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Algae

Algae

Algae (singular: alga) are plants or plantlike organisms that contain chlorophyll (pronounced KLOR-uh-fill) and other pigments (coloring matter) that trap light from the Sun. This light energy is then converted into food molecules in a process called photosynthesis. Most algae store energy as some form of carbohydrate (complex sugars).

Algae can be either single-celled or large, multicellular organisms. They can occur in freshwater or salt water (most seaweeds are algae) or on the surfaces of moist soil or rocks. The multicellular algae lack the true stems, leaves, or roots of the more complex, higher plants, although somelike the giant kelphave tissues that may be organized into structures that serve particular functions. The cell walls of algae are generally made of cellulose and can also contain pectin, which gives algae its slimy feel.

Types of algae

Although the term algae originally referred to aquatic plants, it is now broadly used to include a number of different groups of unrelated organisms. There are seven divisions of organisms that make up the algae. They are grouped according to the types of pigments they use for photosynthesis, the makeup of their cell walls, the types of carbohydrate compounds they store for energy, and the types of flagella (whiplike structures) they use for movement. The colors of the algae types are due to their particular mixtures of photosynthetic pigments, which typically include a combination of one or more of the green-colored chlorophylls as their primary pigments.

Words to Know

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Photosynthesis: Process by which light energy is captured from the Sun by pigment molecules in plants and algae and converted to food.

Phytoplankton: Microscopic algae that live suspended in the water.

Zooplankton: Tiny animals that drift through the upper surface of water bodies and feed on phytoplankton.

Euglenoids (Euglenophyta). The euglenoids, or Euglenophyta, are single-celled, protozoan-like algae, mostly occurring in freshwater. Unlike all other algae, they have no cell wall. Most euglenoids make their own food using light energy from the Sun but are capable of surviving in the dark if fed organic materials. Some species are heterotrophic, meaning they do not produce their own food but feed on organic matter suspended in the water.

Golden-brown algae (Chrysophyta). The Chrysophyta, or golden-brown algae and diatoms, are named for the yellow pigments they possess. These single-celled algae live both in freshwater and salt water. Their cell walls have no cellulose but are composed mostly of pectin, which is often filled with silica, a compound that makes the walls quite rigid. These algae store energy both as a carbohydrate and as large oil droplets. Diatoms have two glass shells made largely of silica that fit together like a pillbox and are exquisitely marked. Their species number from 40,000 to 100,000. When they die, their shells help to form sediments on the sea

bottom. This fine-grained sediment is often used for filtration in liquid purification systems.

Fire algae (Pyrrophyta). Fire algae, or Pyrrophyta, are single-celled algae and include the dinoflagellates (pronounced dye-no-FLAJ-uh-lets), which have two flagella used for locomotion. Most of these microscopic species live in salt water, with some occurring in freshwater. Some species of dinoflagellates emit bright flashes of light when exposed to air, which at night look like fire on the ocean's surface.

Green algae (Chlorophyta). The green algae, or Chlorophyta, occur in freshwater, although some live in the sea. Most green algae are single-celled and microscopic (able to be seen only under a microscope), forming the slimy green scum found in stagnant ponds. Others are larger and more complex, forming spherical (round) colonies composed of many cells or occurring as straight or branched filaments (long, thin series of cells). Green algae are thought to be in the evolutionary line that gave rise to the first land plants.

Red algae (Rhodophyta). The red algae, or Rhodophyta, are marine plants that live mainly in shallow waters and deep tropical seas. A few also occur in freshwater. Their body forms range from single-celled to

branched filaments. The larger species have filaments that are massed together and resemble the leaves and stems of plants. They have no flagella and typically grow attached to a hard surface or on other algae. Some species contain a red pigment; others range in color from green to red, purple, and greenish-black. The cell walls of Coralline red algae become heavily encrusted with minerals and help to cement and stabilize coral reefs.

