Underwater Eucheuma farming in the Philippines

A seaweed farmer in Nusa Lembongan (Indonesia) gathers edible seaweed that has grown on a rope.

Seaweed farming or kelp farming is the practice of cultivating and harvesting seaweed. In its simplest form, it consists of the management of naturally found batches. In its most advanced form, it consists of fully controlling the life cycle of the algae.

The top seven most cultivated seaweed taxa are Eucheuma spp., Kappaphycus alvarezii, Gracilaria spp., Saccharina japonica, Undaria pinnatifida, Pyropia spp., and Sargassum fusiforme. Eucheuma and K. alvarezii are farmed for carrageenan (a gelling agent); Gracilaria is farmed for agar; while the rest are farmed for food. The largest seaweed-producing countries are China, Indonesia, and the Philippines. Other notable producers include South Korea, North Korea, Japan, Malaysia, and Zanzibar (Tanzania).^[1] Seaweed farming has frequently been developed as an alternative to improve economic conditions and to reduce fishing pressure and overexploited fisheries.^[2]

Global production of farmed aquatic plants, overwhelmingly dominated by seaweeds, grew in output volume from 13.5×10^⁶ t (13,300,000 long tons; 14,900,000 short tons) in 1995 to just over 30×10^⁶ t (30,000,000 long tons; 33,000,000 short tons) in 2016.^[3] As of 2014, seaweed was 27% of all marine aquaculture.^[4] Seaweed farming is a carbon negative crop, with a high potential for climate change mitigation .^[4] The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" as a mitigation tactic.^[5]

History

Bundles of brush in the Tama River estuary used for growing Porphyra algae in Japan, c. 1921

Human use of seaweed harvested from the wild date back to at least the Neolithic period.^[1]

Cultivation of gim (laver) in Korea is reported in books from the 15th century, such as Revised and Augmented Survey of the Geography of Korea and Geography of Gyeongsang Province.^[6][7]

Seaweed farming began in Japan as early as 1670 in Tokyo Bay.^[8] In autumn of each year, farmers would throw bamboo branches into shallow, muddy water, where the spores of the seaweed would collect. A few weeks later these branches would be moved to a river estuary. The nutrients from the river would help the seaweed to grow.^[8]

Eucheuma farming in the Philippines

In the 1940s, the Japanese improved this method by placing nets of synthetic material tied to bamboo poles. This effectively doubled the production.^[8] A cheaper variant of this method is called the hibi method — simple ropes stretched between bamboo poles. In the early 1970s, there was a recognized demand for seaweed and seaweed products, outstripping supply, and cultivation was viewed as the best means to increase productions.^[9]

In the tropics, commercial cultivation of Caulerpa lentillifera (sea grapes) was pioneered in the 1950s in Cebu, Philippines, after accidental introduction of C. lentillifera to fish ponds in the island of Mactan.^[10][11] This was further developed by local research, particularly through the efforts of Gavino Trono, since recognized as a National Scientist of the Philippines. Local research and experimental cultures led to the development of the first commercial farming methods for other warm-water algae (since cold-water red and brown edible algae favored in East Asia do not grow in the tropics), including the first successful commercial cultivation of carrageenan-producing algae. These include Eucheuma spp., Kappaphycus alvarezii, Gracilaria spp., and Halymenia durvillei.^{[12][13][14][15]} In 1997, it was estimated that 40,000 people in the Philippines made their living through seaweed farming.^[16] The Philippines was the world's largest producer of carrageenan for several decades, until it was overtaken by Indonesia in 2008.^{[17][18][19][20]}

The practice of seaweed farming has long since spread beyond Japan and the Philippines. Cultivation is also common in all of southeast Asia, Canada, Great Britain, Spain, and the United States.^[21]

In the 2000s, seaweed farming has been getting increasing attention due to its potential for mitigating both climate change and other environmental issues, such as agricultural runoff.^[22][23] Seaweed farming can be mixed with other aquaculture, such as shellfish, to improve water bodies, such as in the practices developed by American non-profit GreenWave.^[22] The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" as a mitigation tactic.^[5]

Methods

The earliest seaweed farming guides in the Philippines recommended the cultivation of Laminaria seaweed and reef flats at approximately one meter's depth at low tide. They also recommended cutting off seagrasses and removing sea urchins before farm construction. Seedlings are then tied to monofilament lines and strung between mangrove stakes pounded into the substrate. This off-bottom method is still one of the primary methods used today.^[24]

There are new long-line cultivation methods that can be used in deeper water approximately 7 meters in depth. They use floating cultivation lines anchored to the bottom and are the primary methods used in the villages of North Sulawesi, Indonesia.^[25][26] Species cultured by long-line include those of the genera Saccharina, Undaria, Eucheuma, Kappaphycus, and Gracilaria.^[27]

