Seaweed
Seaweeds are macroscopic, multicellular, marine algae. They are generally benthic, being anchored to the bottom of the ocean or to solid structures.
Algae (singular alga), a name derived from the Latin word for seaweed, are a large and diverse group of photosynthetic, eukaryotic, plant-like organisms that use chlorophyll in capturing light energy, but lack characteristic plant structures such as leaves, roots, flowers, vascular tissue, and seeds. Although they have historically been regarded as simple plants, they are generally classified in the kingdom Protista, rather than Plantae.
Although algae range from single-celled organisms to multicellular organisms, if they are both multicellular and marine, and are easily seen by the naked eye, they are generally called seaweeds. Single-celled or few-celled organisms are not usually called seaweeds. Seaweeds themselves have many forms, including those that appear as if they are terrestrial plants with leaves and stems, looking like moss, mushrooms, leaf lettuce, or even a palm tree. Some are quite large: the multicellular giant kelp reaches 60 meters in length.
Rather than a specific taxa, seaweed can be one of several types of algae: brown algae, red algae, or green algae. Most of the seaweeds of the warm oceans are red algae. They absorb the deep penetrating blue light, allowing them to exist deeper than other algae. The brown algae include the major seaweeds found on the shores in the temperate zones and the large, offshore beds of kelps. There are few green algae that are seaweeds, but one is the sea lettuce.
Seaweed can be used as food, as fertilizer. The red algae is also a source of agar, a gelatinous polysaccharide that is used as a culture medium for microbiological work, as well as vegetarian gelatin substitute, a thickener for soups, in jellies, ice cream, and so forth.
Taxonomy
Seaweeds are classified into brown algae (Phaeophyta), red algae (Rhodophyta), and green algae (Chlorophyta). Note that in reality, the term algae is mainly used for convenience, rather than taxonomic purposes, as there appears little relationship between the various phyla.
Seaweeds are often confused with other photosynthetic organisms. Seaweeds are popularly described as plants, but biologists typically do not consider them true Plantae. They also should not be confused with seagrasses, which are vascular plants. In addition, a few species of cyanobacteria bear a resemblance to seaweed algae.
Some biologists prefer the term "marine macroalgae" over "seaweeds."
Brown algae
The Phaeophyta, or brown algae (Class Phaeophyceae, Division Heterokontophyta, Kingdom Protista), are a large group of multicellular, mostly marine algae, and include many seaweeds of colder Northern Hemisphere waters.
One example of a brown algae seaweed is Sargassum, which creates unique habitats in the tropical waters of the Sargasso Sea. This is one of the few areas where a large biomass of brown algae may be found in tropical waters.
Kelp are large seaweeds belonging to the brown algae and are classified in the order Laminariales. There are about 30 different genera. Kelp grows in underwater forests (kelp forests) in clear, shallow oceans. They require nutrient rich water below about 20°C. Kelp is known for its high growth rate and is the largest seaweed. Macrocystis, a member of the Laminariales, may reach 60 meters in length and grows up to 30 centimeters per day.
Red algae
The red algae (Phylum Rhodophyta, from Greek rhodon' = rose + phyton = plant, thus red plant) are a large group of mostly multicellular, marine algae, including many notable seaweeds. Red algae are a traditional part of European and Asian cuisine and are used to make other products like agar, carrageenans, and other food additives.
Green algae
The Chlorophyta, or green algae (Division Chlorophyta), include about 8000 species[3] of mostly aquatic organisms. Like the land plants (Bryophyta and Tracheophyta), green algae contain chlorophylls a and b, and store food as starch in their plastids. They contain both unicellular and multicellular species. While most species live in freshwater habitats, and a large number in marine habitats, other species are adapted to a wide range of environments. Few are actually seaweeds, however, either because they are freshwater or microscopic. The sea lettuces (genus genus) are a notable exception.
Structure
Seaweeds' appearance often resembles non-arboreal, terrestrial plants. For example, they have root-like structures (holdfast) that anchor them to the substrate. However, in function they are unlike terrestrial plants, since they do not absorb nutrients, but solely serve to hold the seaweed in place. Examples of similar structures on the algal body (thallus) include:
- thallus: the algal body
- stipe: a stem-like structure (may be absent)
- holdfast: specialized basal structure providing attachment to a surface, often a rock or another alga.
- lamina: a flattened structure that is somewhat leaf-like
- sorus: spore cluster
- on Fucus, air bladders: float-assist organ (on blade)
- on kelp, floats: float-assist organ (between lamina and stipe)
The stipe and blade are collectively known as fronds. Some seaweeds have gas in the fronds that help them to be bouyant and float at or near the surface.
Uses
Food
Seaweeds are extensively used as food by coastal peoples, particularly in Japan and Korea, but also in China, Vietnam, Indonesia, Peru, Taiwan, 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, and in Japan dried, formed into sheets called nori which is widely used in soups, and for wrapping sushi, boiled rice stuffed with bits of raw fish, sea urchin roe, or other ingredients. Chondrus crispus (commonly known as Irish moss or carrageen moss) is another red alga used in producing various food additives, along with Kappaphycus and various gigartinoid seaweeds.
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 [1]. 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. The food industry exploits the gelling, water-retention, emulsifying and other physical properties of these hydrocolloids.
Medicine
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.[citation needed]
A number of research studies have been conducted to investigate claims of seaweed's effects on human health[2]. It has been asserted that seaweeds may have curative properties for tuberculosis, arthritis, colds and influenza, worm infestations and even tumors[3].[dubious — see talk page]
Other uses
Other seaweeds may be used as seaweed fertiliser.
Kelp offers protection to some sea creatures and food for others. Through the 19th Century, the word "kelp" was closely associated with seaweeds that could be burned to obtain soda ash (primarily sodium carbonate). The seaweeds used included species from both the orders Laminariales and Fucales. The word "kelp" was also used directly to refer to these processed ashes.[4]
ReferencesISBN links support NWE through referral fees
Lewis, J.R. 1964. The Ecology of Rocky Shores. The English Universities Press Ltd. Smith, G.M. 1944. Marine Algae of the Monterey Peninsula, California. Stanford Univ., 2nd Edition.</ref>.
External links
- 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 seaweeds. The database is searched 10,000 times a day.
- SeaweedAfrica, information on seaweed utilisation for the African continent.
- NCBI PubMed Search, allows for search on health research studies, including seaweed
- Seaweed Research Links, Many useful seaweed research links
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