Brown algae (Phaeophyta). The brown algae, or Phaeophyta, are shiny brown seaweeds that are especially abundant along rocky coasts, although some float in the open ocean. Brown algae are large in size and include the giant kelps, which are located along the Pacific coast and form forests that provide habitat to a wide range of marine life. Some species of brown algae have structures called holdfasts that anchor the algae to submerged rocks. Attached to the holdfasts are stemlike stalks that support wide leaflike blades. These blades provide the major surface for nutrient exchange and photosynthesis and are lifted up toward the water's surface by air bladders. Brown algae contain an accessory brown-colored pigment that gives the plants their characteristic dark color. Other well-known brown algae are the common rockweed Fucus and Sargassum, which floats in a thick, tangled mass through the Sargasso Seaa huge area of slow currents in the mid-Atlantic Ocean that supports a variety of marine organisms.

Yellow-green algae (Xanthophyta). The yellow-green algae, or Xanthophyta, primarily occur in freshwater. They can be either single celled or form colonies, their cell walls are made of cellulose and pectin compounds that sometime contain silica, they can have two or more flagella for locomotion, and they store their energy as carbohydrates. They derive their yellow-green color from the pigments carotenoids and xanthrophyll.

Ecological importance of algae

Microscopic algae are the source of much of Earth's oxygen. Algae are also very important ecologically because they are the beginning of the food chain for other animals. Phytoplankton, a mostly single-celled type of algae, are eaten by small animals called zooplankton (mostly crustaceans such as tiny shrimp) that drift near the surface of the sea. The zooplankton are in turn fed upon by larger zooplankton, small fish, and some whales. Larger fish eat the smaller ones. At the top of the open-water food web may be fish-eating birds, seals, whales, very large fish such as sharks or bluefin tuna, and humans.

The larger algae provide shelter and habitat for fish and other invertebrate animals. As these algae die, they are consumed by organisms called decomposers (mostly fungi and bacteria). The decomposers feed on decaying plants and release important minerals that are used by other organisms in the food web. In addition, the plant matter partially digested by the decomposers serves as food for worms, snails, and clams.

Algal Blooms

Algal blooms are an overabundance of algae that can severely affect the aquatic ecosystems in which they occur. Some marine species of dinoflagellates grow wildly at times, causing red tides that turn the surrounding sea a deep red color. The great numbers of microorganisms can rob the water of oxygen, causing many fish to suffocate.

The dinoflagellates also produce extremely poisonous chemicals that can kill a wide range of marine animals, as well as humans who eat shellfish containing the toxins. Red tides are natural events, although some scientists believe human interference contributes to their occurrence in certain regions.

Freshwater algae can also cause problems when they are overly abundant. Algal blooms can cause foul tastes in water stored in reservoirs that are used to provide drinking water to nearby communities.

Eutrophication is a major problem that is associated with algal blooms in lakes. A direct result of human interference, eutrophication is caused by the addition of excess nutrients (runoffs of phosphate and nitrate from chemical fertilizers and sewage disposal) to the water that encourage algae to grow abundantly. As the algae die and sink to the bottom, most of the water's oxygen is consumed in breaking down the decaying plant matter. Fish and other animals that require large amounts of oxygen can no longer survive and are replaced by organisms with lower oxygen demands.

Economic products obtained from algae

Brown and red seaweeds provide important economic products in the form of food for people and resources in the manufacturing of industrial products. These seaweeds are mostly harvested from the wild, although efforts are being made to cultivate large algae.

A red alga known as nori is a popular food in Japan. Another alga known as sea kale is consumed dried or cooked into various stews or soups. Sea lettuce and edible kelp are other commonly eaten seaweeds.

Brown seaweeds provide a natural source for the manufacture of chemicals called alginates that are used as thickening agents and stabilizers in the industrial preparation of foods and pharmaceutical drugs. Agar is a seaweed product prepared from certain red algae that is used in the manufacturing of pharmaceuticals and cosmetics, as a culture medium for laboratory microorganisms, and in the preparation of jellied desserts and soups. Carrageenin is an agarlike compound obtained from red algae that is widely used as a stabilizer in paints, pharmaceuticals, and ice cream.

