Dictionary:
al·ga (ăl'gə)  |
Any of various chiefly aquatic, eukaryotic, photosynthetic organisms, ranging in size from single-celled forms to the giant kelp. Algae were once considered to be plants but are now classified separately because they lack true roots, stems, leaves, and embryos.
[Latin, seaweed.]
algal al'gal (ăl'gəl) adj.| 5min Related Video: Alga |
| Sci-Tech Encyclopedia: Algae |
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 also Bryophyta; Plant kingdom.
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).
Classification
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 also Bacillariophyceae; Chrysophyceae; Cryptophyceae; Cyanophyceae; Dinophyceae; Euglenophyceae; Eukaryotae; Eustigmatophyceae; Phaeophyceae; Prasinophyceae; Prochlorophyceae; Prokaryotae; Protozoa; Prymnesiophyceae; Rhodophyceae; Xanthophyceae.
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 also Cell walls (plant).
Characteristics
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 also Cell (biology); Cell plastids; Chromosome;
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 also 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 also 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 also 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.
Occurrence
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 also Lichens; Phytoplankton; Tufa.
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 also 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 also 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. See also Paleobotany; Stromatolite.
| Food and Nutrition: algae |
Simple plants that do not show differentiation into roots, stems, and leaves. They are mostly aquatic—either seaweeds or pond and river-weeds. Some seaweeds, such as dulse and Irish moss, have long been eaten, and a number of unicellular algae, including Chlorella, Scenedesmus, and Spirulina spp. have been grown experimentally as novel sources of food (50-60% of the dry weight is protein).
| Geography Dictionary: algae |
A large and diverse group of simple plants that contain chlorophyll and can therefore photosynthesize. Algae live in aquatic habitats or in moist regions inland.
| Britannica Concise Encyclopedia: algae |
For more information on algae, visit Britannica.com.
| Columbia Encyclopedia: algae |
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.
Bibliography
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).
| Science Dictionary: algae |
Primitive organisms that contain chlorophyll but do not have structures, such as xylem and phloem, to transport fluids. Algae sometimes contain only a single cell, and nowadays they are not considered members of the plant kingdom.
| Veterinary Dictionary: algal |
Pertaining to or caused by algae.
| Gardener's Dictionary: alga |
A flowerless plant of extremely simple structure, usually green but in the seaweeds often beautifully colored. Algae range in size from the microscopic organisms that cover ponds with green scum to the giant kelp, a seaweed more than 100 feet long.
| Wikipedia: Algae |
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 Laurencia, a marine genus of Red Algae from Hawaii.
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Algae (pronounced /ˈældʒiː/; singular alga /ˈælɡə/, Latin for "seaweed") are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in land plants. For that reason they are currently excluded from being considered plants.[3]
Though the prokaryotic Cyanobacteria (commonly referred to as Blue-green Algae) were traditionally included as "Algae" in older textbooks, many modern sources regard this as outdated[4] and restrict the term Algae to eukaryotic organisms.[5] All true algae therefore have a nucleus enclosed within a membrane and chloroplasts bound in one or more membranes.[4][6] Algae constitute a paraphyletic and polyphyletic group,[4] as they do not include all the descendants of the last universal ancestor nor do they all descend from a common algal ancestor, although their chloroplasts seem to have a single origin.[1]
Algae lack the various structures that characterize land plants, such as phyllids and rhizoids in nonvascular plants, or leaves, roots, and other organs that are found in tracheophytes. Many are photoautotrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species rely entirely on external energy sources and have limited or no photosynthetic apparatus.
Nearly all algae have photosynthetic machinery ultimately derived from the Cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dating back to 1.6 to 1.7 billion years ago.[7]
The first alga to have its genome sequenced was Cyanidioschyzon merolae.
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The singular alga is the Latin word for a particular seaweed and retains that meaning in English.[8] The etymology is obscure. Although some speculate that it is related to Latin algēre, "be cold",[9] there is no known reason to associate seaweed with temperature. A more likely source is alliga, "binding, entwining."[10] Since Algae has become a biological classification, alga can also mean one classification under Algae, parallel to a fungus being a species of fungi, a plant being a species of plant, and so on.
The ancient Greek word for seaweed was φῦκος (fūkos or phykos), which could mean either the seaweed, probably Red Algae, or a red dye derived from it. The Latinization, fūcus, meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the Biblical פוך (pūk), "paint" (if not that word itself), a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, blue.[11]
Accordingly the modern study of marine and freshwater algae is called either phycology or algology. The name Fucus appears in a number of taxa.
