Seaweed


Seaweed, or macroalgae, refers to several species of macroscopic, multicellular, marine algae. The term includes some types of Rhodophyta, Phaeophyta and Chlorophyta 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.

Taxonomy

"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 are sometimes considered to be seaweed.

Anatomy

Seaweed's appearance resembles non-arboreal terrestrial plants. Its anatomy includes:
The stipe and blade are collectively known as the frond.

Ecology

Two environmental requirements dominate seaweed ecology. These are seawater 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 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. 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.

Production

As of 2018, the top 10 countries produced 96% of the global total of 2,165,675 metric tons.
CountryThousands metric tons
per year
China699
France617
United Kingdom205
Japan123
Chile109
Philippines96
North Korea71
South Korea67
Indonesia47
Norway41

Uses

Seaweed has a variety of uses, for which it is farmed or foraged.

Food

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, Philippines, and Malaysia, and also in South Africa, Belize, Peru, Chile, the Canadian Maritimes, Scandinavia, South West England, Ireland, Wales, Hawaii and California, and Scotland.
Gim, nori and zicai are sheets of dried Porphyra used in soups, sushi or onigiri. Chondrus crispus is used in food additives, along with Kappaphycus and Gigartinoid seaweed. Porphyra is used in Wales to make laverbread. In northern Belize, seaweed is mixed with milk, nutmeg, cinnamon and vanilla to make "dulce".
Alginate, agar and carrageenan are gelatinous seaweed products collectively known as hydrocolloids or phycocolloids. Hydrocolloids are food additives. 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 herbs

Alginates are used in wound dressings, 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. Sulfated saccharides from red and green algae inhibit some DNA and RNA-enveloped viruses.
Seaweed extract is used in some diet pills. Other seaweed pills exploit the same effect as gastric banding, expanding in the stomach to make the stomach feel more full.

Bioremediation

Algae's strong photosynthesis creates a large affinity for nutrients; this allows the seaweed to be used to remove undesired nutrients from water. Seaweed also generates oxygen, which benefits hypoxic dead zones. 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, 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%.
Seaweed, as opposed to phytoplankton, is used almost universally for filtration purposes because of the need to be able to easily remove 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 and Chaetomorpha are preferred for filtration. Freshwater filtration applications commonly involve species such as Spirogyra.

Climate change

"Ocean afforestation” is a proposal for farming seaweed for carbon removal. After harvesting the seaweed decomposes into biogas, 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.
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 supporting innovations that could aid a seaweed industry.<, ref name=":1" />

Other uses

Other seaweed may be used as fertilizer, compost for landscaping, or to combat beach erosion through burial in beach dunes.
Seaweed is under consideration as a potential source of bioethanol.
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.
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.
Seaweed is sometimes used to build roofs on houses on Læsø in Denmark
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. Adding seaweed to livestock feed can substantially reduce methane emissions from cattle.

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. 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.

Threats

Bacterial disease ice-ice infects Kappaphycus, turning its branches white. The disease caused heavy crop losses in the Philippines, Tanzania and Mozambique.
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.

Genera

The following table lists a very few example genera of seaweed.
GenusAlgae PhylumRemarks
CaulerpaGreenSubmerged
FucusBrownIn intertidal zones on rocky shores.
GracilariaRedCultivated for food
LaminariaBrownAlso known as kelp, 8–30 m under water, cultivated for food.
MacrocystisBrownGiant kelp, forming floating canopies.
MonostromaGreen
PorphyraRedIntertidal zones in temperate climate. Cultivated for food.