ^ Elena Gorgan (6 January 2009). "Appesat, the Seaweed Diet Pill that Expands in the Stomach". softpedia.
  16. ^ Life-giving Slime
  17. ^ a b c d Buck, Holly Jean (April 23, 2019). "The desperate race to cool the ocean before it's too late". MIT Technology Review. Retrieved 2019-04-28.
  18. ^ Rodriguez, Ihosvani (April 11, 2012). "Seaweed invading South Florida beaches in large numbers". South Florida Sun-Sentinel. Retrieved 2012-04-11.
  19. ^ "Seaweed Power: Ireland Taps New Energy Source". 2008-06-24. Retrieved 9 April 2018.
  20. ^ Chen, Huihui; Zhou, Dong; Luo, Gang; Zhang, Shicheng; Chen, Jianmin (2015). "Macroalgae for biofuels production: Progress and perspectives". Renewable and Sustainable Energy Reviews. 47: 427–437. doi:10.1016/j.rser.2015.03.086.
  21. ^ "The promise of Bioyarn from AlgiKnit". MaterialDriven.
  22. ^ "Seaweed Thatch". Retrieved 9 April 2018.
  23. ^ Heuzé V., Tran G., Giger-Reverdin S., Lessire M., Lebas F., 2017. Seaweeds (marine macroalgae). Feedipedia, a programme by INRA, CIRAD, AFZ and FAO. Last updated on May 29, 2017, 16:46
  24. ^ "Seaweed shown to reduce 99% methane from cattle". Retrieved 9 April 2018.
  25. ^ "Algues vertes: la famille du chauffeur décédé porte plainte contre X" AFP, retrieved 2010-04-22 (in French)
  26. ^ Werner, K. A.; Marquart, L.; Norton, S. A. (2012). "Lyngbya dermatitis (toxic seaweed dermatitis)". International Journal of Dermatology. 51 (1): 59–62. doi:10.1111/j.1365-4632.2011.05042.x. PMID 21790555.

Further readingEdit

External linksEdit