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algae (ălˈjē) [plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms. These organisms were previously classified as a primitive subkingdom of the plant kingdom, the thallophytes (plants that lack true roots, stems, leaves, and flowers). More recently, most algae have been classified in the kingdom Protista or in another major group called the eukarya (or eukaryotes), which includes animals and higher plants. The algae have chlorophyll and can manufacture their own food through the process of photosynthesis. They are distributed worldwide in the sea, in freshwater, and in moist situations on land. Nearly all seaweeds are marine algae. Algae that thrive in polluted water, some of which are toxic, can overmultiply, resulting in an algal bloom and seriously unbalancing their ecosystem.

Types of Algae

The simplest algae are single cells (e.g., the diatoms); the more complex forms consist of many cells grouped in a spherical colony (e.g., Volvox), in a ribbonlike filament (e.g., Spirogyra), or in a branching thallus form (e.g., Fucus). The cells of the colonies are generally similar, but some are differentiated for reproduction and for other functions. Kelps, the largest algae, may attain a length of more than 200 ft (61 m). Euglena and similar genera are free-swimming one-celled forms that contain chlorophyll but that are also able, under certain conditions, to ingest food in an animallike manner. The green algae include most of the freshwater forms. The pond scum, a green slime found in stagnant water, is a green alga, as is the green film found on the bark of trees. The more complex brown algae and red algae are chiefly saltwater forms; the green color of the chlorophyll is masked by the presence of other pigments. Blue-green algae have been grouped with other prokaryotes in the kingdom Monera and renamed cyanobacteria.

See the separate phyla (divisions) Chlorophyta, Euglenophyta, Dinoflagellata, Chrysophyta, Phaeophyta, Rhodophyta.

Uses of Algae

Algae, the major food of fish (and thus indirectly of many other animals), are a keystone in the aquatic food chain of life; they are the primary producers of the food that provides the energy to power the whole system. They are also important to aquatic life in their capacity to supply oxygen through photosynthesis. Seaweeds, e.g., the kelps (kombu) and the red algae Porphyra (nori), have long been used as a source of food, especially in Asia. Both cultivated and naturally growing seaweeds have been harvested in the Pacific Basin for hundreds of years. Kelp are also much used as fertilizer, and kelp ash is used industrially for its potassium and sodium salts. Other useful algae products are agar and carrageen, which is used as a stabilizer in foods, cosmetics, and paints.


See H. C. Bold and M. J. Wynne, Introduction to the Algae: Structure and Reproduction (1985); C. A. Lembi and J. R. Waaland, Algae and Human Affairs (1988); C. van den Hoek, Algae: an Introduction to Phycology (1994).

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An informal assemblage of predominantly aquatic organisms that carry out oxygen-evolving photosynthesis but lack specialized water-conducting and food-conducting tissues. They may be either prokaryotic (lacking an organized nucleus) and therefore members of the kingdom Monera, or eukaryotic (with an organized nucleus) and therefore members of the kingdom Plantae, constituting with fungi the subkingdom Thallobionta. They differ from the next most advanced group of plants, Bryophyta, by their lack of multicellular sex organs sheathed with sterile cells and by their failure to retain an embryo within the female organ. Many colorless organisms are referable to the algae on the basis of their similarity to photosynthetic forms with respect to structure, life history, cell wall composition, and storage products. The study of algae is called algology (from the Latin alga, meaning sea wrack) or phycology (from the Greek phykos, seaweed). See Bryophyta, Plant kingdom, Thallobionta

General form and structure

Algae range from unicells 1–2 micrometers in diameter to huge thalli [for example, kelps often 100 ft (30 m) long] with functionally and structurally distinctive tissues and organs. Unicells may be solitary or colonial, attached or free-living, with or without a protective cover, and motile or nonmotile. Colonies may be irregular or with a distinctive pattern, the latter type being flagellate or nonmotile. Multicellular algae form packets, branched or unbranched filaments, sheets one or two cells thick, or complex thalli, some with organs resembling roots, stems, and leaves (as in the brown algal orders Fucales and Laminariales). Coenocytic algae, in which the protoplast is not divided into cells, range from microscopic spheres to thalli 33 ft (10 m) long with a complex structure of intertwined siphons (as in the green algal order Bryopsidales).