[See also Food web and food chain ]

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Algae

Algae

Algae are a diverse group of all photosynthetic organisms that are not plants. Algae are important in marine, freshwater, and some terrestrial ecosystems . Seaweeds are large marine algae. The study of algae is called phycology.

Algae may be unicellular, colonial, or multicellular. Some algae, like the diatoms, are microscopically small. Other algae, like kelp, are as big as trees. Some algae, the phytoplankton , drift in the water. Other algae, the epiphitic or benthic algae, grow attached to rocks, docks, plants, and other solid objects.

Classification

The major groups of eukaryotic algae are the green algae, diatoms, red algae, brown algae, and dinoflagellates. They are classified as protista. Another group, the blue-green algae, is the cyanobacteria. Some authorities do not consider the blue-green algae to be true algae because they are prokaryotes , not eukaryotes.

Green Algae. Green algae are the algae most closely related to plants. They have the same pigments (chlorophyll a and b and carotenoids), the same chemicals in their cell walls (cellulose), and the same storage product (starch) as plants. Green algae may be unicellular or form filaments, nets, sheets, spheres, or complex mosslike structures. There are both freshwater and marine species. Some species of green algae live on snow, or in symbiotic associations as lichens, or with sponges or other aquatic animals. Edible green algae include Chlorella and sea lettuce. There are at least seventeen thousand species of green algae.

Diatoms. Diatoms are often regarded as the most beautiful of the algae. Each diatom has a cell wall made of glass that is very finely etched with a species-specific pattern of dots and lines. The patterns on the diatom cell walls are so precise that they were used for years to test the optics of new microscopes. Diatoms are also the most abundant algae in the open ocean and responsible for about one-quarter of all the oxygen gas produced on the earth each year. Diatom populations often bloom in lakes in the spring, providing a major food for zooplankton, forming the base of the aquatic food chain. There are over one hundred thousand species of diatoms.

Red Algae. Red algae are almost exclusively marine and include many edible and economically important species, including nori and laver. Red algae are also the source of carageenan and agar , which are used as food thickeners and stabilizers. Red algae are mostly large, complex seaweeds. There are four thousand to six thousand species.

Brown Algae. Brown algae are almost exclusively marine and include the largest and most complex seaweeds. Kelp, for example, may be more than 60 meters (200 feet) tall, and forms dense underwater forests off the California coast. Other important brown algae include the rockweeds and Sargassum, for which the Sargasso Sea is named. There are about fifteen hundred species of brown algae.

Dinoflagellates. Dinoflagellates are unicellular algae with armor made of cellulose and flagella that cause them to spin as they swim. Dinoflagellates are found in both freshwater and marine ecosystems. Some species of dinoflagellates emit an eerie blue light when disturbed, called bioluminescence . Other dinoflagellates are toxic and responsible for red tides and outbreaks of shellfish poisoning. There are two thousand to four thousand species of dinoflagellates.

Life Cycles

Life cycles among the algae are incredibly varied. In fact, almost any type of life cycle one can imagine is displayed by some member of the algae. In an asexual life cycle, individuals reproduce by splitting. Some dinoflagellates reproduce primarily by asexual division. There are three types of sexual life cycles, which involve at some stage the fusion of gametes : gametic meiosis , zygotic meiosis, and sporic meiosis.

Gametic Meiosis. In the gametic meiosis life cycle (which is employed by humans), meiosis produces the gametes, so the only haploid cells in the life cycle are the gametes. The individual that one sees is made of diploid cells. Diatoms have gametic meiosis.

Zygotic Meiosis. In zygotic meiosis, the zygote undergoes meiosis, so the only cell that is diploid is the zygote. All the other cells in the organism are haploid. Many of the green algae, including sea lettuce, have zygotic meiosis.

