This article is about the aquatic plant-like algae. For its culinary uses, see
Edible seaweed. For the similarly-named plant, see
Seagrass. For other uses, see
Seaweed (disambiguation).
Seaweed, or macroalgae, refers to thousands of species of
macroscopic,
multicellular,
marinealgae. The term includes some types of Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) 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 and producing at least 50% of Earth's oxygen.[3]
Natural seaweed ecosystems are sometimes under threat from human activity. For example, mechanical
dredging of kelp destroys the resource and dependent fisheries. Other forces also threaten some seaweed ecosystems; for example, a
wasting disease in predators of
purple urchins has led to an urchin population surge which has destroyed large
kelp forest regions off the coast of California.[4]
"Seaweed" lacks a formal definition, but seaweed generally lives in the ocean and is visible to the naked eye. The term refers to both flowering plants submerged in the ocean, like
eelgrass, as well as larger marine algae. Generally, it is one of several groups of multicellular
algae;
red,
green and
brown.[7] They lack one common multicellular ancestor, forming a
polyphyletic group. In addition, blue-green algae (
Cyanobacteria) are occasionally considered in seaweed literature.[8]
The number of seaweed species is still a topic of discussed among scientists, but it is most likely that there are several thousand species of seaweed.[9]
Genera
The following table lists a very few example genera of seaweed.
Holdfast: basal structure providing attachment to a substrate
Haptera: finger-like extension of the holdfast that anchors to a
benthic substrate
The stipe and blade are collectively known as the
frond.
Ecology
Two environmental requirements dominate seaweed
ecology. These are seawater (or at least
brackish water) and light sufficient to support
photosynthesis. Another common requirement is an attachment point, and therefore seaweed most commonly inhabits the
littoral zone (nearshore waters) and within that zone, on rocky shores more than on sand or shingle. In addition, there are few
genera (e.g., Sargassum and Gracilaria) which do not live attached to the sea floor, but float freely.
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.[12] 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.[13]
Macroalgae and macroalgal detritus have also been shown to be an important food source for benthic organisms, because macroalgae shed old
fronds.[14]
These macroalgal fronds tend to be utilized by
benthos in the
intertidal zone close to the shore.[15][16]
Alternatively,
pneumatocysts (gas filled "bubbles") can keep the macroalgae
thallus afloat; fronds are transported by wind and currents from the coast into the deep ocean.[14] It has been shown that
benthic organisms also at several 100 m tend to utilize these macroalgae remnants.[16]
As macroalgae takes up
carbon dioxide and releases
oxygen in the
photosynthesis, macroalgae fronds can also contribute to
carbon sequestration in the ocean, when the macroalgal fronds drift offshore into the deep
ocean basins and sink to the sea floor without being remineralized by organisms.[14] The importance of this process for the
Blue Carbon storage is currently a topic of discussion among scientists.[17][18][19]
Biogeographic expansion
Nowadays a number of vectors - e.g., transport on ship hulls, exchanges among shellfish farmers, global warming, opening of trans-oceanic canals - all combine to enhance the transfer of exotic seaweeds to new environments. Since the piercing of the Suez Canal, the situation is particularly acute in the Mediterranean Sea, a 'marine biodiversity hotspot' that now registers over 120 newly introduced seaweed species -the largest number in the world.[20]
Production
As of 2019, 35,818,961 tonnes were produced, of which 97.38% were produced in Asian countries.[21]
Seaweed farming or
kelp farming is the practice of
cultivating and
harvesting seaweed. In its simplest form farmers gather from natural beds, while at the other extreme farmers fully control the crop's
life cycle.
The largest seaweed-producing countries as of 2022 are
China (58.62%) and
Indonesia (28.6%); followed by
South Korea (5.09%) and the
Philippines (4.19%). Other notable producers include
North Korea (1.6%),
Japan (1.15%),
Malaysia (0.53%),
Zanzibar (
Tanzania, 0.5%), and
Chile (0.3%).[24][25] Seaweed farming has frequently been developed to improve economic conditions and to reduce fishing pressure.[26]
The
Food and Agriculture Organization (FAO), reported that world production in 2019 was over 35 million tonnes. North America produced some 23,000 tonnes of wet seaweed. Alaska, Maine, France, and Norway each more than doubled their seaweed production since 2018. As of 2019, seaweed represented 30% of marine
aquaculture.[27]
Alginate,
agar and
carrageenan are gelatinous seaweed products collectively known as
hydrocolloids or
phycocolloids. Hydrocolloids are food additives.[36] 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.
Seaweeds are used as animal feeds. They have long been grazed by sheep, horses and cattle in Northern Europe, even though their nutritional benefits are questionable. Their protein content is low and their heavy metal content is high, especially for arsenic and iodine, which are respectively toxic and nutritious.[37][38]
They are valued for fish production.[39] Adding seaweed to livestock feed can substantially reduce
methane emissions from cattle,[40] but only from their feedlot emissions. As of 2021, feedlot emissions account for 11% of overall emissions from cattle. [41]
Alginates are used in wound dressings (see
alginate dressing), 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.[42] Sulfated saccharides from red and green algae inhibit some
DNA and
RNA-enveloped
viruses.[43]
Seaweed extract is used in some diet pills.[44] Other seaweed pills exploit the same effect as
gastric banding, expanding in the stomach to make the stomach feel more full.[45][46]
Seaweed cultivation in the open ocean can act as a form of carbon sequestration to mitigate climate change.[47][48] Studies have reported that nearshore seaweed forests constitute a source of
blue carbon, as seaweed detritus is carried into the middle and deep ocean thereby sequestering carbon.[30][29][49][50][51]Macrocystis pyrifera (also known as giant kelp) sequesters carbon faster than any other species. It can reach 60 m in length and grow as rapidly as 50 cm a day.[52] According to one study, covering 9% of the world’s oceans with kelp forests could produce “sufficient
biomethane to replace all of today’s needs in fossil fuel energy, while removing 53 billion tons of CO2 per year from the atmosphere, restoring pre-industrial levels”.[53][54]
Seaweed is under consideration as a potential source of
bioethanol.[56][57]
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 (a textile).[58]
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[59]
Small plots being used to farm seaweed in
Indonesia, with each rectangle belonging to a different family
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.[60] It can cause vomiting and diarrhea.[61]
Bacterial disease
ice-ice infects Kappaphycus (red seaweed), turning its branches white. The disease caused heavy crop losses in the Philippines, Tanzania and Mozambique.[63]
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.[63]
See also
Algaculture – Aquaculture involving the farming of algae
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