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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).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.

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.


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.PhotosynthesisPlant 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.

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).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. 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.

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.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.Food web Sewage treatment

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.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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