Sporic Meiosis. In sporic meiosis, there are both haploid individuals and diploid individuals within the life cycle. Meiosis produces haploid spores, which then divide to produce an individual that is made entirely of haploid cells. This individual produces gametes by mitosis . Two gametes unite and form a diploid zygote. The zygote divides to produce an individual that is made entirely of diploid cells. This individual produces spores by meiosis to complete the cycle. Because the life cycle includes two generations of individuals, a haploid generation and a diploid generation, it is called "alternation of generations." Plants and many of the green, red, and brown algae have sporic meiosis.

In Japan, Korea, and China, the production of nori is a billion-dollara-year industry, but because the two generations in the nori life cycle look completely unlike each other it was not until the early twentieth century that the second generation was discovered. This discovery radically improved the ability of humans to grow nori, and there is a memorial park in Japan dedicated to the British scientist, Kathleen Drew Baker, who discovered it.

Economic and Ecological Importance

Algae are the base of the aquatic food chain. Humans also eat many types of algae. The marine algae nori and kelp have been harvested in China for over two thousand years. Spirulina, a blue-green algae that is rich in protein and vitamin B, is harvested from Lake Chad in Africa. The photosynthesis done by algae is very important to the biosphere because it reduces the amount of carbon dioxide and increases the amount of oxygen in the atmosphere.

Some types of algae can cause environmental problems such as red tides and fishy-tasting water. These problems are usually caused by the excessive release of nutrients from farms, sewage, and other human activities. The outbreak of the nerve-toxin-producing Pfiesteria (a dinoflagellate) on the Atlantic coast, for example, has been linked to overflowing sewage lagoons.

see also Alternation of Generations; Cell Wall; Chloroplast; Evolution of Plants; Lichen; Life Cycles; Limnologist; Ocean Ecosystems: Hard Bottoms; Ocean Ecosystems: Open Ocean; Ocean Ecosystems: Soft Bottoms; Photosynthesis; Plankton; Plant; Protista

Virginia Card

Bibliography

Lembi, Carole A., and J. Robert Waaland. Algae and Human Affairs. Cambridge: Cambridge University Press, 1988.

Raven, Peter H., Ray F. Evert, and Susan E. Eichorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

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algae

algae (ăl´jē) [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.

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).

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algae

algae (sing. alga) A group of unrelated simple organisms that contain chlorophyll (and can therefore carry out photosynthesis) and live in aquatic habitats and in moist situations on land. The algal body may be unicellular or multicellular (filamentous, ribbon-like, or platelike). Formerly regarded as plants, algae are now classified as members of the kingdom Protoctista; they are assigned to separate phyla based primarily on the composition of the cell wall, the nature of the stored food reserves, and the other photosynthetic pigments present. See Bacillariophyta; Chlorophyta; Chrysomonada; Phaeophyta; Rhodophyta.

The organisms formerly known as blue-green algae are now classified as bacteria (see Cyanobacteria).

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alga

alga (pl. algae) Common (non-taxonomic) name for a relatively simple type of plant which is never differentiated into root, stem, and leaves; which contains chlorophyll a as the primary photosynthetic pigment; which has no true vascular (water-conducting) system; and in which there is no sterile layer of cells surrounding the reproductive organs. The algae range in form from single cells (Protista) to plants many metres in length; algae can be found in most habitats on Earth, although the majority occur in freshwater or marine environments. See BACILLARIOPHYCEAE; CHAROPHYCEAE; CHLOROPHYCEAE; CHRYSOPHYCEAE; DINOPHYCEAE; PHAEOPHYCEAE; and RHODOPHYCEAE.

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AILSA ALLABY and MICHAEL ALLABY. "alga." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 27 Aug. 2016 <http://www.encyclopedia.com>.

