algae

—algal, adj.

/al"jee/, n.pl., sing. alga /-geuh/.

any of numerous groups of chlorophyll-containing, mainly aquatic eukaryotic organisms ranging from microscopic single-celled forms to multicellular forms 100 ft. (30 m) or more long, distinguished from plants by the absence of true roots, stems, and leaves and by a lack of nonreproductive cells in the reproductive structures: classified into the six phyla Euglenophyta, Crysophyta, Pyrrophyta, Chlorophyta, Phaeophyta, and Rhodophyta. Cf. blue-green algae.

[ < NL, pl. of L alga seaweed]

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Members of a group of mostly aquatic, photosynthetic organisms (see photosynthesis) that defy precise definition.

They range in size from the microscopic flagellate Micromonas to giant kelp that reach 200 ft (60 m) in length. Algae provide much of Earth's oxygen, serve as the food base for almost all aquatic life, and provide foods and industrial products, including petroleum products. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. The classification of algae is changing rapidly because new taxonomic information is being discovered. Algae were formerly classified into three major groups

the red, brown, and green seaweeds

based on the pigment molecules in their chloroplasts. Many more than three groups are now recognized, each sharing a common set of pigment types. Algae are not closely related to each other in an evolutionary sense. Specific groups can be distinguished from protozoans and fungi (see fungus) only by the presence of chloroplasts and by their ability to carry out photosynthesis; these specific groups thus have a closer evolutionary relationship with the protozoa or fungi than with other algae. Algae are common on "slimy" rocks in streams (see diatoms) and as green sheens on pools and ponds. Use of algae is perhaps as old as humankind; many species are eaten by coastal societies.

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▪ biology

Introduction

singular  alga 

  members of a group of predominantly aquatic, photosynthetic organisms of the kingdom Protista (protist). They range in size from the tiny flagellate Micromonas that is 1 micrometre (0.00004 inch) in diameter to giant kelps (kelp) that reach 60 metres (200 feet) in length. Algae provide much of the Earth's oxygen, they are the food base for almost all aquatic life, they are a source of crude oil, and they provide foods and pharmaceutical and industrial products for humans. The algae have many types of life cycles. Their photosynthetic pigments are more varied than those of plants (plant), and their cells have features not found among plants and animals (animal). Some groups of algae are ancient, whereas other groups appear to have evolved more recently. The taxonomy of algae is subject to rapid change because new information is constantly being discovered. The study of algae is termed phycology, and one who studies algae is known as a phycologist.

      In this article the algae are defined as eukaryotic (eukaryote) (nucleus-bearing) organisms that photosynthesize but lack the specialized reproductive structures of plants, which always have multicellular reproductive structures that contain fertile gamete-producing cells surrounded by sterile cells. Algae lack true roots, stems, and leaves—features they share with the plant division Bryophyta (e.g., mosses (moss) and liverworts (liverwort)).

      The algae as treated in this article do not include the prokaryotic (nucleus-lacking) blue-green algae (cyanobacteria) or prochlorophytes. Beginning in the 1970s, some scientists suggested that the study of the prokaryotic algae should be incorporated into the study of bacteria because of certain shared cellular features. However, other scientists consider the oxygen-producing photosynthetic capability of blue-green and prochlorophyte algae to be as significant as cell structure. Therefore, these organisms continue to be classified as algae.

 Beginning in the 1830s, algae were classified into major groups based on colour (e.g., red, brown, and green). The colours are a reflection of different chloroplast pigments, such as chlorophylls (chlorophyll), carotenoids (carotenoid), and phycobiliproteins. Many more than three groups of pigments are recognized, and each class of algae shares a common set of pigment types distinct from those of all other groups.

      The algae are not closely related in an evolutionary sense. Specific groups of algae share features with protozoa (protozoan) and fungi (fungus) that, without the presence of chloroplasts and photosynthesis as delimiting features, make them difficult to distinguish from certain protozoa and fungi. Thus, some algae appear to have a closer evolutionary relationship with the protozoa or fungi than they do with other algae, and, conversely, some protozoa or fungi are more closely related to algae than to other protozoa or fungi.

 Knowledge and use of algae are perhaps as old as humankind. Seaweeds (seaweed) are still eaten by some coastal peoples, and algae are considered acceptable foods in many restaurants. Many slimy rocks are covered with algae such as diatoms (diatom) or cyanophytes, and algae are the cause of green or golden sheens on pools and ponds. Algae are the base of the food chain for all marine organisms since few other kinds of plants live in the oceans.

      This article discusses the algae in terms of their morphology, ecology, and evolutionary features. For a discussion of the related protists, see the articles protozoan and protist. For a more complete discussion of photosynthesis, see the articles photosynthesis and plant.

