5.3 Economic importance and applications of marine algae

Economic Importance and Applications of Marine Algae

Marine algae are commercially valuable organisms used across the food, cosmetics, pharmaceutical, and agricultural industries. Understanding their applications matters because global seaweed aquaculture is one of the fastest-growing sectors in marine biology, worth over $16 billion annually. This section covers the major commercial species, how they're cultivated and harvested, their key properties, and the challenges of managing these resources sustainably.

Commercial Species of Marine Algae

The commercially important marine algae fall into three major groups, each with distinct species and uses.

Red algae (Rhodophyta)

• Nori (Porphyra) is the most recognizable edible seaweed. It's the dark, papery wrap around sushi rolls and is also ground into seasoning flakes. Japan, China, and South Korea are the largest producers.
• Carrageenan-producing species (Chondrus crispus, Kappaphycus alvarezii, Eucheuma denticulatum) are harvested for carrageenan, a polysaccharide extracted from their cell walls. Carrageenan acts as a thickening and gelling agent in ice cream, yogurt, toothpaste, and pharmaceutical capsules.

Brown algae (Phaeophyceae)

• Kelp (Laminaria, Saccharina, Macrocystis) is used in food products, fertilizers, and animal feed. Macrocystis pyrifera (giant kelp) can grow up to 60 cm per day, making it one of the fastest-growing organisms on Earth.
• Wakame (Undaria pinnatifida) is consumed as a vegetable in miso soup and seaweed salads, particularly in East Asian cuisine.
• Alginate-producing species (Ascophyllum nodosum, Laminaria hyperborea, Macrocystis pyrifera) yield alginate, another polysaccharide used as a thickening and stabilizing agent in food, textiles, wound dressings, and pharmaceuticals.

Green algae (Chlorophyta)

• Sea lettuce (Ulva lactuca) has thin, bright green blades and is eaten in salads and soups or used as a nutritional supplement.
• Chlorella (Chlorella vulgaris, Chlorella pyrenoidosa) is a unicellular microalga sold as a dietary supplement (tablets, powders) and added to animal feed for its high protein content.

Cultivation and Harvesting of Seaweeds

Different species require different cultivation methods. Here are the main approaches:

Nori cultivation:

1. Spores are collected and seeded onto nets or ropes.
2. The seeded nets are suspended in shallow, nutrient-rich seawater (often in bays or estuaries).
3. Blades grow over several weeks and are harvested when mature.
4. Harvested nori is washed, shredded, and dried into thin sheets.

Kelp cultivation:

1. Juvenile sporophytes are produced in a hatchery by allowing spores to settle onto seed string.
2. Seed string is wound around longlines and deployed in open water.
3. Kelp grows on the suspended longlines over several months.
4. Mature blades are cut from the lines, typically by hand or with mechanical harvesters.

Carrageenan and alginate production: These species are obtained through either wild harvesting from natural seaweed beds or cultivation on nets, ropes, or rafts in seawater. Small-scale operations harvest by hand, while larger commercial operations use mechanical methods. After harvest, the seaweed is dried and processed to extract the target polysaccharide (carrageenan or alginate).

Properties and Applications of Marine Algae

Nutritional properties

Marine algae are nutrient-dense. They contain vitamins A, B, C, E, and K, along with high concentrations of minerals like iodine, calcium, iron, and magnesium. They also provide essential fatty acids (omega-3 and omega-6) and are a good source of dietary fiber. Nori, for example, contains up to 50% protein by dry weight, which is unusually high for a plant-like organism.

Medicinal properties

Research into the bioactive compounds in marine algae has revealed several promising medicinal applications:

1. Antioxidant compounds such as polyphenols and carotenoids may help prevent chronic diseases by neutralizing free radicals that damage cells.
2. Antiviral and antibacterial properties have been identified in extracts from several species, showing potential for treating infections and supporting immune function.
3. Anti-inflammatory compounds like fucoidan (from brown algae) and phlorotannins show promise in treating arthritis and other inflammatory conditions. Fucoidan in particular has attracted significant research interest for its range of biological activities.

Management of Marine Algal Resources

Challenges

• Overharvesting of wild seaweed populations can deplete natural beds faster than they regenerate.
• Habitat degradation from pollution and coastal development reduces suitable growing areas.
• Climate change alters ocean temperatures and chemistry, shifting where seaweeds can thrive. Ocean acidification and marine heatwaves are particular threats.
• Invasive species like Undaria pinnatifida (outside its native range) can outcompete native seaweeds and disrupt local ecosystems.

Opportunities

• Sustainable cultivation practices offer alternatives to wild harvesting. Integrated multi-trophic aquaculture (IMTA) grows seaweeds alongside fish and shellfish so that the seaweed absorbs excess nutrients from the animal species, reducing pollution while producing a crop. Offshore and land-based seaweed farms are also expanding.
• Ecosystem services from seaweed beds are substantial. They sequester carbon (absorbing $CO_2$ from seawater), cycle nutrients, and provide habitat and nursery grounds for marine life.
• Emerging applications include biofuels and bioplastics derived from algal biomass, wastewater treatment through bioremediation (algae absorb heavy metals and excess nutrients), and new formulations for animal feed and agricultural fertilizers.