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alga

alga(pl. algae) The common (non-taxonomic) name for a relatively simple type of eukaryotic plant which is never differentiated into root, stem, and leaves, contains chlorophyll a as the primary photosynthetic pigment, has no true vascular (water-conducting) system, and in which there is no sterile layer of cells surrounding the reproductive organs. The algae range in form from eukaryotic single cells to plants many metres in length. Algae can be found in most habitats on Earth, although the majority occur in freshwater or marine environments.

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MICHAEL ALLABY. "alga." A Dictionary of Ecology. 2004. Encyclopedia.com. 27 Aug. 2016 <http://www.encyclopedia.com>.

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alga

alga Common (non-taxonomic) name for a relatively simple type of eukaryotic plant which is never differentiated into root, stem and leaves; which contains chlorophyll a as the primary photo-synthetic pigment; which has no true vascular system; and in which there is no sterile layer of cells surrounding the reproductive organs. The algae range in form from eukaryotic single cells to plants many metres in length. Algae can be found in most habitats on Earth, although the majority occur in freshwater or marine environments.

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MICHAEL ALLABY. "alga." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 27 Aug. 2016 <http://www.encyclopedia.com>.

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algae

algae Large group of essentially aquatic photosynthetic organisms found in salt and freshwater worldwide. Algae are a primary source of food for molluscs, fish, and other aquatic animals. Algae are directly important to humans as food and fertilizers. They range in size from unicellular microscopic organisms, such as those that form green pond scum, to huge brown seaweeds more than 45m (150ft) long. Algae belong to the kingdom Protoctista. See also green algae; photosynthesis; protoctist; red algae

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Algae

Algae

Scientists' concepts of which organisms should be termed algae (alga, singular; algae, plural; algal, adjective) have changed radically over the past two centuries. The term algae originally referred to almost all aquatic, photosynthetic organisms. But, as more has been learned about the evolutionary

EUKARYOTIC ALGAE
Division Common Name Pigments Habitats General Morphology
Glaucophyta Chlorophyll a Freshwater Unicellular flagellates
Phycocyanin
Rhodophyta Red algae Chlorophyll a Mostly marine Unicells, filaments, thalli; no flagellated stages; some calcified, some mucilaginous
Phycoerythrin
Phycocyanin
Cryptophyta Cryptomonads Chlorophyll a Marine and freshwater Mostly unicells
Chlorophyll c
Phycocyanin
Phycoerythrin
Heterokontophyta
(Ochrophyta)
Chrysophyceae Golden brown algae Chlorophyll a Freshwater Mostly unicells or colonies; biflagellate
Chlorophyll c
Fucoxanthin
Xanthophyceae Chlorophyll Chlorophyll a Mostly freshwater and terrestrial; some marine Coccoid, flagellate, or amoeboid unicells; colonies, uni- and multinucleate filaments; biflagellate
(Tribophyceae) Chlorophyll c
Eustigmatophyceae Chlorophyll a Freshwater and marine Unicells and coccoid; uni- or biflagellate
Violaxanthin
Bacillariophyceae Diatoms Chlorophyll a Freshwater and marine Unicells and colonial coccoids; no flagella
Chlorophyll c
Fucoxanthin
Raphidophyceae Chlorophyll a Freshwater and marine Unicellular biflagellates
Chlorophyll c
Fucoxanthin (Marine species only)
Diadinoxanthin
Vaucheriaxanthin
Heteroxanthin
Dictyochophyceae Silicoflagellates Chlorophyll a Marine Unicellular uniflagellates
Chlorophyll c
Fucoxanthin
Phaeophyceae Brown algae Chlorophyll a Marine Multicellular; reproductive cells biflagellate
Chlorophyll c
Fucoxanthin
Dinophyta (Pyrrhophyta) Dinoflagellates Chlorophyll a Mostly marine Mostly unicells, some coccoids and filaments; biflagellate
Chlorophyll c
Haptophyta Chlorophyll a Mostly marine Unicellular biflagellates
Chlorophyll c
Euglenophyta Euglenoids Chlorophyll a Mostly freshwater Unicellular uniflagellates
Chlorophyll b
Chlorophyta Green algae Chlorophyll a
Chlorophyll b
Prasinophyceae Marine and freshwater Unicells; 1-8 flagella
Chlorophyceae Mostly freshwater; some terrestrial and marine Unicellular, coccoid, or colonial flagellates; multicellular or multinucleate filaments; bi- or tetraflagellate
Ulvophyceae Marine or subaerial Uni- or multicellular or multinucleate filaments; reproductive cells bi-or tetraflagellate
Pleurastrophyceae Subaerial Coccoid or filament; reproductive cells biflagellate
Charophyceae Stoneworts or brittleworts; desmids Fresh or brackish water or subaerial Coccoid or filament, reproductive cells biflagellate or with no flagella; or multinucleate cells, complex thalli with biflagellate male gametes