While Cyanobacteria have been traditionally included among the Algae, recent works usually exclude them due to large differences such as the lack of membrane-bound organelles, the presence of a single circular chromosome, the presence of peptidoglycan in the cell walls, and ribosomes different in size and content from those of the Eukaryotes.[12][13]. Rather than in chloroplasts, they conduct photosynthesis on specialized infolded cytoplasmic membranes called thylakoid membranes. Therefore, they differ significantly from the Algae despite occupying similar ecological niches.
By modern definitions Algae are Eukaryotes and conduct photosynthesis within membrane-bound organelles called chloroplasts. Chloroplasts contain circular DNA and are similar in structure to Cyanobacteria, presumably representing reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of Algae, reflecting different endosymbiotic events. The table below describes the composition of the three major groups of Algae. Their lineage relationships are shown in the figure in the upper right. Many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost plastids entirely.
Phylogeny:[14]
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| Supergroup affiliation | Members | Endosymbiont | Summary |
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| Primoplantae/ Archaeplastida |
Cyanobacteria | These Algae have primary chloroplasts, i.e. the chloroplasts are surrounded by two membranes and probably developed through a single endosymbiotic event. The chloroplasts of Red Algae have chlorophylls a and d (often), and phycobilins, while those of Green Algae have chloroplasts with chlorophyll a and b. Higher plants are pigmented similarly to Green Algae and probably developed from them, and thus Chlorophyta is a sister taxon to the plants; sometimes they are grouped as Viridiplantae. | |
| Excavata and Rhizaria | Green Algae |
These groups have green chloroplasts containing chlorophylls a and b [12]. Their chloroplasts are surrounded by four and three membranes, respectively, and were probably retained from ingested Green Algae. Chlorarachniophytes, which belong to the phylum Cercozoa, contain a small nucleomorph, which is a relict of the algae's nucleus. Euglenids, which belong to the phylum Euglenozoa, live primarily in freshwater and have chloroplasts with only three membranes. It has been suggested that the endosymbiotic Green Algae were acquired through myzocytosis rather than phagocytosis. |
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| Chromista and Alveolata | Red Algae |
These groups have chloroplasts containing chlorophylls a and c, and phycobilins. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with the Red Algae suggest a relationship there. In the first three of these groups (Chromista), the chloroplast has four membranes, retaining a nucleomorph in Cryptomonads, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether the Heterokonts, Haptophyta, and Cryptomonads are in fact more closely related to each other than to other groups.[2][15] The typical dinoflagellate chloroplast has three membranes, but there is considerable diversity in chloroplasts within the group, and it appears there were a number of endosymbiotic events.[1] The Apicomplexa, a group of closely related parasites, also have plastids called apicoplasts. Apicoplasts are not photosynthetic but appear to have a common origin with Dinoflagellate chloroplasts.[1] |
W.H.Harvey (1811—1866) was the first to divide the Algae into four divisions based on their pigmentation. This is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: Red Algae (Rhodophyta), Brown Algae (Heteromontophyta), Green Algae (Chlorophyta) and Diatomaceae.[16]
The first plants on earth evolved from shallow freshwater algae much like Chara some 400 million years ago. These probably had an isomorphic alternation of generations and were probably filamentous. Fossils of isolated land plant spores suggest land plants may have been around as long as 475 million years ago.[17][18]
A range of algal morphologies are exhibited, and convergence of features in unrelated groups is common. The only groups to exhibit three dimensional multicellular thalli are the reds and browns, and some chlorophytes.[19] Apical growth is constrained to subsets of these groups: the florideophyte reds, various browns, and the charophytes.[19] The form of charophytes is quite different to those of reds and browns, because have distinct nodes, separated by internode 'stems'; whorls of branches reminiscent of the horsetails occur at the nodes.[19] Conceptacles are another polyphyletic trait; they appear in the coralline algae and the Hildenbrandiales, as well as the browns.[19]
Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and non-motile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the life cycle of a species, are
In three lines even higher levels of organization have been reached, with full tissue differentiation. These are the brown algae,[20]—some of which may reach 50 m in length (kelps)[21]—the red algae,[22] and the green algae. [23] The most complex forms are found among the green algae (see Charales and Charophyta), in a lineage that eventually led to the higher land plants. The point where these non-algal plants begin and algae stop is usually taken to be the presence of reproductive organs with protective cell layers, a characteristic not found in the other alga groups.
Some species of algae form symbiotic relationships with other organisms. In these symbioses, the algae supply photosynthates (organic substances) to the host organism providing protection to the algal cells. The host organism derives some or all of its energy requirements from the algae. Examples are as follows.