Cultivation of seaweed in Asia is a relatively low-technology business with a high labor requirement. There have been many attempts in various countries to introduce high technology to cultivate detached plants growth in tanks on land in order to reduce labor, but they have yet to attain commercial viability.^[24]

There has been considerable discussion as to how seaweeds can be cultivated in the open ocean as a means to regenerate decimated fish populations and contribute to carbon sequestration. Notably, Tim Flannery has highlighted how growing seaweeds in the open ocean, facilitated by artificial upwelling and substrate, can enable carbon sequestration if seaweeds are sunk below a depth of one kilometer.^[28][29][30] Similarly, the NGO Climate Foundation and a number of permaculture experts have posited that the offshore mariculture of seaweed ecosystems can be conducted in ways that embody the core principles of permaculture, thereby constituting marine permaculture.^{[31][32][33][34][35]} The concept envisions using artificial upwelling and floating, submerged platforms as substrate to replicate natural seaweed ecosystems that provide habitat and the basis of a trophic pyramid for marine life.^[36] Following the principles of permaculture, seaweeds and fish can be sustainably harvested while sequestering atmospheric carbon. As of 2020, a number of successful trials have taken place in Hawaii, the Philippines, Puerto Rico and Tasmania.^[37][38] The idea has received substantial public attention, notably featuring as a key solution covered by Damon Gameau’s documentary 2040 and in the book Drawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming edited by Paul Hawken.

Environmental and ecological impacts

Aerial view of seaweed farms in South Korea

Several environmental problems can result from seaweed farming. Sometimes seaweed farmers cut down mangroves to use as stakes for their ropes. This, however, negatively affects farming since it reduces the water quality and mangrove biodiversity due to depletion. Farmers may also sometimes remove eelgrass from their farming areas. This, however, is also discouraged, as it adversely affects water quality.^[39]

In the UK, Maine and British Columbia, only native varieties of seaweed may be grown.^[40]

Seaweed farming helps to preserve coral reefs^[16] by increasing diversity where the algae and seaweed have been introduced, and it also provides an added niche for local species of fish and invertebrates. Farming may be beneficial by increasing the production of herbivorous fishes and shellfish in the area.^[2] Pollnac 1997b reported an increase in Siginid population after the start of extensive farming of eucheuma seaweed in villages in North Sulawesi, Indonesia.^[26]

Seaweed culture can also be used to capture, absorb, and eventually incorporate excessive nutrients into living tissue. "Nutrient bioextraction" is the preferred term for bioremediation involving cultured plants and animals. Nutrient bioextraction (also called bioharvesting) is the practice of farming and harvesting shellfish and seaweed to remove nitrogen and other nutrients from natural water bodies.^[41] (See main article Nutrient pollution.)

There has been considerable attention to how large-scale seaweed cultivation in the open ocean can act as a form of carbon sequestration to mitigate climate change.^[42][43] A number of academic studies have demonstrated that nearshore seaweed forests constitute a source of blue carbon, as seaweed detritus is carried by wave currents into the middle and deep ocean thereby sequestering carbon.^{[5][4][44][45][46]} Moreover, nothing on earth sequesters carbon faster than Macrocystis pyrifera (also known as giant kelp) which can grow up to 60 m in length and as rapidly as 50 cm a day in ideal conditions.^[47] It has therefore been suggested that growing seaweeds at scale can have a significant impact on climate change. 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”.^[48] As well as climate change mitigation, seaweed farming may be an initial step towards adapting to inevitable environmental constraints that may arise as a result of climate change in the near future. These include essential shoreline protection through the dissipation of wave energy, especially important to mangrove coasts. Carbon dioxide intake would lower pH locally, which will be highly beneficial to calcifiers (e.g. crustaceans) or in preventing the irreversibility of coral bleaching. Finally, seaweed farming and regenerative ocean farming would provide a strong oxygen input to coastal waters, thus countering the effects of ocean deoxygenation through the rising ocean temperature.^[4][49]

Harvesting seaweed in North Cape (Canada)

Socioeconomic aspects

In Japan alone, the annual production value of nori amounts to US$2 billion and is one of the world's most valuable crops produced by aquaculture. The high demand for seaweed production provides plentiful opportunities and work for the local community. A study conducted by the Philippines showed that plots of approximately one hectare could have a net income from Eucheuma farming that was 5 to 6 times that of the minimum average wage of an agriculture worker. In the same study, they also saw an increase in seaweed exports from 675 metric tons (MT) in 1967 to 13,191 MT in 1980, which doubled to 28,000 MT by 1988.^[50]