history of algae, which spans at least five hundred million years, the definition has narrowed considerably. For instance, the assemblage of organisms traditionally called the blue-green algae will not be discussed here. These organisms are now known as cyanobacteria , a name that more accurately reflects their nature as prokaryotes . The algae are now generally considered to include only eukaryotic organisms.

Even after narrowing the group by excluding cyanobacteria, a succinct, precise definition of algae is not really possible. It would be accurate to say that algae are eukaryotic, photosynthetic autotrophs (and their colorless relatives), and that most are aquatic (there are some terrestrial species). The algae include organisms ranging in size from the microscopic to those reaching lengths as tall as a six-story building (e.g., the giant kelp, Macrocystis, which exists off the California coast), but no species of alga achieves the morphological complexity of true land plants; furthermore, sexual reproduction in algae is completely different than that of true land plants. Although not as beautiful to most people as roses and redwood trees, the algae are arguably the most important photosynthesizing eukaryotes on Earth.

Data analysis based on morphological, biochemical, and molecular research has led many systematists (scientists who study relationships among organisms) to conclude that traditional classification schemes for algae, and plants in general, do not reflect natural groupings and so should be abolished. It is useful, nevertheless, to have a classification system that provides a structure for comparing and discussing the various groups in terms that phycologists (scientists who study algae) and other scientists who work with algae (such as ecologists and biochemists) and students can understand. The accompanying table compares different groups of algae at the taxonomic level of division using a scheme that is generally accepted by many phycologists.

Of the eight divisions of algae in the table, only the group called the Chlorophyta is considered to be closely related to green plants. The organisms in the other groups are considered to be more closely related to protists than to green plants.

An ancient unicellular green alga gave rise to all algae in the Chlorophyta lineage . Green algae within the Chlorophyta are further split into two groups, one that contains the charophycean algae and another that consists of all other green algae. It is generally accepted among botanists (scientists who study plants) that a charophycean alga is the closest ancestor to the higher green plants.

Without the ancestral green algae, there would be no land plants, and without the algae and land plants, life as we know it would not be possible. Algae are primary producers in any aquatic environment. They are the basis of the food web, forming the very bottom of the food chain, meaning that they provide, as a byproduct of photosynthesis, a majority of the oxygen humans and animals breathe.

Some algae form symbiotic relationships with other organisms. Specific algae, in association with various types of fungi, form lichens of many different species, one of which is a major food source for reindeer in arctic regions. Algae can also form symbiotic relationships with animals, as evidenced by the very successful association of some reef-forming corals and the dinoflagellate algae of the species Symbiodinium.

Many algae are of economic importance. The fossilized remains of diatoms , known as diatomaceous earth, are used in cleaning products and as filtering and inert processing agents. Algal polysaccharides provide agar, used to prepare media for culturing bacteria, fungi, and plant tissues and in the purification and separation of nucleic acids and proteins. In Asia, certain algae are a major source of food. A tour through an Asian food store will turn up innumerable products made with algae, including the red alga, Porphyra (also known as nori or laver), which is used as a wrapper for sushi; prepared packets of dried soups featuring green algae; and several species of red and brown algae that are packaged, dried, salted, refrigerated, pickled, or frozen. The red alga Chondrus crispus provides carrageenan, used in the food industry as a thickener and emulsifier in many brands of ice cream, pudding, baby food, and chocolate milk. Brown algae provide alginates, also used as thickeners and stabilizers in numerous industries including food, paints, and cosmetics. Algal seaweeds are also collected and used as fodder for livestock in many parts of the world.