Sixteen major phyletic lines (classes) are distinguished on the basis of differences in pigmentation, storage products, cell wall composition, flagellation of motile cells, and structure of such organelles as the nucleus, chloroplast, pyrenoid, and eyespot. These classes are interrelated to varying degrees, the interrelationships being expressed by the arrangement of classes into divisions (the next-higher category). Among phycologists there is far greater agreement on the number of major phyletic lines than on their arrangement into divisions.

  • Superkingdom Prokaryotae
    • Kingdom Monera
      • Division Cyanophycota (= Cyanophyta, Cyanochloronta)
        • Class Cyanophyceae, blue-green algae
      • Division Prochlorophycota (= Prochlorophyta)
        • Class Prochlorophyceae
  • Superkingdom Eukaryotae
    • Kingdom Plantae
      • Subkingdom Thallobionta
        • Division Rhodophycota (= Rhodophyta, Rhodophycophyta)
          • Class Rhodophyceae, red algae
        • Division Chromophycota (= Chromophyta)
          • Class: Chrysophyceae, golden or golden-brown algae
          • Prymnesiophyceae (= Haptophyceae)
          • Xanthophyceae (= Tribophyceae), yellow-green algae
          • Eustigmatophyceae
          • Bacillariophyceae, diatoms
          • Dinophyceae, dinoflagellates
          • Phaeophyceae, brown algae
          • Raphidophyceae, chloromonads
          • Cryptophyceae, cryptomonads
        • Division Euglenophycota (= Euglenophyta, Euglenophycophyta)
          • Class Euglenophyceae
      • Division Chlorophycota (= Chlorophyta, Chlorophycophyta)
        • Class: Chlorophyceae, green algae
        • Charophyceae, charophytes
        • Prasinophyceae

Placing more taxonomic importance on motility than on photosynthesis, zoologists traditionally have considered flagellate unicellular and colonial algae as protozoa, assigning each phyletic line the rank of order. See Protozoa

Although some unicellular algae are naked or sheathed by mucilage or scales, most are invested with a covering (wall, pellicle, or lorica) of diverse composition and construction. These coverings consist of at least one layer of polysaccharide (cellulose, alginate, agar, carrageenan, mannan, or xylan), protein, or peptidoglycan that may be impregnated or encrusted with calcium carbonate, iron, manganese, or silica. They are often perforated and externally ornamented. Diatoms have a complex wall composed almost entirely of silica. In multicellular and coenocytic algae, most reproductive cells are naked, but vegetative cells have walls whose composition varies from class to class. See Cell walls (plant)


Prokaryotic algae lack membrane-bounded organelles. Eukaryotic algae have an intracellular architecture comparable to that of higher plants but more varied. Among cell structures unique to algae are contractile vacuoles in some freshwater unicells, gas vacuoles in some planktonic blue-green algae, ejectile organelles in dinoflagellates and cryptophytes, and eyespots in motile unicells and reproductive cells of many classes. Chromosome numbers vary from n = 2 in some red and green algae to n ≥ 300 in some dinoflagellates. The dinoflagellate nucleus is in some respects intermediate between the chromatin region of prokaryotes and the nucleus of eukaryotes and is termed mesokaryotic. Some algal cells characteristically are multinucleate, while others are uninucleate. Chloroplasts, which always originate by division of preexisting chloroplasts, have the form of plates, ribbons, disks, networks, spirals, or stars and may be positioned centrally or along the cell wall. Photosynthetic membranes (thylakoids) are arranged in distinctive patterns and contain pigments diagnostic of individual classes. See Cell (biology), Cell plastids, Chromosome, Photosynthesis, Plant cell

In all classes of algae except Prochlorophyceae, there are cells that are capable of movement. The slow, gliding movement of certain blue-green algae, diatoms, and reproductive cells of red algae presumably results from extracellular secretion of mucilage. Ameboid movement, involving pseudopodia, is found in certain Chrysophyceae and Xanthophyceae. An undulatory or peristaltic movement occurs in some Euglenophyceae. The fastest movement is produced by flagella, which are borne by unicellular algae and reproductive cells of multicellular algae representing all classes except Cyanophyceae, Prochlorophyceae, and Rhodophyceae.

