Aquaculture, the farming of aquatic organisms, has seen a growing focus on the cultivation of marine plants known as macroalgae, or seaweed. Seaweed farming involves actively managing the growth of these large, multicellular organisms in controlled environments, typically in coastal ocean waters. This practice has emerged as a significant sector in the blue economy, moving beyond traditional coastal harvesting to a managed production system. The increasing global interest is driven by seaweed’s utility as a sustainable raw material for a wide array of products, from human food to industrial compounds.
Defining Seaweed and Global Cultivation
Seaweed is a term used to describe thousands of species of marine macroalgae. Macroalgae are classified into three primary groups based on their pigmentation: brown, red, and green algae. Each group contains species of commercial importance, with a small number of species accounting for the majority of global production.
The brown algae class includes kelp species like Saccharina japonica and Undaria pinnatifida (wakame). Red algae, such as Pyropia (nori) and Kappaphycus, are cultivated extensively for both food and industrial extracts. Green algae, like Ulva (sea lettuce), are also farmed but represent a smaller fraction of the market. Historically and currently, the global cultivation of seaweed is dominated by East and Southeast Asian countries, with China, Indonesia, and the Philippines leading world production by a substantial margin.
Methods of Cultivating Marine Algae
The physical techniques for growing seaweed are adapted to the specific species and the local marine environment, but generally revolve around providing a stable substrate for the algae to attach and grow. The cultivation process begins in a land-based hatchery where spores are collected and allowed to settle onto fine threads or ropes under controlled conditions. Once the juvenile sporophytes reach a few millimeters in length, these seeded lines are then ready for deployment in the ocean.
Line and Raft Systems
One of the most widespread methods is line cultivation, which includes monoline and long-line systems where seeded ropes are suspended horizontally beneath the water surface. In the monoline technique, seaweed fragments are tied individually to a single rope anchored at both ends, allowing the algae to grow freely in the water column.
More complex raft systems utilize a grid of floating structures, often made of bamboo or plastic pipe, from which multiple ropes are hung vertically into the water. Raft cultivation is particularly common for red algae like Kappaphycus, where a frame supports numerous growing lines.
Another variation is the net or cage system, which involves growing seaweed within tubular nets or as part of an integrated multi-trophic aquaculture (IMTA) system attached to the rings of fish cages. These methods provide a degree of protection from grazing fish and strong currents, and they allow for dense cultivation in a relatively small area.
Harvesting
When the seaweed reaches marketable maturity, which can take as little as six weeks for some fast-growing species, it is ready for harvest. Harvesting is typically conducted manually, with farmers using knives to cut the mature fronds from the cultivation ropes. Manual harvesting is selective, allowing the basal portion of the algae to remain intact for regrowth, which supports multiple harvests from a single seeding. For large-scale operations or certain species like giant kelp, mechanical harvesting vessels equipped with cutting and collection systems may be used, prioritizing speed and volume over the selective cutting of manual methods.
Commercial and Industrial Applications
The economic value of farmed seaweed is driven by its diverse applications across several major industries. The most traditional application is direct human consumption, particularly in Asian cuisine. Species such as Pyropia are processed into thin sheets of nori for sushi rolls, while Undaria pinnatifida and Saccharina japonica are widely used as vegetables in salads and soups.
Beyond direct food use, a significant portion of the global harvest is dedicated to the extraction of hydrocolloids, which are gelatinous polysaccharides with thickening and gelling properties.
- Red algae species yield agar and carrageenan, utilized as stabilizing agents in dairy products, jellies, and pharmaceuticals.
- Brown algae (kelp) are the source of alginates, compounds used as emulsifiers and stabilizers.
- Alginates are found in products ranging from processed foods and cosmetics to dental impression materials.
The third category involves agricultural and emerging industrial uses. Seaweed biomass is increasingly used as a feedstock for fertilizers and soil conditioners, as it is rich in micronutrients and growth hormones that enhance crop yields. Furthermore, research is exploring its potential as a sustainable feed additive for livestock to reduce methane emissions, and as a raw material for the production of biofuels and bioplastics, offering a renewable alternative to petroleum-based products.
Ecological Functions of Farm Sites
Seaweed farms function as biological filters, providing ecosystem services to coastal waters. As the macroalgae grow, they absorb dissolved inorganic nutrients, primarily excess nitrogen and phosphorus, directly from the surrounding seawater. This nutrient stripping process can mitigate the effects of coastal eutrophication, which is the over-enrichment of water bodies that often leads to harmful algal blooms and low-oxygen “dead zones.”
The photosynthetic activity of the seaweed draws dissolved inorganic carbon from the water, which is then converted into organic carbon and stored in the algal biomass. This local uptake of carbon dioxide can help to raise the \(text{pH}\) of the surrounding water, offering a localized buffering effect against ocean acidification. While a significant portion of the stored carbon is released when the seaweed is harvested and consumed, the farm biomass provides a pathway for carbon export, particularly if unharvested or waste portions sink to deep ocean sediments.
The physical structures of the farm, including the lines, rafts, and nets, also serve as artificial habitats. These structures create new three-dimensional environments that can attract and shelter various marine organisms, including small fish, crustaceans, and invertebrates. This aggregation of marine life can enhance local biodiversity, essentially creating a managed, temporary reef structure within the farm site.