Some algae are of concern to humans because of the problems they cause. Some algae grow on the sides of buildings and on statues or other structures, forming unsightly discoloration. Rarely, and generally only in immuno-compromised individuals, certain species of green algae invade human tissues, initially gaining entry through a cut or abrasion on the skin and then proliferating. The green alga Cephaleuros virescens can become parasitic on the leaves of economically important plants such as coffee and tea. But, by far, the most destructive algal incidents are harmful algal blooms (HABs), the consequences of which can cost millions of dollars and cause serious health problems to livestock, fish, and even humans. HABs can occur in freshwater, contaminating watering sources for livestock and killing fish, or in marine environments. The marine HAB known as red tide is caused by certain toxin -producing dinoflagellates. The toxin can poison fish and shell-fish, and shellfish contaminated by the toxin can cause mild to severe illness, even death, in humans who consume them. The alga Pfiesteria has caused toxic reactions in fish and humans in estuaries in the southeastern United States. Many HABs can be attributed to pollution, especially runoff into waterways that causes a nutrient-rich environment conducive to the rapid growth of algae.

Divisions of Algae

The type of chlorophyll and other pigments is characteristic of certain groups of algae. For instance, the Chlorophyta (and the pigmented members of the euglenoids) have both chlorophylls a and b. These pigments are contained in chloroplasts that are the result of endosymbiotic events; that is, during the evolutionary history of the algae, photosynthetic, prokaryotic organisms survived being ingested by their algal hosts and became an integral part of them. The main features distinguishing the algal divisions are listed in the accompanying table. Here are a few more details:

Glaucophyta.

The glaucophytes are unusual unicells in which the plastids are recent endosymbionts.

Cryptophyta.

The cryptomonads are unicells with phycobiliprotein pigments like the red algae, but the pigments are located in a different position within the chloroplast.

Haptophyta.

The haptophytes are distinguished by the haptonema, an anterior filament that sometimes serves to attach the unicells to a substrate or to catch prey. The haptophytes include the coccolithophorids, the scales of which formed the white cliffs of Dover on the coast of England.

Dinophyta (or Pyrrhophyta).

The dinoflagellates provide a good example of the problems in classifying algae, as many species do not have chloroplasts and, thus, live heterotrophically. As discussed above, some species are notorious for causing HABs, including red tides.

Euglenophyta.

The euglenoids are motile unicells often found in organically enriched waters; like the dinoflagellates, some species of euglenoids are colorless heterotrophs.

Heterokontophyta (or Ochrophyta).

This large division includes the brown algae (class Phaeophyceae) and the diatoms (Bacillariophyceae). Brown algae are mostly seaweeds, very diverse in form and habitat. They range in size from microscopic filaments to kelps 50 or 60 meters in length. Sargassum floats freely in the Sargasso Sea, but some kelp, such as Laminaria, have a holdfast that attaches to a substrate, leaving the stem and leafy blade to undulate in the current.

Diatoms are noted for their siliceous walls, which can form many intricate and beautiful shapes. Diatoms are very abundant in both freshwater and marine environments and are important primary producers.

Rhodophyta.

The red algae are mostly seaweeds, and they form some of the most beautiful, exotic shapes of all algae. Some species are calcified and resemble corals.

Chlorophyta.

The very diverse green algae form two major lineages. The charophycean algae have complex morphologies and ultrastructural and genetic features that indicate they are ancestral to land plants. The other lineage comprises all other green algae, which range from unicells to large multinucleate filaments.

see also Aquatic Ecosystems; Cyanobacteria; Endosymbiosis; Evolution of Plants.