Physical and ecological features of algae

Size range and diversity of structure

 The size range of the algae spans seven orders of magnitude. Many algae consist of only one cell, others have two or more cells, and the largest have millions of cells. In large, macroscopic algae, groups of cells are specialized for specific functions, such as anchorage, transport, photosynthesis, and reproduction. Specialization involving thousands of cells indicates a measure of complexity and evolutionary advancement.

  The algae can be divided into several types based on the morphology of their vegetative, or growing, state. Filamentous forms have cells arranged in chains like strings of beads. Some filaments (e.g., Spirogyra) are unbranched, whereas others (e.g., Stigeoclonium) are branched and bushlike. In many red algae (e.g., Palmaria), numerous adjacent filaments joined laterally create the gross morphological form of the alga. Parenchymatous (tissuelike) forms, such as the giant kelp Macrocystis, can be very large, measuring many metres in length. Coenocytic forms of algae grow to large sizes without forming distinct cells. Coenocytic algae are essentially unicellular, multinucleated algae in which the protoplasm (cytoplasmic and nuclear content of a cell) is not subdivided by cell walls. The green seaweed Codium, which has been called dead-man's-fingers, is an example of this. Some algae have flagella (flagellum) and swim through the water. These flagellates (flagellate) range from single cells, such as Ochromonas, to colonial organisms with thousands of cells, such as Volvox. Coccoid organisms, such as Scenedesmus, normally have an exact number of cells per colony, produced by a series of rapid cell divisions when the organism is first formed; once the exact cell number is obtained, the organism grows in size but not in cell number. Capsoid organisms, such as Chrysocapsa, have variable numbers of cells. These cells are found in clusters that increase gradually in cell number and are embedded in transparent gel.

Distribution and abundance

      Algae are almost ubiquitous throughout the world, being most common in aquatic habitats. They can be categorized ecologically by their habitats. Planktonic (plankton) microscopic algae grow suspended in the water, whereas neustonic (neuston) algae grow on the water surface. Cryophilic algae occur in snow and ice; thermophilic algae live in hot springs (hot spring); edaphic algae live on or in soil; epizoic algae grow on animals, such as turtles (turtle) and sloths (sloth); epiphytic algae grow on fungi, land plants, or other algae; corticolous algae grow on the bark of trees; epilithic algae live on rocks; endolithic algae live in porous rocks; and chasmolithic algae grow in rock fissures. Some algae live inside other organisms, and in a general sense these are called endosymbionts. Specifically, endozoic endosymbionts live in protozoa or other, larger animals, whereas endophytic endosymbionts live in fungi, plants, or other algae.

 Algal abundance and diversity vary from one environment to the next, just as land plant abundance and diversity vary from tropical forests (tropical rainforest) to deserts (desert). Terrestrial vegetation (plants and algae) is influenced most by precipitation and temperature, whereas aquatic vegetation (primarily algae) is influenced most by light and nutrients. When nutrients are abundant, as in some polluted waters, algal cell numbers can become great enough to produce obvious patches of algae called “blooms” (water bloom) or “red tides,” (red tide) usually linked to favourable growing conditions, including an abundance of nutrients.

Ecological and commercial importance

 Algae form organic (organic compound) food molecules from carbon dioxide and water through the process of photosynthesis, in which they capture energy from sunlight. Similar to land plants, algae are at the base of the food chain, and the existence of nonphotosynthetic organisms is dependent upon the presence of photosynthetic organisms. Nearly three-fourths of Earth is covered by water, and since the so-called higher plants are virtually absent from the major water sources (e.g., the oceans (ocean)), the existence of nearly all marine life—including whales (whale), seals (seal), fishes (fish), turtles (turtle), shrimps (shrimp), lobsters (lobster), clams (clam), octopuses (octopus), starfish (sea star), and worms (worm)—ultimately depends upon algae. In addition to making organic molecules, algae produce oxygen as a by-product of photosynthesis. Algae produce an estimated 30 to 50 percent of the net global oxygen available to humans and other terrestrial animals for respiration.

       crude oil and natural gas are the remnants of photosynthetic products of ancient algae, which were subsequently modified by bacteria. The North Sea oil deposits are believed to have been formed from coccolithophore algae (class Prymnesiophyceae), and the Colorado oil shales by an alga similar to Botryococcus (a green alga (green algae)). Today, Botryococcus produces blooms in Lake Baikal (Baikal, Lake) where it releases so much oil onto the surface of the lake that it can be collected with a special skimming apparatus and used as a source of fuel. Several companies have grown oil-producing algae in high-salinity ponds and have extracted the oil as a potential alternative to fossil fuels (fossil fuel).

