Aquaculture of giant kelp, Macrocystis pyrifera, is the growing of kelp for uses such as food, dietary supplements, or potash. Giant kelp contains iodine, potassium, other minerals vitamins and carbohydrates.

History

At the start of the 20th century, California kelp beds were collected for their potash. Business interest grew during the 1970s and 1980s because of the production of alginates and for using kelp as a food source for animals due to the energy crisis. However, commercial production of M. pyrifera did not develop. When the energy crisis ended and alginate prices dropped, research on farming Macrocystis decreased.

The supply of M. pyrifera for alginate production depended heavily on restoring and managing natural kelp beds in the early 1990s. Other uses, such as stabilizing the seafloor, were studied in California, where the “Kelp bed project” moved adult kelp plants that were 3 to 6 meters tall to strengthen harbor stability and increase biodiversity.

Research is exploring the use of M. pyrifera as food for other aquatic animals, such as fish.

China and Chile are the largest producers of aquatic plants and algae, each making over 300,000 tonnes in 2007. How much of this total comes from M. pyrifera is unknown. Both countries grow many species; in Chile, 50% of production includes Phaeophytes, and the other 50% includes Rhodophytes. China grows a wider range of seaweeds, including chlorophytes. Experiments in Chile are testing hybrids of M. pyrifera and M. integrifolia.

Kelp farming has continued in Oregon and British Columbia. Startups in Alaska have begun renting ocean space to grow M. pyrifera on a large scale.

In the 2010s, Northern California lost 95% of its kelp ecosystems because of marine heatwaves.

Kelp recovery efforts in California mainly focus on removing sea urchins, done by scuba divers and by sea otters, which naturally eat sea urchins.

A type of brown algae called Sargassum horneri, an invasive species first found in 2003, has also been a concern.

Researchers at the Bodega Marine Laboratory of UC Davis are creating replanting plans, and volunteers from the Orange County Coastkeeper group are replanting giant kelp. Humboldt State University started growing bull kelp in its research farm in 2021.

In July 2020, state-level research efforts were announced to prevent kelp forest loss in California.

At the federal level, H.R. 4458, the Keeping Ecosystems Living and Productive (KELP) Act, introduced on July 29, 2021, aims to create a new grant program through NOAA for kelp forest restoration.

Ocean Rainforest, a company based in the Faroe Islands, received $4.5 million in U.S. government funding to grow giant kelp on an 86-acre farm near Santa Barbara, California.

Methods

The most common way to grow M. pyrifera was created in China during the 1950s. This method is called the long line cultivation system. It begins with sporelings grown in a cooled water greenhouse. These sporelings are then attached to long lines and placed in the ocean. They grow at different depths.

M. pyrifera has a life cycle that changes between two forms: a large sporophyte and a tiny gametophyte. The sporophyte is the part harvested as seaweed. When mature, sporophytes develop reproductive structures called sori on the underside of their leaves. These sori release motile zoospores, which grow into gametophytes.

To encourage sporulation, plants are dried for up to 12 hours and placed in a container filled with seawater at 9–10°C, 30% salinity, and a pH of 7.8–7.9. Light exposure is carefully controlled during sporulation and growth. After sporulation, a synthetic twine (2–6 mm in diameter) is placed in the container. Zoospores attach to the twine and grow into male and female gametophytes. When mature, these gametophytes release sperm and egg cells that join in the water. The fertilized cells attach to the twine and grow into young sporophytes over 60 days.

These young sporophytes are either wrapped around or cut into pieces and attached to larger cultivation ropes. The ropes are about 60 meters long and have floating buoys spaced 2–3 meters apart. In China, ropes are anchored to wooden pegs in the ocean. In Chile, M. pyrifera is grown at a depth of 2 meters using buoys to maintain a constant depth. Ropes are usually spaced 50 cm apart.

Challenges include managing the transition from spores to gametophytes and young sporophytes in controlled terrestrial facilities. Water flow, temperature, nutrients, and light must be carefully managed. In Japan, a forced cultivation method is used, where two years of growth occur in one season by controlling environmental factors.

In China, a project tested offshore cultivation by pumping nutrients from deep water into the beds. This helped avoid size limits in shallow waters. However, issues with farm design and operations limited further development of this method.

The time needed to grow M. pyrifera varies by region and farming intensity. It is typically harvested after two growing seasons (2 years). In China, cultivated M. pyrifera is harvested using a pulley system on boats that pull ropes for cleaning. In the United States, wild M. pyrifera is harvested by boats that collect the surface canopy multiple times per year. This works because the seaweed grows quickly without damaging its vegetative or reproductive parts.

Applications

In the United Kingdom, laws classify giant kelp as a nuisance. Invasive plants are removed by machines.

The main uses for M. pyrifera include making fertilizers, cleaning the environment, and providing food for abalone and sea urchins.

To reduce current carbon emissions, about 50 trillion trees would be needed. Another option is growing kelp forests. Kelp grows 2 feet per day, 30 times faster than land plants. Covering 10% of the world’s oceans (about 4.5 times the size of Australia) with kelp could achieve the same carbon reduction. This would also help produce 2 million tons of fish yearly and lower ocean acidification. Large-scale kelp farming in open oceans would need special materials and added nutrients.

A company in Maine, Running Tide Technologies, aims to grow large amounts of kelp to store carbon in the ocean floor.

A kelp nursery and planting project is being developed near Catalina Island. Methods using a "kelp elevator" have shown promise for creating biofuel through thermochemical liquefaction. This research is funded by ARPA-E.

On a smaller scale, kelp is used as a substitute for kale. Groups like GreenWave, based in Connecticut, help connect ocean farmers with buyers to support the kelp market.