The largest of the green algae, Ulva (sea lettuce), grows to a ribbon or sheet 3 ft (91 cm) long. It provides food for many sea creatures, and its broad surface releases a large amount of oxygen. Fucus, called rockweed or bladderwrack, is a tough, leathery brown alga (though it often looks olive-green) that clings to rocks and has flattened, branched fronds buoyed by air bladders at the tips.
Seaweeds, especially species of the red algae Porphyra (nori) and Chondrus, form an important part of the diet and are farmed for food in China and Japan; other species (often called laver) are eaten in the British Isles and Iceland. Commercial agar (vegetable gelatin) is obtained from species of red algae and is the most valuable seaweed product. Irish moss or carrageen ( Chondrus crispus ), a red alga, is one of the few seaweeds used commercially in the United States. After being bleached in the sun the fronds contain a high proportion of gelatin, which is used for cooking, textile sizing, making cosmetics, and other purposes. In Japan it is made into a shampoo to impart gloss to the hair.
The kelps generally include the many large brown seaweeds and are among the most familiar forms found on North American coasts. Some have fronds up to 200 ft (61 m) long, e.g., the Pacific coast Nereocystis and Macrocystis, found also off the Cape of Good Hope. Common Atlantic species include Laminaria and Agarum (devil's apron). The kelps are a source of salts of iodine and potassium and, to a lesser extent, other minerals. When the seaweed is burned, the soluble mineral compounds are removed from the ashes (also called kelp) by washing. They are used chiefly as chemical reagents and for dietary deficiencies in people and in livestock. Kelp is also a commercial source of potash, fertilizer, and medicines made from its vitamin and mineral content. Kelps are especially abundant in Japan, and various foods known as kombu are made from them.
The brown algae of the genus Sargassum are called gulfweed. They inhabit warm ocean regions and are commonly found floating in large patches in the Sargasso Sea and in the Gulf Stream. Gulfweed was observed by Columbus. Although it was formerly thought to cover the whole Sargasso Sea, making navigation impossible, it has since been found to occur only in drifts. Numerous berrylike air sacs keep the branching plant afloat. The thick masses of gulfweed provide the environment for a distinctive and specialized group of marine forms, many of which are not found elsewhere.