 Algae, as processed and unprocessed food, have an annual commercial value of several billion dollars. Algal extracts are commonly used in preparing foods and other products, and the direct consumption of algae has existed for centuries in the diets of East Asian and Pacific Island societies. Many species of algae, including Porphyra umbilicalis (nori, or laver) and Palmaria palmata ( dulse), are eaten by humans. The red alga (red algae) Porphyra (laver) is the most important commercial food alga. In Japan alone approximately 100,000 hectares (247,000 acres) of shallow bays and seas are farmed. Porphyra has two major stages in its life cycle: the Conchocelis stage and the Porphyra stage. The Conchocelis is a small, shell-boring stage that can be artificially propagated by seeding on oyster shells that are tied to ropes or nets and set out in special marine beds for further development. The conchospores that germinate grow into the large blades of Porphyra plants, which in due course are removed from the nets, washed, sometimes chopped, and pressed into sheets to dry.

 Palmaria palmata, another red alga, is eaten primarily in the North Atlantic region. Known as dulse in Canada and the United States, as duileasg (dulisk) in Scotland, as duileasc (dillisk) in Ireland, and as söl in Iceland, it is harvested by hand from intertidal rocks during low tide. Species of Laminaria, Undaria, and Hizikia (a type of brown algae) are also harvested from wild beds along rocky shores, particularly in Japan, Korea, and China, where they may be eaten with meat or fish and in soups. The green algae Monostroma and Ulva look somewhat like leaves of lettuce (their common name is sea lettuce) and are eaten as salads or in soups, relishes, and meat or fish dishes.

      The microscopic, freshwater green alga Chlorella is cultivated as a food supplement and is eaten in Taiwan, Japan, Malaysia, and the Philippines. It has a high protein content (53 to 65 percent) and has even been considered as a possible food source during extended space travel.

      The cell walls of many seaweeds contain phycocolloids (algal colloids (colloid)) that can be extracted by hot water. The three major phycocolloids are alginates, agars (agar), and carrageenans. Alginates are extracted primarily from brown seaweeds, and agar and carrageenan are extracted from red seaweeds. These phycocolloids are polymers of chemically modified sugar molecules, such as galactose in agars and carrageenans, or organic acids, such as mannuronic acid and glucuronic acid in alginates. Most phycocolloids can be safely consumed by humans and other animals, and many are used in a wide variety of prepared foods, such as “ready-mix” cakes, “instant” puddings and pie fillings, and artificial dairy toppings.

      Alginates, or alginic acids, commercially extracted from brown seaweeds, such as Macrocystis, Laminaria, and Ascophyllum, are used in ice creams (ice cream) to limit ice crystal formation (producing a smooth texture), in syrups as emulsifiers and thickeners, and in candy bars and salad dressings as fillers.

      Agars (agar), extracted primarily from species of red algae, such as Gelidium, Gracilaria, Pterocladia, Acanthopeltis, and Ahnfeltia, are used in instant pie fillings, canned meats or fish, and bakery icings and for clarifying beer and wine.

      Carrageenans are extracted from various red algae, including Eucheuma in the Philippines, Chondrus (also called Irish moss) in the United States and the Canadian Maritime Provinces, and Iridaea in Chile. It is estimated that the average human consumption of carrageenans is 250 mg (0.01 ounce) a day in the United States, where they are used for thickening and stabilizing dairy products (dairy product), imitation creams, puddings, syrups, and canned pet foods.

      In addition to their important role as food products, phycocolloids have industrial uses. They are relatively inert and are used as creams and gels in medical drugs and insecticides. Agar is used extensively in laboratory research as a substrate for growing bacteria, fungi, and algae in pure cultures (pure culture) and as a substrate for eukaryotic cell culture and tissue culture. Carrageenans are used in the manufacture of shampoos, cosmetics, and medicines.

 The diatoms (diatom) (class Bacillariophyceae) played an important role in industrial development during the 20th century. The frustules, or cell walls, of diatoms are made of opaline silica and contain many fine pores. Large quantities of frustules are deposited in some ocean and lake sediments, and their fossilized (fossil) remains are called diatomite. Diatomite contains approximately 3,000 diatom frustules per cubic millimetre (50 million diatom frustules per cubic inch). When geologic uplifting brings deposits of diatomite above sea level, the diatomite is easily mined. A deposit located in Lompoc, Calif., U.S., for example, covers 13 square kilometres (5 square miles) and is up to 425 metres (1,400 feet) deep.

      Diatomite is relatively inert and has a high absorptive capacity, large surface area, and low bulk density. It consists of approximately 90 percent silica, and the remainder consists of compounds such as aluminum and iron oxides. The fine pores in the diatom frustules make diatomite an excellent filtering material for beverages (e.g., fruit juices, soft drinks (soft drink), beer, and wine), chemicals (e.g., sodium hydroxide, sulfuric acid, and gold salts), industrial oils (e.g., those used as lubricants in rolling mills or for cutting), cooking oils (e.g., vegetable and animal), sugars (sugar) (e.g., cane, beet, and corn), water supplies (e.g., municipal, swimming pool, waste, and boiler), varnishes (varnish), lacquers, jet fuels, and antibiotics (antibiotic), as well as many other products. Its relatively low abrasive properties make it suitable for use in toothpaste, sink cleansers, polishes (for silver and automobiles), and buffing compounds.