Internal movement also occurs in algae in the form of cytoplasmic streaming and light-induced orientation of chloroplasts. See Cell motility, Cilia and flagella

Sexual reproduction is unknown in prokaryotic algae and in three classes of eukaryotic unicells (Eustigmatophyceae, Cryptophyceae, and Euglenophyceae), in which the production of new individuals is by binary fission. In sexual reproduction, which is found in all remaining classes, the members of a copulating pair of gametes may be morphologically indistinguishable (isogamous), morphologically distinguishable but with both gametes motile (anisogamous), or differentiated into a motile sperm and a relatively large nonmotile egg (oogamous). Gametes may be formed in undifferentiated cells or in special organs (gametangia), male (antheridia) and female (oogonia). Sexual reproduction may be replaced or supplemented by asexual reproduction, in which special cells (spores) capable of developing directly into a new alga are formed in undifferentiated cells or in distinctive organs (sporangia). See Reproduction (plant)

Most algae are autotrophic, obtaining energy and carbon through photosynthesis. All photosynthetic algae liberate oxygen and use chlorophyll a as the primary photosynthetic pigment. Secondary (accessory) photosynthetic pigments, which capture light energy and transfer it to chlorophyll a, include chlorophyll b (Prochlorophyceae, Euglenophyceae, Chlorophycota), chlorophyll c (Chromophycota), fucoxanthin among other xanthophylls (Chromophycota), and phycobiliproteins (Cyanophyceae, Rhodophyceae, Cryptophyceae). Other carotenoids, especially β-carotene, protect the photosynthetic pigments from oxidative bleaching. Except for different complements of accessory pigments (resulting in different action spectra), photosynthesis in algae is identical to that in higher plants. Carbon is predominantly fixed through the C3 pathway. See Carotenoid, Chlorophyll

The source of carbon for most photosynthetic algae is carbon dioxide (CO2), but some can use bicarbonate. Many photosynthetic algae are also able to use organic substances (such as hexose sugars and fatty acids) and thus can grow in the dark or in the absence of CO2. Colorless algae obtain both energy and carbon from a wide variety of organic compounds in a process called oxidative assimilation.

Numerous substances are liberated into water by living algae, often with marked ecological effects. These extracellular products include simple sugars and sugar alcohols, wall polysaccharides, glycolic acid, phenolic substances, and aromatic compounds. Some secreted substances inhibit the growth of other algae and even that of the secreting alga. Some are toxic to fishes and terrestrial animals that drink the water.


Algae are predominantly aquatic, inhabiting fresh, brackish, and marine waters without respect to size or degree of permanence of the habitat. They may be planktonic (free-floating or motile) or benthic (attached). Benthic marine algae are commonly called seaweeds. Substrates include rocks (outcrops, boulders, cobbles, pebbles), plants (including other algae), animals, boat bottoms, piers, debris, and less frequently sand and mud. Some species occur on a wide variety of living organisms, suggesting that the hosts are providing only space. Many species, however, have a restricted range of hosts and have been shown to be (or are suspected of being) at least partially parasitic. All reef-building corals contain dinoflagellates, without which their calcification ability is greatly reduced. Different phases in a life history may have different substrate preferences. Many fresh-water algae have become adapted to a nonaquatic habitat, living on moist soil, masonry and wooden structures, and trees. A few parasitize higher plants (expecially in the tropics), producing diseases in such crops as tea, coffee, and citrus. Thermophilic algae (again, chiefly blue-greens) live in hot springs at temperatures up to 163°F (73°C), forming a calcareous deposit known as tufa. One of the most remarkable adaptations of certain algae (blue-greens and greens) is their coevolution with fungi to form a compound organism, the lichen. See Lichens, Phytoplankton

