Seaweed--actually a complex form of algae--lives only in saltwater and is essential to the survival of all ocean life. It has also been harvested for centuries by many coastal peoples who use it for its health benefits and delightful culinary properties.
Adapted to the Ocean
Most plants, needing a balance of direct sunlight and freshwater, can't live in the ocean. Seaweed, however, despite being a simple plant with no root systems or flowers, is well-adapted to marine life. It has a thick, rubbery exterior that protects it from intaking too much salt and it grows on top of rocks or other stable surfaces in order to absorb sunlight, which it uses for photosynthesis.
Over 400 Species
Earth's oceans contain over 400 species of seaweed. Seaweed is at the bottom of the saltwater food chain, and is therefore essential to ocean health.
Seaweed as Food
Seaweed is part of many world cuisines. It's well-known that in Japan, seaweed is used in soups and salads, as well as to wrap sushi. Seaweed, though, is used in cooking in many countries with seaweed-rich coastlines, including Ireland, Iceland and Norway. Seaweed has many health benefits, as it is rich in calcium, magnesium and iodine.
Agar and other gelatinous substances are extracted from seaweed. Agar is used as a culture medium in microbiology. The other gelatinous substances are used in dyes, gels and adhesives.