      Diatomite is also widely used as a filler and extender in paint, paper, rubber, and plastic products. The gloss and sheen of “flat” paints can be controlled by the use of various additions of diatomite. During the manufacture of plastic bags, diatomite can be added to the newly formed sheets to act as an antiblocking agent so that the plastic (polyethylene) can be rolled while it is still hot. Because it can absorb approximately 2.5 times its weight in water, it also makes an excellent anticaking carrier for powders used to dust roses or for cleansers used to clean rugs. Diatomite is also used in making welding rods, battery boxes, concrete, explosives, and animal foods.

       chalk is another fossilized deposit of remains of protists. It consists in part of calcium carbonate scales, or coccoliths (coccolith), from the coccolithophore members of the class Prymnesiophyceae. Chalk deposits, such as the white cliffs in Dover, Kent, Eng., contain large amounts of coccoliths, as well as the shells of foraminiferan protozoa. Coccoliths can be observed in fragments of ordinary blackboard chalk examined under a light microscope.

      By the end of the 18th century, kelps (kelp) (class Phaeophyceae) were harvested and burned to produce soda. When mineral deposits containing soda were discovered in Salzburg, Austria, and elsewhere, the use of kelp ash declined. Kelps were again harvested in abundance during the 19th century when salts and iodine were extracted for commercial use, although the discovery of cooking salt and iodides led to a demise of the kelp industry. During World War I the United States used seaweeds to produce potash, a plant fertilizer, and acetone, a necessary component in the manufacture of smokeless gunpowder.

      For many centuries, seaweeds (seaweed) around the world have been widely used as agricultural fertilizers. (fertilizer) Coastal farmers collect seaweeds by cutting them from seaweed beds growing in the ocean or by gathering them from masses washed up on shores after storms. The seaweeds are then spread over the soil. Dried seaweed, although almost 50 percent mineral matter, contains a large amount of nitrogenous organic matter. Commercial extracts of seaweed sold as plant fertilizers contain a mixture of macronutrients, micronutrients, and trace elements that promote robust plant growth.

      The green unicellular flagellate Dunaliella, which turns red when physiologically stressed, is cultivated in saline ponds for the production of carotene and glycerol. These compounds can be produced in large amounts and extracted and sold commercially.

Toxicity

 Some algae can be harmful to humans. A few species produce toxins that may be concentrated in shellfish and finfish, which are thereby rendered unsafe or poisonous for human consumption. The dinoflagellates (dinoflagellate) (class Dinophyceae) are the most notorious producers of toxins. Paralytic shellfish poisoning is caused by saxitoxin or any of at least 12 related compounds. Saxitoxin is probably the most toxic compound known; it is 100,000 times more toxic than cocaine. Saxitoxin and saxitoxin-like compounds are nerve toxins that interfere with neuromuscular function. Alexandrium tamarense and Gymnodinium catenatum are the two species most often associated with paralytic shellfish poisoning. Diarrheic shellfish poisoning is caused by okadaic acids that are produced by several kinds of algae, especially species of Dinophysis. Neurotoxic shellfish poisoning, caused by toxins produced in Gymnodinium breve, an organism associated with red tides (red tide), is notorious for fish kills and shellfish poisoning along the coast of Florida in the United States. When the red tide blooms are blown to shore, wind-sprayed toxic cells can cause health problems for humans and other animals that breathe the air.

      Ciguatera is a disease of humans caused by consumption of tropical fish that have fed on the alga Gambierdiscus or Ostreopsis. Unlike many other dinoflagellate toxins, ciguatoxin and maitotoxin are concentrated in finfish rather than shellfish. Levels as low as one part per billion in fish can be sufficient to cause human intoxication.

      Not all shellfish poisons are produced by dinoflagellates. Amnesic shellfish poisoning is caused by domoic acid, which is produced by diatoms (class Bacillariophyceae), such as Nitzschia pungens f. multiseries and Nitzschia pseudodelicatissima. Symptoms of this poisoning in humans progress from abdominal cramps to vomiting to memory loss to disorientation and finally to death.

      Several algae produce toxins lethal to fish. Prymnesium parvum (class Prymnesiophyceae) has caused massive die-offs in ponds where fish are cultured, and Chrysochromulina polylepis (class Prymnesiophyceae) has caused major fish kills along the coasts of the Scandinavian (Scandinavia) countries. Other algae, such as Heterosigma (class Raphidophyceae) and Dictyocha (class Dictyochophyceae), are suspected fish killers as well.