Geographic distribution

Fresh-water algae, which are distributed by spores or fragments borne by the wind or by birds, tend to be widespread if not cosmopolitan, their distribution being limited by the availability of suitable habitats. Certain species, however, are characteristic of one or another general climatic zone, such as cold-temperate regions or the tropics. Marine algae, which are spread chiefly by water-borne propagules or reproductive cells, often have distinctive geographic patterns. Many taxonomic groups are widely distributed, but others are characteristic of particular climatic zones or geographic areas. See Plant geography

Economic importance

Numerous red, brown, and green seaweeds as well as a few species of fresh-water algae are consumed by the peoples of eastern Asia, Indonesia, Polynesia, and the North Atlantic. Large brown seaweeds may be chopped and added to poultry and livestock feed or applied whole as fertilizer for crop plants. The purified cell-wall polysaccharides of brown and red algae (alginate, agar, carrageenan) are used as gelling, suspending, and emulsifying agents in numerous industries. Some seaweeds have specific medicinal properties, such as effectiveness against worms. Petroleum is generally believed to result from bacterial degradation of organic matter derived primarily from planktonic algae.

Planktonic algae, as the primary producers in oceans and lakes, support the entire aquatic trophic pyramid and thus are the basis of the fisheries industry. Concomitantly, their production of oxygen counteracts its uptake in animal respiration. The ability of certain planktonic algae to assimilate organic nutrients makes them important in the treatment of sewage. See Food web

On the negative side, algae can be a nuisance by imparting tastes and odors to drinking water, clogging filters, and making swimming pools, lakes, and beaches unattractive. Sudden growths (blooms) of planktonic algae can produce toxins of varying potency. In small bodies of fresh water, the toxin (usually from blue-green algae) can kill fishes and livestock that drink the water. In the ocean, toxins produced by dinoflagellate blooms (red tides) can kill fishes and render shellfish poisonous to humans.

Fossil algae

At least half of the classes of algae are represented in the fossil record, usually abundantly, in the form of siliceous, calcareous, or organic remains, impressions, or indications. Blue-green algae were among the first inhabitants of the Earth, appearing in rocks at least as old as 2.3 billion years. Their predominance in shallow Precambrian seas is indicated by the extensive development of stromatolites.

All three classes of seaweeds (reds, browns, and greens) were well established by the close of the Precambrian, 600 million years ago (mya). By far the greatest number of fossil taxa belong to classes whose members are wholly or in large part planktonic. Siliceous frustules of diatoms and endoskeletons of silicoflagellates, calcareous scales of coccolithophorids, and highly resistant organic cysts of dinoflagellates contribute slowly but steadily to sediments blanketing ocean floors, as they have for tens of millions of years. Cores obtained in the Deep Sea Drilling Project have revealed an astounding chronology of the appearance, rise, decline, and extinction of a succession of species and genera. From this chronology, much can be deduced about the climate, hydrography, and ecology of particular geological periods.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a group of lower, autotrophic, and usually aquatic plants.

Algae contain chlorophyll and other pigments and form organic substances during the process of photosynthesis. They have no flowers or seeds. The spores, as a rule, are without a hard covering. The body of an alga (the thallus) is simpler in structure than that of mosses, ferns, and other land plants; often there is no differentiation of cells into tissue. In the most primitive algae (Cyanophyceae) the cells are without formal nuclei and chromatophores. The cells of some algae contain many nuclei; there are also algae with a noncellular structure (botrydium, siphon algae). Algae chromatophores are lamellate, stellate, striate, reticular, or small discoid (the last are characteristic of higher representatives of a number of types of algae). Many algae have dense formations known as pyrenoids and pyrenoid-like bodies; in higher algae (almost all Phaeophyceae and the majority of Rhodophyceae) they are absent. The cell covering consists of cellulose, pectin substances, organic silicon compounds (in diatomaceous algae), algin, and fucin (in Phaeophyceae). Storage substances include starch, glycogen, polysaccharides, and, less often, fat.

