Seaweed

Ascophyllum nodosum exposed to the sun in Nova Scotia, Canada

Dead man's fingers (Codium fragile) off the Massachusetts coast in the United States

The top of a kelp forest in Otago, New Zealand

Seaweed refers to several species of macroscopic, multicellular, marine algae.^[1]

The term includes some types of red, brown, and green algae. Seaweed can also be classified by use (as food, medicine,^[2] fertilizer, filtration, industrial, etc.).

Taxonomy

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

Structure

Seaweed's appearance somewhat resembles non-arboreal terrestrial plants.

• thallus: the algal body
  • lamina or blade: a flattened structure that is somewhat leaf-like
    • sorus: a spore cluster
    • on Fucus, air bladder: a flotation-assisting organ on the blade
    • on kelp, float: a flotation-assisting organ between the lamina and stipe
  • stipe: a stem-like structure, may be absent
  • holdfast: a specialized basal structure providing attachment to a surface, often a rock or another alga
  • haptera: a finger-like extension of the holdfast anchoring to a benthic substrate

The stipe and blade are collectively known as the frond.

Ecology

Seaweed cover this rocky seabed on the east coast of Australia

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, although some genera such as Sargassum and Gracilaria have species that float freely. As a result, seaweed most commonly inhabit the part of a sea that is close to the shore (the littoral zone) and within that zone more frequently on rocky shores than on sand or shingle. Seaweed 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 seaweed can extend several miles out to sea. The limiting factor in such cases is sunlight availability. 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 even occasional drying.^[3]

Uses

Onigiri and wakame miso soup, Japan

Laver and toast

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^[4] or foraged from the wild.^[5]

At the beginning of 2011, Indonesia produced 3 million tonnes of seaweed and surpassed the Philippines as the world's largest seaweed producer. By 2011, the production was estimated to have reached 10 million tonnes.^[6]

Food

Seaweed is consumed by coastal people, particularly in East Asia, e.g. Japan, China, Korea, Taiwan, and Southeast Asia, e.g. Brunei, Singapore, Thailand, Burma, Cambodia, Vietnam, Indonesia, Philippines, and Malaysia, and also in South Africa, Belize, Peru, Chile, the Canadian Maritimes, Scandinavia, South West England,^[7] Ireland, Wales, California, and Scotland.

In Asia, Gim (Korean food) (김, Korea), nori (海苔, Japan), zicai (紫菜, China) are sheets of dried Porphyra used in soups, sushi wrap or onigiri (rice balls). Chondrus crispus (commonly known as 'Irish moss' or carrageenan moss) is another red alga used in producing food additives, along with Kappaphycus and gigartinoid seaweed. Porphyra is a red alga used in Wales to make laver. Laverbread, made from oats and the laver, is a popular dish there. In northern Belize, edible seaweed are mixed with milk, nutmeg, cinnamon, and vanilla to make a common beverage affectionately called "dulce" (or "sweet").

Seaweed 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.^[8] 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.

Herbalism

Seaweed-covered rocks in the United Kingdom

Seaweed on rocks in Long Island

Alginates are commonly used in wound dressings, and production of dental moulds. In microbiology research, agar — a plant-based goo similar to gelatin and made from seaweed — is extensively used as culture medium. Carrageenans, alginates and agaroses (the latter are prepared from agar by purification), with other lesser-known macroalgal polysaccharides, have several important biological activities or applications in biomedicine.

Seaweed extract is used in some diet pills.^[9][10] Other seaweed pills exploit the same effect as gastric banding, expanding in the stomach to make the body feel more full.^[11][12]

Filtration

The strong photosynthesis of algae creates a large affinity for nutrients; this allows the seaweed to be used purposely to remove undesired nutrients from water. 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 (the seaweed being consumed by fish and invertebrates), and this filtering process is duplicated in man-made seaweed filters such as algae scrubbers.

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 it from the water than macroalgae does; macroalgae is simply pulled out.

When used for filtration, saltwater algae commonly grows species of Cladophora, Ulva (sea lettuce), and Chaetomorpha. Freshwater filtration applications are useful, too, and will commonly grow species such as Spirogyra.

Other uses

Other seaweed may be used as fertilizer, compost for landscaping, or a means of combating beach erosion through burial in beach dunes.^[13] Seaweed is under consideration as a potential source of bioethanol.^[14][15]

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

Seaweed is an ingredient in toothpaste, cosmetics and paints.^[4] 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. Research suggests that the Australian seaweed Delisea pulchra may interfere with bacterial colonization.^[16] Sulfated saccharides from both red and green algae have been known to inhibit some DNA and RNA enveloped viruses.^[2]

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.

Seaweed is sometimes used to build roofs on houses on Læsø in Denmark ^[17]

Seaweeds are also used as animal feeds. They have long been grazed by sheep, horses and cattle in Northern Europe. They are currently particularly valuable for fish production.^[18] Adding seaweed to livestock feed can substantially reduce methane emissions from cattle.^[19]

Health risks

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.^[20] It can cause vomiting and diarrhea.

Genera

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 also

Claudea elegans tetrasporangia

• Algaculture
• Algae fuel
• Aonori, Hijiki, Kombu, Mozuku, Nori, Ogonori, and Wakame - seaweed preparations used in Japan, and occasionally Korea and parts of Oceania
• Cochayuyo, a form of kelp used as a vegetable in Chile
• Sea lettuce
• Seaweed cultivator
• Seaweed dermatitis

References

Further reading

• Christian Wiencke, Kai Bischof [editors]: Seaweed Biology: Novel Insights into Ecophysiology, Ecology & Utilization. Springer, 2012. ISBN 978-3-642-28450-2 (print); ISBN 978-3-642-28451-9 (eBook)

External links

Wikimedia Commons has media related to Seaweed.

• Michael Guiry's Seaweed Site information on all aspects of algae, seaweed and marine algal biology
• SeaweedAfrica, information on seaweed utilisation for the African continent.
• Seaweed. A chemical industry in Brittany, in the past and today.
• AlgaeBase, a searchable taxonomic, image, and utilization database of freshwater, marine and terrestrial algae, including seaweed.