How can seaweed provide a source of iodine for industry?

Certain species of seaweed are capable of concentrating iodine from seawater, where it otherwise exists in very low concentrations, making commercial extraction viable.1

To recover iodine from seaweed, the plant material is first dried and burned. The resulting ash is leached with water, and impurities such as sodium chloride and sodium sulfate are removed through crystallization. The remaining solution is concentrated by evaporation. To release the iodine, the solution is treated with sulfuric acid to decompose sulfites and sulfides, followed by the addition of manganese dioxide. The liberated iodine is then vaporized and purified via sublimation. Alternatively, the addition of cupric sulfate can be used to produce cuprous iodide.2