It is considered that there are approximately 30,000 species of algae. Based on differences in the selection of pigments, morphological traits, and biochemistry (the composition of cell envelope and storage substances), ten types (divisions) of algae are distinguished: blue-green (Cyanophyta), golden-brown (Chrysophyta), dinoflagellates (Pyrrophyta), diatoms (Bacillariophyta), yellow-green (Xanthophyta), flagellates (Euglenophyta), green (Chlorophyta), stoneworts (Charophyta), brown (Phaeophyta), and red (Rhodophyta). All these algae types evolved independently. Algae (evidently Chlorophyta) formed a basis for land plants.

Algae vary in size from one micron (Coccolithophoridae and some Bacillariophyta) to 40 meters(Macrocystis). Many algae are single-celled plants; among them there are mobile creatures that can make sliding movements (Bacillariophyta, Desmidiaceae, Cyanophyta); their ambulatory mechanism has not been definitively explained. There are also algae equipped with flagella, in many ways similar to the protozoa Mastigophora, but distinct from them in that the plants have chlorophyll and chromatophores. They can lose chlorophyll (in the dark) and become colorless and exist by absorbing organic substances in the water; there are also species of single-cell algae which can, like protozoa, capture organic particles (some Pyrrophyta). Single-celled algae often unite in colonies by means of mucus or outgrowths.

There are microscopic as well as large multicellular algae. The most simply organized look like branched threads, consisting of one row of cells; others have a thallus that may be crustlike, cordlike, spherical, lamellate, or fruticose with “leaves” that have veins (Sargassum). Some Cyanophyta, Chlorophyta, and Rhodophyta have calcium compounds in the thallus that make it hard. Algae have no roots and absorb needed substances from the water with their entire surface. Large seabed algae have organs for attaching to surfaces—a sole (a thickened wide part of the base) or rhizoids (branched outgrowths). In some algae, shoots spread along the ocean floor and start new thalluses.

Algae reproduce vegetatively, asexually, and sexually. Many single-celled algae reproduce by mitosis. Large algae reproduce vegetatively, by means of parts of the thallus or with the aid of special buds (Sphacerlaria). Some multicellular algae do not reproduce sexually; the majority form spores and gametes either in ordinary cells (Chlorophylla, some Rhodophylla) or in special formations called sporangia and gametangia (Phaeophyta). Spores and gametes are either nonmotile (Rhodophylla, conjugates) or motile, with flagella. All forms of the sexual process are observed in algae: isogamy, heterogamy, oogamy, and conjugation (the fusion of protoplasts of two vegetative cells). The zygote formed as a result of the sexual process divides at once or after a period of rest. Meiosis can occur simultaneously in the zygote. In primitive algae the same individual yields gametes or spores depending upon external conditions. In other algae the functions of sexual and asexual reproduction are fulfilled by different individuals (sporophytes and gametophytes). They can grow simultaneously under identical conditions (Furcellaria), simultaneously but under different conditions (Bangia), or in the same place but at different seasons. In a number of algae there is a strict alternation of gametophytes and sporophytes; this process is called alternation of generations. During this process in higher algae, either the zygote sprouts on the gametophyte, as does the sporophyte (Laminaria) while the gametophyte dies away, or the spore, without separating from the sporophyte, grows into the gametophyte that is developing on the sporophyte (Fucaceae). The Soviet algologist M. M. Gollerbakh proposed the term “succession of growth forms” for this phenomenon; this term more accurately reflects the essence of the process. In algae the sporophyte is often diploid (the nuclei contain a double set of chromosomes), while the gametophyte is haploid (the nuclei have one set of chromosomes). In a number of cases the gametophyte and sporophyte are found in the nuclear phase: both are haploid (Bangia) or both are diploid (some cladophores and Fucaceae).

