- For the band, see; Seaweed (band)
- For the rock musician, see; Seaweed (musician)
Seaweeds are any of a large number of marine benthic algae. They are macroscopic and multicellular, in contrast with most other algae.  Seaweeds areoften found in the seashore biome.
Seaweeds consist 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 paraphyletic group. In addition, tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered as seaweeds.
Seaweeds are popularly described as plants, but only red and green algae belong to the kingdom Plantae). They should not be confused with aquatic plants such as seagrasses (which are vascular plants).
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.
The stipe and blade are collectively known as fronds.
The ecology of seaweeds is dominated by two specific environmental requirements. These are the presence of sea-water (or at least brackish water) and the presence of light sufficient to drive photosynthesis. A very common requirement is also to have a firm point of attachment. As a result, seaweeds are most commonly found in the littoral zone and within that zone more frequently on rocky shores than on sand or shingle. The ecological niches utilised by seaweeds are wide ranging. At the highest level are those that inhabit the zone that is only wetted by the tops of sea spray, the deepest living are those that are attached to the sea-bed under several meters of water. In some parts of the world, the area colonized by littoral seaweeds can extend for several miles away from the shore. The limiting factor in such cases is the availability of sufficient sun-light to support photosynthesis. The deepest living sea-weeds are the various kelps.
In addition to the familiar sea-shore seaweeds, a number of species have adapted to a fully planktonic niche and are free-floating, often with the assistance of gas filled sacs. Sargassum is one of the better know examples of this type of seaweed.
A number of species have adapted to the specialised environment of tidal rock pools. In this niche seaweeds are able to withstand rapidly changing temperature and salinity and even occasional drying. 
Seaweeds are extensively used as food by coastal peoples, particularly in East Asia, e.g. Japan, China, Korea, Taiwan, and Vietnam, but also in Indonesia, Peru, the Canadian Maritimes, Scandinavia, Ireland, Wales, Philippines, and Scotland, among other places. For example, Porphyra is a red alga used in Wales to make laverbread. In Asia, nori is a food composed of sheets of dried Porphyra and is 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. Laverbread made from oats and the seaweed laver is a popular dish in Wales.
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, especially in food production as food additives.  The food industry exploits the gelling, water-retention, emulsifying and other physical properties of these hydrocolloids. Agar is used in foods such as confectionery, meats and poultry products, desserts and beverages and moulded foods. Carrageenan is used in preparation of salad dressings and sauces, dietetic foods, and as a preservative in meat and fish products, dairy items and baked goods. Alginates enjoy many of the same uses as carrageenan, but are also used in production of industrial products such as paper coatings, adhesives, dyes, gels, explosives and in processes such as paper sizing, textile printing, hydro-mulching and drilling.
In the biomedicine and pharmaceutical industries, alginates are used in wound dressings, and production of dental moulds and have a host of other applications. In microbiology research, agar 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. It has been asserted that seaweeds may have curative properties for tuberculosis, arthritis, colds and influenza, worm infestations and even tumors .[dubious ] A number of research studies have been conducted to investigate these claims and other effects of seaweed on human health .
See also Fucoidan
Other seaweeds may be used as seaweed fertilizer.
- The Seaweed Site, information on all aspects of 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.
- NCBI PubMed Search, allows for search on health research studies, including seaweed
- Fountain of Youth Equals Phorphyra Sp. (It Wouldn't Hurt) - A Review from the Science Creative Quarterly
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