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Lichens are defined by the International Association for Lichenology to be "an association of a fungus and a photosynthetic symbiont resulting in a stable vegetative body having a specific structure."[24] The fungi, or mycobionts, are from the Ascomycota with a few from the Basidiomycota. They are not found alone in nature but when they began to associate is not known.[25] One mycobiont associates with the same phycobiont species, rarely two, from the Green Algae, except that alternatively the mycobiont may associate with the same species of Cyanobacteria (hence "photobiont" is the more accurate term). A photobiont may be associated with many specific mycobionts or live independently; accordingly, lichens are named and classified as fungal species.[26] The association is termed a morphogenesis because the lichen has a form and capabilities not possessed by the symbiont species alone (they can be experimentally isolated). It is possible that the photobiont triggers otherwise latent genes in the mycobiont.[27]
Coral reefs are accumulated from the calcareous exoskeletons of marine invertebrates of the Scleractinia order; i.e., the Stony Corals. As animals they metabolize sugar and oxygen to obtain energy for their cell-building processes, including secretion of the exoskeleton, with water and carbon dioxide as byproducts. As the reef is the result of a favorable equilibrium between construction by the corals and destruction by marine erosion, the rate at which metabolism can proceed determines the growth or deterioration of the reef.
Algae of the Dinoflagellate phylum are often endosymbionts in the cells of marine invertebrates, where they accelerate host-cell metabolism by generating immediately available sugar and oxygen through photosynthesis using incident light and the carbon dioxide produced in the host. Endosymbiont algae in the Stony Corals are described by the term zooxanthellae, with the host Stony Corals called on that account hermatypic corals, which although not a taxon are not in healthy condition without their endosymbionts. Zooxanthellae belong almost entirely to the genus Symbiodinium.[28] The loss of Symbiodinium from the host is known as coral bleaching, a condition unless corrected leading to the deterioration and loss of the reef.
Green Algae live close to the surface of some sponges, for example, breadcrumb sponge (Halichondria panicea). The alga is thus protected from predators; the sponge is provided with oxygen and sugars which can account for 50 to 80% of sponge growth in some species.[29]
Rhodophyta, Chlorophyta and Heterokontophyta, the three main algal Phyla, have life-cycles which show tremendous variation with considerable complexity. In general there is an asexual phase where the seaweed's cells are diploid, a sexual phase where the cells are haploid followed by fusion of the male and female gametes. Asexual reproduction is advantageous in that it permits efficient population increases, but less variation is possible. Sexual reproduction allows more variation but is more costly because among other things. Often there is no strict alternation between the sporophyte and also because there is often an asexual phase, which could include the fragmentation of the thallus.[21][30][31]
The Algal Collection of the U.S. National Herbarium (located in the National Museum of Natural History) consists of approximately 320500 dried specimens, which, although not exhaustive (no exhaustive collection exists), gives an idea of the order of magnitude of the number of algal species (that number remains unknown).[32] Estimates vary widely. For example, according to one standard textbook,[33] in the British Isles the UK Biodiversity Steering Group Report estimated there to be 20000 algal species in the UK. Another checklist reports only about 5000 species. Regarding the difference of about 15000 species, the text concludes: "It will require many detailed field surveys before it is possible to provide a reliable estimate of the total number of species ...."
Regional and group estimates have been made as well: 5000—5500 species of Red Algae worldwide, "some 1300 in Australian Seas,"[34] 400 seaweed species for the western coastline of South Africa,[35] 669 marine species from California (U.S.A.), [36] 642 in the check-list of Britain and Ireland,[37] and so on, but lacking any scientific basis or reliable sources, these numbers have no more credibility than the British ones mentioned above. Most estimates also omit the microscopic Algae, such as the phytoplankta, entirely.
The topic of distribution of algal species has been fairly well studied since the founding of phytogeography in the mid-19th century AD.[38] Algae spread mainly by the dispersal of spores analogously to the dispersal of Plantae by seeds and spores. Spores are everywhere in all parts of the Earth: the waters fresh and marine, the atmosphere, free-floating and in precipitation or mixed with dust, the humus and in other organisms, such as humans. Whether a spore is to grow into an organism depends on the combination of the species and the environmental conditions.
The spores of fresh-water Algae are dispersed mainly by running water and wind, as well as by living carriers.[39] The bodies of water into which they are transported are chemically selective. Marine spores are spread by currents. Ocean water is temperature selective, resulting in phytogeographic zones, regions and provinces.[40]
To some degree the distribution of Algae is subject to floristic discontinuities caused by geographical features, such as Antarctica, long distances of ocean or general land masses. It is therefore possible to identify species occurring by locality, such as "Pacific Algae" or "North Sea Algae". When they occur out of their localities, it is usually possible to hypothesize a transport mechanism, such as the hulls of ships. For example, Ulva reticulata and Ulva fasciata travelled from the mainland to Hawaii in this manner.