      Algae can cause human diseases (human disease) by directly attacking human tissues, although the frequency is rare. Protothecosis, caused by the chloroplast-lacking green alga, Prototheca, can result in waterlogged skin lesions, in which the pathogen grows. Prototheca organisms may eventually spread to the lymph glands from these subcutaneous lesions. Prototheca is also believed to be responsible for ulcerative dermatitis in the platypus. Very rarely, similar infections in humans and cattle can be caused by chloroplast-bearing species of Chlorella.

      Some seaweeds contain high concentrations of arsenic and when eaten may cause arsenic poisoning. For example, the brown alga Hizikia contains sufficient arsenic to be used as a rat poison.

      Diatoms have been used in forensic medicine. In cases in which death by drowning is suspected, lung tissue and blood vessels are examined; the presence of siliceous diatom walls, transported in the bloodstream of the dying persons, is evidence for death by drowning. Certain diatom species can even be used to pinpoint the location of death insofar as they are characteristic for a given body of water.

Form and function of algae

The algal cell

 Eukaryotic algal cells contain three types of double-membrane-bound organelles (organelle): the nucleus, the chloroplast, and the mitochondrion (cell). In most algal cells there is only a single nucleus, although some cells are multinucleate. In addition, some algae are siphonaceous, with many nuclei not separated by cell walls. The nucleus contains most of the genetic material, or deoxyribonucleic acid ( DNA), of the cell. In most algae, the molecules of DNA exist as linear strands that are condensed into obvious chromosomes (chromosome) only at the time of nuclear division ( mitosis); however, there are two classes of algae, Dinophyceae and Euglenophyceae, in which the nuclear DNA is always condensed into chromosomes. In all algae, the two membranes that surround the nucleus are referred to as the nuclear envelope. The nuclear envelope typically has specialized nuclear pores that regulate the movement of molecules into and out of the nucleus.

 Chloroplasts (chloroplast) are the sites of photosynthesis, the complex set of biochemical reactions that use the energy of light to convert carbon dioxide and water into sugars. Each chloroplast contains flattened, membranous sacs, called thylakoids, that contain the photosynthetic light-harvesting pigments, the chlorophylls, carotenoids, and phycobiliproteins (see below Photosynthesis (algae)).

      The mitochondria (mitochondrion) are the sites where food molecules are broken down and carbon dioxide, water, and chemical bond energy are released, a process called cellular respiration (see below Cellular respiration (algae)). Photosynthesis and respiration are approximately opposite processes, the former building sugar molecules and the latter breaking them down. The inner membrane of the mitochondrion is infolded to a great extent, and this provides the surface area necessary for respiration. The infoldings, called cristae, have three morphologies: (1) flattened or sheetlike, (2) fingerlike or tubular, and (3) paddlelike. The mitochondria of land plants and animals, by comparison, generally have flattened cristae.

      Chloroplasts and mitochondria also have their own DNA. However, this DNA is not like nuclear DNA in that it is circular (or, more correctly, in endless loops) rather than linear and therefore resembles the DNA of prokaryotes. The similarity of chloroplastic and mitochondrial DNA to prokaryotic DNA has led many scientists to accept the hypothesis of endosymbiosis, which states that these organelles developed as a result of a long and successful symbiotic association of prokaryote cells inside eukaryote host cells.

      Algal cells also have several single-membrane-bound organelles, including the endoplasmic reticulum, Golgi apparatus, lysosomes (lysosome), peroxisomes, contractile or noncontractile vacuoles (vacuole), and, in some, ejectile organelles. The endoplasmic reticulum is a complex membranous system that forms intracellular compartments, acts as a transport system within the cell, and serves as a site for synthesizing fats, oils, and proteins. The Golgi apparatus, a series of flattened, membranous sacs that are arranged in a stack, performs four distinct functions: it sorts many molecules synthesized elsewhere in the cell; it produces carbohydrates, such as cellulose or sugars, and sometimes attaches the sugars to other molecules; it packages molecules in small vesicles; and it marks the vesicles so that they are routed to the proper destination. The lysosome is a specialized vacuole that contains digestive enzymes (enzyme) that break down old organelles, cells or cellular components during certain developmental stages, and particulate matter that is ingested in species that can engulf food. Peroxisomes (peroxisome) specialize in metabolically breaking down certain organic molecules and dangerous compounds, such as hydrogen peroxide, that may be produced during some biochemical reactions. Vacuoles (vacuole) are membranous sacs that store many different substances, depending on the organism and its metabolic state. Contractile vacuoles (contractile vacuole) are specialized organelles that regulate the water content of cells and are therefore not involved in the long-term storage of substances. When too much water enters the cells, contractile vacuoles serve to “squirt” it out. Some algae have special ejectile organelles that apparently act as protective structures. The Dinophyceae has harpoonlike trichocysts (trichocyst) beneath the cell surface that can explode from a disturbed or irritated cell. Trichocysts may serve to attach prey to algae cells before the prey is consumed. Ejectosomes are structures that are analogous to ejectile organelles and are found in the class Cryptophyceae. Several classes of algae in the division Chromophyta have mucous organelles that secrete slime. Gonyostomum semen, a freshwater member of the class Raphidophyceae, has numerous mucocysts, which, when such cells are collected in a plankton net, discharge and render the net and its contents somewhat gummy.

