Macroalgae

This article is about aquatic plant-like algae. For use in food, see edible seaweed. For other uses, see Seaweed (disambiguation).



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.). As with other algae, seaweed are not plants.

Taxonomy

A seaweed 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 to be seaweeds — "seaweed" is a colloquial term and lacks a formal definition.

Structure

Seaweeds' 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 floatation-assisting organ on the blade
      • on kelp, float: a floatation-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

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]

Uses


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 million tonnes of seaweed and surpassed the Philippines as the world's largest seaweed producer. By 2012 the production will hit 10 million tonnes.[5]

Food

Main article: Edible seaweed

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

In Asia, Nori (海苔, Japan), Zicai (紫菜, China), and Gim (김, 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. In northern Belize, edible seaweeds are mixed with milk, nutmeg, cinnamon, and vanilla to make a common beverage affectionately called "Dulce" (or "sweet").

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.

Herbalism


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), together with other lesser-known macroalgal polysaccharides, also have several important biological activities or applications in biomedicine.

Seaweed is a source of iodine,[8] necessary for thyroid function and to prevent goitre. However, an excess of iodine is suspected in the heightened cancer risk in Japanese who consume a lot of the plant, and even bigger risks in post-menopausal women.[9]

Seaweeds may have curative properties for tuberculosis, arthritis, colds and influenza, worm infestations and even tumors.[10][dubious ] In Japan, seaweed eaten as nori is known as a remedy for radiation poisoning.

Seaweed extract is used in some diet pills.[11][12][13] Other seaweed pills exploit the same effect as gastric banding, expanding in the stomach to make the body feel more full.[14][15]

Other uses

Other seaweeds may be used as fertilizer, compost for landscaping, or a means of combating beach erosion through burial in beach dunes.[16] Seaweed is currently under consideration as a potential source of bioethanol.[17][18] 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. Research suggests that the Australian seaweed Delisea pulchra may interfere with bacterial colonization.[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 diarrhoea.

Genera

Genus Algae type Remarks
Caulerpa Green Under water
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

References

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. A chemical industry in Brittany, in the past and today.
  • Sea Greens. An ingenious new business provides Maine’s lobster crews with a self-kelp manual for their downtime in winter. Portland Magazine, October 2013
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