Mapping is possible for select species only: "there are many valid examples of confined distribution patterns."[41] For example, Clathromorphum is an arctic genus and is not mapped far south of there.[42] On the other hand, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."[43]
Algae are prominent in bodies of water, common in terrestrial environments and are found in unusual environments, such as on snow and on ice. Seaweeds grow mostly in shallow marine waters, under 100 metres (330 ft); however some have been recorded to a depth of 360 metres (1,200 ft)[44]
The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column (phytoplankton) provide the food base for most marine food chains. In very high densities (algal blooms) these algae may discolor the water and outcompete, poison, or asphyxiate other life forms.
Algae are variously sensitive to different factors, which has made them useful as biological indicators in the Ballantine Scale and its modification.
Agar, an Algae derivative, has a number of commercial uses.[45]
Between 100,000 and 170,000 wet tons of Macrocystis are harvested annually in California for alginate extraction and abalone feed.[46][47]
To be competitive and independent from fluctuating support from (local) policy on the long run, biofuels should equal or beat the cost level of fossil fuels. Here, algae based fuels hold great promise, directly related to the potential to produce more biomass/ha-year than any other form of biomass. The break-even point for algae-based biofuels should be within reach in about ten years.
For centuries seaweed has been used as a fertilizer; George Owen of Henllys writing in the 16th century referring to drift weed in South Wales:[48]
This kind of ore they often gather and lay on great heapes, where it heteth and rotteth, and will have a strong and loathsome smell; when being so rotten they cast on the land, as they do their muck, and thereof springeth good corn, especially barley ... After spring-tydes or great rigs of the sea, they fetch it in sacks on horse backes, and carie the same three, four, or five miles, and cast it on the lande, which doth very much better the ground for corn and grass.
Today Algae are used by humans in many ways; for example, as fertilizers, soil conditioners and livestock feed.[49] Aquatic and microscopic species are cultured in clear tanks or ponds and are either harvested or used to treat effluents pumped through the ponds. Algaculture on a large scale is an important type of aquaculture in some places. Maerl is commonly used as a soil conditioner.
Naturally growing seaweeds are an important source of food, especially in Asia. They provide many vitamins including: A, B1, B2, B6, niacin and C, and are rich in iodine, potassium, iron, magnesium and calcium.[50] In addition commercially cultivated microalgae, including both Algae and Cyanobacteria, are marketed as nutritional supplements, such as Spirulina,[51] Chlorella and the Vitamin-C supplement, Dunaliella, high in beta-carotene.
Algae are national foods of many nations: China consumes more than 70 species, including fat choy, a cyanobacterium considered a vegetable; Japan, over 20 species;[52] Ireland, dulse; Chile, cochayuyo.[53] Laver is used to make "laver bread" in the British Isles; in Korea, gim; in Japan, nori and aonori. It is also used along the west coast of North America from California to British Columbia, in Hawaii and by the Maoris of New Zealand. Sea lettuce and badderlocks are a salad ingredient in Scotland, Ireland, Greenland and Iceland.
The oils from some Algae have high levels of unsaturated fatty acids. For example, Arachidonic acid is very high in Parietochloris incisa, where it reaches up to 47% of the triglyceride pool.[54] Some varieties of Algae favored by vegetarianism and veganism contain the long-chain, essential omega-3 fatty acids, Docosahexaenoic acid (DHA) and Eicosapentaenoic acid (EPA), in addition to vitamin B12. The vitamin B12 in algae is not biologically active. Fish oil contains the omega-3 fatty acids, but the original source is algae, which are eaten by marine life such as copepods and are passed up the food chain.[55]
The natural pigments produced by algae can be used as an alternative to chemical dyes and coloring agents.[57]
Carrageenan, from the red alga Chondrus crispus, is used as a stabiliser in milk products.
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| Translations: Algae |
Français (French)
n. pl. - algues (npl)
Deutsch (German)
n. pl. - Alge
Ελληνική (Greek)
n. pl. - άλγες, φύκια
Português (Portuguese)
n. pl. - algas (m pl) (Bot.)
Español (Spanish)
n. pl. - alga
Svenska (Swedish)
n. pl. - alg, tång
中文（简体）(Chinese (Simplified))
藻类, 海藻
中文（繁體）(Chinese (Traditional))
n. pl. - 藻類, 海藻
한국어 (Korean)
n. pl. - 해초 , 말무리
العربيه (Arabic)
(الجمع) طحالب
עברית (Hebrew)
n. pl. - אצות
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