seaweed

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seaweed name commonly used for the multicellular marine algae . Simpler forms, consisting of one cell (e.g., the diatom ) or of a few cells, are not generally called seaweeds; these tiny plants help to make up plankton. The more highly developed types of seaweed usually have a basal disk, called a holdfast, and a frond of varying length and shape, which often resembles a plant in having stemlike and leaflike parts.

Types of Seaweed

The simplest of the seaweeds are among the cyanobacteria , formerly called the blue-green algae, and green algae (division Chlorophyta ), found nearest the shore in shallow waters and usually growing as threadlike filaments, irregular sheets, or branching fronds. The brown algae (division Phaeophyta ), in which brown pigment masks the green of the chlorophyll, are the most numerous of the seaweeds of temperate and polar regions. They grow at depths of 50 to 75 ft (15–23 m). The red seaweeds (division Rhodophyta ), many of them delicate and fernlike, are found at the greatest depths (up to 879 ft/268 m); their red pigment enables them to absorb the blue and violet light present at those depths.

Reproduction in Seaweeds

Seaweeds reproduce in a variety of ways. Lower types reproduce asexually. More advanced kinds produce motile zoospores that swim off, anchor themselves, and grow into new individuals, or they reproduce sexually by forming sex cells (gametes) that, after fusing, follow the same pattern. Sometimes pieces of a seaweed break off and form new plants; in a few species there is a cycle of asexual and sexual reproduction foreshadowing the alternation of generations characteristic of plants.

Common Species and Their Uses

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.

seaweeds are the most familiar of all marine plants. They are large algae, non-flowering plants. With the exception of sargasso weed, seaweeds are restricted to shallow inshore waters where they can anchor themselves to the seabed. They are limited to depths that are shallow enough for sufficient sunlight to penetrate to support their photosynthesis. They are classified according to their pigmentation into reds, browns, greens, and blue-greens. On rocky shores around Europe ebbing low spring tides reveal a zonation of the brown algae, mostly wracks. The species most tolerant of desiccation flourish at the top of the shore but are replaced down the shore by a succession of more vigorously growing species that are less and less tolerant of being exposed at low tide. Lowest down the shore are the kelps or oarweeds (e.g. Laminaria) that are anchored to rocks by their rootlike holdfasts from which grow their stipes (stalks) and leaflike blades. They form dense forests in the sublittoral (the zone just below the lowest tide mark), which are home to a rich fauna of invertebrate animals and fishes. Off California and Chile giant kelps can grow to lengths of 100 metres (325 ft), each year renewing their blades, whereas the stipes are perennial, They are grazed by sea urchins, types of echinoderms, which in turn are eaten by sea otters (see marine mammals). When the sea otters have gone into decline because of over-exploitation or pollution, the urchins are unchecked and graze down the kelp forest so the numbers of other species inhabiting the region decline markedly. Molecular techniques have shown that clumps of seaweeds can be surprisingly old. Off the coast of Brittany quite modest clumps of bladder wrack (Ascophyllum nodosum) have been shown to be as much as 70 years old.

On coral reefs eutrophication, one of today's environmental issues, can lead to seaweeds overgrowing and destroying the coral by shading it. So what can be good for a rich diversity of species in one habitat can be bad for it in others.

Some seaweeds are used for food and they are extensively cultured in China. In Britain the red alga (Chrondrus crispus), known as carragheen or Irish moss, is traditionally used to make blancmanges and jellies. In south Wales another species, Porpyra umbilicalis, is eaten fried with bacon or rolled in oatmeal as laverbread. Alginin, extracted from kelp, is used as an emulsifier in the manufacture of ice cream.

See also marine biology; marine pharmaceuticals.

M. V. Angel

seaweed Any of numerous species of brown, green or red algae, found in greatest profusion in shallow waters on rocky coasts. Kelps are the largest forms. Many species are important for the manufacture of fertilizers or food, or as a valuable source of chemicals such as iodine. Kingdom Protoctista.

seaweed Marine algae of interest as food include Irish moss, laver bread, and kelp, which are eaten to some extent in different communities and serve as a mineral supplement in animal feed.

seaweed The common name for a macroscopic marine alga. Seaweeds belong to the groups Rhaeophyta (brown seaweeds), Rhodophyta (red seaweeds), and Chlorophyta (green seaweeds).

seaweeds Large multicellular algae living in the sea or in the intertidal zone. They are commonly species of the Chlorophyta, Phaeophyta, and Rhodophyta.

sea·weed / ˈsēˌwēd/ • n. large algae growing in the sea or on rocks below the high-water mark.

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