Small free-floating algae become part of plankton, and as they develop into large quantities, they “color” the water. Benthic algae attach themselves to the bottom of a body of water or to other algae. There are algae that become embedded in shells and limestone (borers); there are also parasitic algae (among Rhodophyta). Large marine algae, mainly Phaeophyta, often form entire submarine forests. The majority of algae live between the surface of the water and a depth of 20-40 m; isolated species (of Rhodophyta and Phaeophyta) go as deep as 200 m if the water is very clear. Algae often live in large quantity on the surface and in the upper layers of soil; some of them fix atmospheric nitrogen, while others adapt to life on the roots of trees, fences, walls of houses, and rocks. Microscopic algae give a red or yellow “coloration” to snow high in the mountains and in the polar regions. Some algae enter into symbiotic relationships with fungi (lichens) and animals.

Algae are the main producers of organic substances in the water environment. Approximately 80 percent of all the organic substances created annually on the earth are created by algae and other aquatic plants. Algae serve either directly or indirectly as the source of food for all aquatic animals. There are rocks (diatomites, combustible shales, some limestones) that are the result of the activity of algae in earlier geological epochs. Algae are involved in the formation of medicinal muds. Some algae, particularly marine types, are used as food (kelp, red laver, green laver). In maritime regions algae are used as animal feed and fertilizer. In many countries algae are cultivated for large quantities of biomass, used in cattle feed and in the food industry. Many algae are as important component of the process of biological purification of sewage. Algae are used to obtain the gelatin- and mucus-forming substances agar-agar (Ahnfeltia, Gelidium), agaroids (Phyllophora, Gracillaria), carrageen (Chondrus, Gigartina, Furcellaria), algins (Laminaria and Fucaceae), and fodder that contains trace elements and iodine. Algae are widely used in experiments on photosynthesis and the role of the nucleus and other cell components. Experiments have been undertaken to use several fast-reproducing and undemanding algae (for example, Chlorella, which quickly and in great quantities synthesizes proteins, fats, carbohydrates, and vitamins and sufficiently fully absorbs substances excreted by humans and animals) to recycle substances in the living quarters of spaceships. The science of algae is called algology.


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The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


General name for the chlorophyll-bearing organisms in the plant subkingdom Thallobionta.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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1,497 68 0 1,565 Egeria densa 0 9 0 9 Filamentous algae 1,381 14,636 372 16,389 Hydrilla verticillata 0 0 19 19 Hydrocotyle sp.
The prevalence of filamentous algae in protrusion and crevice microhabitats explained the remainder of the variance (Fig.
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This difference is likely accounted for by the copious layer of filamentous algae (cyanobacteria) covering most of the sandy/muddy substrate in Apoyo, in which we observed the majority of these snails were found.
Densities and biovolumes of filamentous algae were determined using a Palmer cell at 100x until 500 algal cells were counted or the entire Palmer cell was scanned.
fasciatus was able to consume, assimilate and grow using three distinct food types: detritus (in the form of preconditioned leaves - both coarse particulate organic matter and fine particulate organic matter), filamentous algae (Cladophora spp.) and animal material (dead chironomids).
In different localities of the Baltic Sea similar trends in macroalgal vegetation have been reported associated to coastal eutrophication: decreased occurrence of perennial algae and increased occurrence of fast-growing filamentous algae and loose-lying algal mats causing anoxia (Eriksson et al., 1998 and references therein; Kotta et al., 2000, 2008b; Middelboe & Sand-Jensen, 2000; Lehvo & Back, 2001; Torn et al., 2006).
The northeastern inlet (Blanca NE), is relatively shallow with a smoothly-sloped substrate including dead coral reef elements, flat stones cover by filamentous algae and abundant organic matter.
Werner (1994) similarly found a significant positive correlation between tadpole mass and the mass of macro-vegetation (macrophytes and filamentous algae) in pond enclosures.
gryllotalpa was also observed to build more tubes in live filamentous algae. Morphology is therefore an important factor explaining our preference hierarchy (Fig.