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Microbiology => Agar => Seaweed
Seaweed
Seaweed, any of the larger, multicellular forms of algae living in fresh and salt water, especially along marine coastlines. The three main phyla, or divisions, are the brown algae, such as the kelps; the red algae, such as Irish moss; and the green algae, such as the sea lettuces, all of which are commonly seen at low tide along rocky shores of northern seas. Seaweeds differ from plants in that they lack the true stems, leaves, roots, and vascular systems of higher plants. Instead, they anchor themselves to solid objects by holdfasts and absorb nutrients directly from the water, manufacturing their food by photosynthesis. The pigments of red and brown algae mask the predominant green photosynthetic pigment, chlorophyll, and probably aid in photosynthetic metabolism by absorbing and transferring light energy to the chlorophyll.
Seaweeds abound in shallow waters from the midtide line down to depths of 50 m (165 ft). Along damp cold-water shores, they are able to withstand several hours of exposure to the sun, and they cover rocks high into the intertidal zone. In the Tropics, seaweeds are confined to the zone between the low-tide line and a depth of about 200 m (about 660 ft); red algae predominate, especially in lagoons and around coral reefs.
The brown algae, commonly called kelp, comprise the largest seaweeds. Pacific species can reach 65 m (213 ft) in length and have structures that superficially resemble leaves and stems, as well as large air-filled bladders and strong holdfasts that anchor them against heavy surf. Other brown algae are the common rockweed and the gulfweed, which floats in great masses in the Gulf Stream and the Sargasso Sea.
Among the red algae are several species of Irish moss, which is commonly seen along northern Atlantic coasts as a matted carpet in the sublittoral zone. Red algae are abundant in clear tropical waters, where their red pigment, phycoerythrin, enables them to carry on photosynthesis at deeper levels than is possible for ordinary green algae.
Seaweed is a commercially important food, especially in Japan, where it is called nori and is harvested mainly from red algae, extensively cultivated on bamboo screens submerged in estuaries. Agar, also derived from red algae, is consumed as a delicacy in Asia and is used as a laboratory medium for culturing microorganisms. Red algae are probably of little nutritive value to humans, however, other than for their limited protein, vitamin, and mineral (especially iodine) content. Brown algae are used as fertilizer and as an ingredient for livestock meal. Alginic acid, found in kelp, has wide industrial uses. It can be made into a silklike thread or a plastic material, insoluble in water, that is used to make films, gels, rubber, and linoleum, and as a colloid in cosmetics, car polishes, and paints. Organic derivatives of alginates are used as food gums in making ice cream, puddings, and processed cheeses.
Scientific classification: Brown algae make up the phylum Phaeophyta, red algae the phylum Rhodophyta, and green algae the phylum Chlorophyta. Large Pacific brown algae include those species classified in the genera Macrocystis and Nereocystis. Rockweed makes up the genus Fucus, and gulfweed the genus Sargassum. Irish moss makes up the genus Chondrus. The species most commonly cultivated for food in Japan is Porphyra tenera.
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