      The nonmembrane-bound organelles of algae include the ribosomes (ribosome), pyrenoids, microtubules (microtubule), and microfilaments. Ribosomes are the sites of protein synthesis, where genetic information in the form of messenger ribonucleic acid (m RNA) is translated into protein. The ribosomes accurately interpret the genetic code of the DNA so that each protein is made exactly to the genetic specifications. The pyrenoid, a dense structure inside or beside chloroplasts of certain algae, consists largely of ribulose biphosphate carboxylase, one of the enzymes necessary in photosynthesis for carbon fixation and thus sugar formation. starch, a storage form of glucose, is often found around pyrenoids. Microtubules, (microtubule) tubelike structures formed from tubulin proteins, are present in most cells. In many algae, microtubules appear and disappear as needed. Microtubules provide a rigid structure, or cytoskeleton, in the cell that helps determine and maintain the shape of the cell, especially in species without cell walls. Microtubules also provide a sort of “rail” system along which vesicles are transported. The spindle apparatus, which separates the chromosomes during nuclear division, consists of microtubules. Finally, certain kinds of microtubules also form the basic structure, or axoneme, of a flagellum, and they are a major component of the root system that anchors a flagellum within the cell. Microfilaments are formed by the polymerization of proteins such as actin, which can contract and relax and therefore function as tiny muscles inside the cells.

Flagella (flagellum)

 A flagellum is structurally complex, containing more than 250 types of proteins. Each flagellum consists of an axoneme, or cylinder, with nine outer pairs of microtubules surrounding two central microtubules. The axoneme is surrounded by a membrane, sometimes beset by hairs or scales. The outer pairs of microtubules are connected to the axoneme by a protein called nexin. Each of the nine outer pairs of microtubules has an a tubule and a b tubule. The a tubule has numerous molecules of a protein called dynein that are attached along its length. Extensions of dynein, called dynein arms, connect neighbouring tubules, forming dynein cross-bridges. Dynein is involved in converting the chemical energy of adenosine triphosphate (ATP) into the mechanical energy that mediates flagellar movement. In the presence of ATP, dynein molecules are activated, and the flagellum bends as dynein arms on one side of a dynein cross-bridge become activated and move up the microtubule. This creates the power stroke. The dynein arms on the opposite side of the dynein cross-bridge are then activated and slide up the opposite microtubule. This causes the flagellum to bend in the opposite direction during the recovery stroke. Although scientists are working to discover the additional mechanisms that are involved in producing the whiplike movement characteristic of many eukaryotic flagella, the importance of dynein activation in this process has been established.

      The flagellum membrane is also complex. It may contain special receptors called chemoreceptors that respond to chemical stimuli and allow the algal cell to recognize a multitude of signals, ranging from signals carrying information about changes in the alga's environment to signals carrying information about mating partners. On some flagella, superficial scales and hairs may aid in swimming. Certain swellings and para-axonemal structures, such as crystalline rods and noncrystalline rods and sheets, may be involved in photoreception, providing the swimming cell with a means for detecting light. The flagellum membrane merges into the cell membrane, where the nine pairs of axonemal microtubules enter the main body of the cell. At this junction, each pair of microtubules is joined by an additional microtubule, forming nine triplets. This cylinder of nine triplets, constituting the basal body, anchors the flagellum in the cell membrane. The anchorage provided by the basal body is strengthened by musclelike fibres and special microtubules called microtubular roots. Most flagellate cells have two flagella, and therefore two basal bodies, each with microtubular roots. The orientation of the flagella and the arrangement of the musclelike fibres and microtubular roots are important taxonomic features that can be used to classify algae and are especially important in the classification of the Chlorophyta.

mitosis

  mitosis, or the process of replication and division of the nucleus that results in the production of genetically identical daughter cells, is relatively similar among plants and animals, but the algae have a wide diversity of mitotic features that not only set the algae apart from plants and animals but also set certain algae apart from other algae. The nuclear envelope breaks apart in some algal groups but remains intact in others. The spindle microtubules remain outside the nucleus in some algae, enter the nucleus through holes in the nuclear envelope in other algae, and form inside the nucleus and nuclear envelope in still other algae. The diversity and complexity of algal mitosis provide clues to a better understanding of how mitosis operates in higher plants and animals.