Russell L. Chapman

Debra A. Waters

Bibliography

Bold, Harold C., and Michael J. Wynne. Introduction to the Algae, 2nd ed. Upper Saddle River, NJ: Prentice-Hall, 1985.

Graham, Linda E., and Lee W. Wilcox. Algae. Upper Saddle River, NJ: Prentice-Hall, 2000.

Lembi, Carole A., and J. Robert Waaland, eds. Algae and Human Affairs. Cambridge: Cambridge University Press, 1988.

Sze, Philip. A Biology of the Algae, 3rd ed. Boston: WCB McGraw-Hill, 1998.

van den Hoek, C., D. G. Mann, and H. M. Jahns. Algae: An Introduction to Phycology.

Cambridge: Cambridge University Press, 1995.

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algae

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).

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alga

al·ga / ˈalgə/ • n. (usu. in pl. algae / -jē/ ) a simple nonflowering plant of a large assemblage that includes the seaweeds and many single-celled forms. Algae contain chlorophyll but lack true stems, roots, leaves, and vascular tissue. DERIVATIVES: al·gal / -gəl/ adj.

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alga

alga pl. algae XVI. — L. alga seaweed.

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alga

algablagger, bragger, dagger, flagger, Jagger, lagger, nagger, quagga, saggar, shagger, stagger, swagger •alga, realgar, Trafalgar •anger, clangour (US clangor), Katanga, languor, manga, panga, sangar, tanga, Tauranga, Zamboanga •sandbagger • carpetbagger • Erlanger •Aga, Braga, dagga, dargah, laager, lager, naga, Onondaga, raga, saga •beggar, eggar, Gregor, mega, Megger •Edgar • Helga • Heidegger •bootlegger •Jaeger, maigre, Meleager, Noriega, Ortega, rutabaga, Sagar •Antigua, beleaguer, bodega, eager, intriguer, leaguer, meagre (US meager), reneger, Riga, Seeger, Vega •chigger, configure, digger, figure, Frigga, jigger, ligger, rigger, rigor, rigour, snigger, swigger, transfigure, trigger, vigour (US vigor) •churinga, finger, linger, malinger •gravedigger • ladyfinger • forefinger •omega • vinegar • Honegger •outrigger • Minnesinger •Auriga, Eiger, liger, saiga, taiga, tiger

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algae

algae •haji • algae • Angie •argy-bargy, Panaji •edgy, sedgy, solfeggi, veggie, wedgie •cagey, stagy •mangy, rangy •Fiji, gee-gee, squeegee •Murrumbidgee, ridgy, squidgy •dingy, fringy, mingy, stingy, whingy •cabbagy • prodigy • effigy • villagey •porridgy • strategy • cottagey •dodgy, podgy, splodgy, stodgy •pedagogy •Georgie, orgy •ogee • Fuji •bhaji, budgie, pudgy, sludgy, smudgy •bulgy •bungee, grungy, gungy, scungy, spongy •allergy, analogy, genealogy, hypallage, metallurgy, mineralogy, tetralogy •elegy •antilogy, trilogy •aetiology (US etiology), amphibology, anthology, anthropology, apology, archaeology (US archeology), astrology, biology, campanology, cardiology, chronology, climatology, cosmology, craniology, criminology, dermatology, ecology, embryology, entomology, epidemiology, etymology, geology, gynaecology (US gynecology), haematology (US hematology), hagiology, horology, hydrology, iconology, ideology, immunology, iridology, kidology, meteorology, methodology, musicology, mythology, necrology, neurology, numerology, oncology, ontology, ophthalmology, ornithology, parasitology, pathology, pharmacology, phraseology, phrenology, physiology, psychology, radiology, reflexology, scatology, Scientology, seismology, semiology, sociology, symbology, tautology, technology, terminology, theology, topology, toxicology, urology, zoology • eulogy • energy • synergy • apogee • liturgy • lethargy •burgee, clergy •zymurgy • dramaturgy

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