Seaweed is a member of any of a number of species of brown, green, or red algae, found in greatest profusion in shallow waters on rocky coasts. Kelps are the largest forms. The giant kelp (Macrocystis sp.) exceeds 45 m (150 ft) in length.

 

Derivatives of seaweed

Many species of seaweed are important for the manufacture of fertilizers or food, or as a valuable source of chemicals such as iodine. Algin is a polysaccharide extracted from brown seaweeds. A derivative, sodium alginate, is a thickening agent and emulsion stabilizer used in the food industry, especially in ice cream manufacture, and as sizing in paper and textiles.