cellular respiration

  cellular respiration in algae, as in all organisms, is the process by which food molecules are metabolized to obtain chemical energy for the cell. Most algae are aerobic (i.e., they live in the presence of oxygen), although a few Euglenophyceae can live anaerobically in environments without oxygen. The biochemical pathways for respiration in algae are similar to those of other eukaryotes; the initial breakdown of food molecules, such as sugars, fatty acids (fatty acid), and proteins, occurs in the cytoplasm, but the final high-energy-releasing steps occur inside the mitochondria.

photosynthesis and light-absorbing pigments

  photosynthesis is the process by which light energy is converted to chemical energy whereby carbon dioxide and water are converted into organic molecules (organic compound). The process occurs in almost all algae, and in fact much of what is known about photosynthesis was first discovered by studying the green alga Chlorella.

 Photosynthesis comprises both light reactions and dark reactions (or Calvin cycle). During the dark reactions, carbon dioxide is bound to ribulose bisphosphate, a 5-carbon sugar with two attached phosphate groups, by the enzyme ribulose bisphosphate carboxylase. This is the initial step of a complex process leading to the formation of sugars. During the light reactions, light energy is converted into the chemical energy needed for the dark reactions.

      The light reactions of many algae differ from those of land plants because some of them use different pigments to harvest light. Chlorophylls (chlorophyll) absorb primarily blue and red light, whereas carotenoids (carotenoid) absorb primarily blue and green light, and phycobiliproteins absorb primarily blue or red light. Since the amount of light absorbed depends upon the pigment composition and concentration found in the alga, some algae absorb more light at a given wavelength, and therefore, potentially, those algae can convert more light energy of that wavelength to chemical energy via photosynthesis. All algae use chlorophyll a to collect photosynthetically active light. green algae and euglenophytes also use chlorophyll b. In addition to chlorophyll a, the remaining algae also use various combinations of other chlorophylls, chlorophyllides, carotenoids, and phycobiliproteins to collect additional light from wavelengths of the spectrum not absorbed by chlorophyll a or b. The chromophyte algae, dinoflagellates (dinoflagellate), cryptomonads (cryptomonad) (class Cryptophyceae), and the class Micromonadophyceae, for example, also use chlorophyllides. (Chlorophyllides, often incorrectly called chlorophylls, differ from true chlorophylls in that they lack the long, fat-soluble phytol tail that is characteristic of chlorophylls.) Some green algae use carotenoids for harvesting photosynthetically active light, but the Dinophyceae and chromophyte algae almost always use carotenoids. Phycobiliproteins, which appear either blue (phycocyanins) or red (phycoerythrins), are found in red algae and cryptomonads.

The effects of water on light absorption

      Red wavelengths are absorbed in the first few metres of water. Blue wavelengths are more readily absorbed if the water contains average or abundant amounts of organic material. Thus, green wavelengths are often the most common light in deep water.

      Chlorophylls absorb red and blue wavelengths much more strongly than they absorb green wavelengths, which is why chlorophyll-bearing plants appear green. The carotenoids and phycobiliproteins, on the other hand, strongly absorb green wavelengths. Algae with large amounts of carotenoid appear yellow to brown, those with large amounts of phycocyanin appear blue, and those with large amounts of phycoerythrin appear red.

      At one time it was believed that algae with specialized green-absorbing accessory pigments outcompeted green algae in deeper water. Some green algae, however, grow as well as other algae in deep water, and the deepest attached algae include green algae. The explanation of this paradox is that the cell structure of the deepwater green algae is designed to capture virtually all light, green or otherwise. Thus, while green-absorbing pigments are advantageous in deeper waters, evolutionary changes in cell structure can evidently compensate for the absence of these pigments.

Nutrient storage

      As in land plants, the major carbohydrate storage product of the green algae is usually starch in the form of amylose or amylopectin. These starches are polysaccharides (polysaccharide) in which the monomer, or fundamental unit, is glucose. Green algal starch comprises more than 1,000 sugar molecules, joined by alpha linkages between the number 1 and number 4 carbon atoms. The cell walls of many, but not all, algae contain cellulose. Cellulose is formed from similar glucose molecules but with beta linkages between the number 1 and 4 carbons.

      The Cryptophyceae also store amylose and amylopectin. These starches are stored outside the chloroplast but within the surrounding membranes of the chloroplast endoplasmic recticulum. Most Dinophyceae store starch outside the chloroplast, often as a cap over a bulging pyrenoid. The major carbohydrate storage product of red algae is a type of starch molecule (Floridean starch) that is more highly branched than amylopectin. Floridean starch is stored as grains outside the chloroplast.