Tanzania

Seaweed farming has had widespread socio-economic impacts in Tanzania, and has become a very important source of resources for women, and is the third biggest contributor of foreign currency to the country.^[51] 90% of the farmers are women, and much of it is used by the skincare and cosmetics industry.^[52]

Uses

Farmed seaweed is used in a number of different industrial products: directly as food, as an ingredient in animal feed and as a source material for biofuels.^[53]

Chemicals

Many seaweeds are used to produced derivative chemicals that can be used for various industrial, pharmaceutical or food products. Two major derivative products are carrageenan and agar. However, there are a wide range of bioactive ingredients that can be used for a variety of industries, such as the pharmaceutical industry,^[54] industrial food,^[55] and the cosmetic industry.^[56]

Carrageenan

Agar

Food

A dish of pickled spicy seaweed

Edible seaweed, or sea vegetables, are seaweeds that can be eaten and used for culinary purposes. They typically contain high amounts of fiber.^[72][73] They may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae.^[72]

Seaweeds are also harvested or cultivated for the extraction of polysaccharides^[74] such as alginate, agar and carrageenan, gelatinous substances collectively known as hydrocolloids or phycocolloids. Hydrocolloids have attained commercial significance, especially in food production as food additives.^[75] The food industry exploits the gelling, water-retention, emulsifying and other physical properties of these hydrocolloids.^[76]

Most edible seaweeds are marine algae whereas most freshwater algae are toxic. Some marine algae contain acids that irritate the digestion canal, while some others can have a laxative and electrolyte-balancing effect.^[77] Most marine macroalgae are nontoxic in normal quantities, but members of the genus Lyngbya are potentially lethal.^[78] Typically poisoning is caused by eating fish which have fed on Lyngbya or on other fish which have done so.^[78] This is called ciguatera poisoning.^[78] Handling Lyngbya majuscula can also cause seaweed dermatitis.^[79] Some species of Desmarestia are highly acidic, with vacuoles of sulfuric acid that can cause severe gastrointestinal problems.^[78]

The dish often served in western Chinese restaurants as 'Crispy Seaweed' is not seaweed but cabbage that has been dried and then fried.^[80]

Fuel

A conical flask of "green" jet fuel made from algae

Algae fuel, algal biofuel, or algal oil is an alternative to liquid fossil fuels that uses algae as its source of energy-rich oils. Also, algae fuels are an alternative to commonly known biofuel sources, such as corn and sugarcane.^[81][82] When made from seaweed (macroalgae) it can be known as seaweed fuel or seaweed oil.

Several companies and government agencies are funding efforts to reduce capital and operating costs and make algae fuel production commercially viable.^[83][84] Like fossil fuel, algae fuel releases CO₂ when burnt, but unlike fossil fuel, algae fuel and other biofuels only release CO₂ recently removed from the atmosphere via photosynthesis as the algae or plant grew. The energy crisis and the world food crisis have ignited interest in algaculture (farming algae) for making biodiesel and other biofuels using land unsuitable for agriculture. Among algal fuels' attractive characteristics are that they can be grown with minimal impact on fresh water resources,^[85][86] can be produced using saline and wastewater, have a high flash point,^[87] and are biodegradable and relatively harmless to the environment if spilled.^[88][89] Algae cost more per unit mass than other second-generation biofuel crops due to high capital and operating costs,^[90] but are claimed to yield between 10 and 100 times more fuel per unit area.^[91] The United States Department of Energy estimates that if algae fuel replaced all the petroleum fuel in the United States, it would require 15,000 square miles (39,000 km²), which is only 0.42% of the U.S. map,^[92] or about half of the land area of Maine. This is less than 1⁄7 the area of corn harvested in the United States in 2000.^[93]

The head of the Algal Biomass Organization stated in 2010 that algae fuel could reach price parity with oil in 2018 if granted production tax credits.^[94] However, in 2013, Exxon Mobil Chairman and CEO Rex Tillerson said that after committing to spend up to $600 million over 10 years on development in a joint venture with J. Craig Venter's Synthetic Genomics in 2009, Exxon pulled back after four years (and $100 million) when it realized that algae fuel is "probably further" than 25 years away from commercial viability.^[95] In 2017, Synthetic Genomics and ExxonMobil reported a breakthrough in the joint research into advanced biofuels.^[96] The breakthrough was that they managed to double lipid content (from 20% in its natural form to 40-55 percent) in a genetically engineered strain of Nannochloropsis gaditana.^[97] On the other hand, Solazyme,^[98] Sapphire Energy,^[99] and Algenol,^[100] among others have begun commercial sale of algal biofuel in 2012 and 2013, and 2015, respectively. By 2017, most efforts had been abandoned or changed to other applications, with only a few remaining.^[101]