      The major carbohydrate storage product of the chromophyte algae and Euglenophyceae is formed from glucose molecules interconnected with beta linkages between the number 1 and 3 carbons. These polysaccharide compounds are always stored outside the chloroplast. The number of glucose units in each storage product varies among the algal classes, and each type is given a special name—i.e., chrysolaminarin in diatoms, laminarin in brown algae, leucosin in chrysophytes, and paramylon in euglenophytes. The exact chemical constituency of the major polysaccharide storage products is unknown for the classes Bicosoecophyceae, Dictyochophyceae, Eustigmatophyceae, Raphidophyceae, Synurophyceae, and Xanthophyceae. In the chromophyte algae, the molecules are usually small (16–40 units of sugar) and are stored in solution in vacuoles, whereas in the euglenophyte algae, the molecules of paramylon are large (approximately 150 units of sugar) and are stored as grains.

Alternative methods of nutrient absorption

      Not all algae have chloroplasts and photosynthesize. “Colourless” algae can obtain energy and food by oxidizing organic molecules, which they absorb from the environment or digest from engulfed particles. They are classified as algae, rather than fungi (fungus) or protozoa (protozoan), because in most other features they resemble photosynthetic algae. Algae that rely on ingestion and oxidation of organic molecules are referred to as heterotrophic algae because they depend on the organic materials produced by other organisms.

      Algae also produce many other kinds of sugars and sugar alcohols, such as rhamnose, trehalose, and xylose, and some algae can generate energy by oxidizing these molecules.

Reproduction and life histories

 Algae regenerate by sexual reproduction, involving male and female gametes (gamete) (sex cells), by asexual reproduction, or by both ways.

      Asexual reproduction is the production of progeny without the union of cells or nuclear material. Many small algae reproduce asexually by ordinary cell division or by fragmentation, whereas larger algae reproduce by spores (spore). Some red algae produce monospores (walled, nonflagellate, spherical cells) that are carried by water currents and upon germination produce a new organism. Some green algae produce nonmotile spores called aplanospores. In contrast, zoospores lack true cell walls and bear one or more flagella. These flagella allow zoospores to swim to a favourable environment, whereas monospores and aplanospores have to rely on passive transport by water currents.

 Sexual reproduction is characterized by the process of meiosis, in which progeny cells receive half of their genetic information from each parent cell. Sexual reproduction is usually regulated by environmental events. In many species, when temperature, salinity, inorganic nutrients (e.g., phosphorus, nitrogen, and magnesium), or day length become unfavourable, sexual reproduction is induced. A sexually reproducing organism typically has two phases in its life cycle. In the first stage, each cell has a single set of chromosomes and is called haploid, whereas in the second stage each cell has two sets of chromosomes and is called diploid. When one haploid gamete fuses with another haploid gamete during fertilization, the resulting combination, with two sets of chromosomes, is called a zygote. Either immediately or at some later time, a diploid cell directly or indirectly undergoes a special reductive cell-division process (meiosis). Diploid cells in this stage are called sporophytes (sporophyte) because they produce spores. During meiosis the chromosome number of a diploid sporophyte is halved, and the resulting daughter cells are haploid. At some time, immediately or later, haploid cells act directly as gametes. In algae, as in plants, haploid cells in this stage are called gametophytes (gametophyte) because they produce gametes.

 The life cycles of sexually reproducing algae vary; in some, the dominant stage is the sporophyte, in others it is the gametophyte. For example, Sargassum (class Phaeophyceae) has a diploid (sporophyte) body, and the haploid phase is represented by gametes. Ectocarpus (class Phaeophyceae) has alternating diploid and haploid vegetative stages, whereas Spirogyra (class Charophyceae) has a haploid vegetative stage, and the zygote is the only diploid cell.

 In freshwater organisms especially, the fertilized egg, or zygote, often passes into a dormant state called a zygospore. Zygospores generally have a large store of food reserves and a thick, resistant cell wall. Following an appropriate environmental stimulus, such as a change in light, temperature, or nutrients, the zygospores are induced to germinate and start another period of growth.

      Most algae can live for days, weeks, or months. Small algae are sometimes found in abundance during a short period of the year and remain dormant during the rest of the year. In some species, the dormant form is a resistant cyst, whereas other species remain in the vegetative state but at very low population numbers. Some large, attached species are true perennials. They may lose the main body at the end of the growing season, but the attachment part, the holdfast, produces new growth only at the beginning of the next growing season.

      The red algae, as exemplified by Polysiphonia, have some of the most complex life cycles known for living organisms. Following meiosis, four haploid tetraspores are produced, which germinate to produce either a male or a female gametophyte. When mature, the male gametophyte produces special spermatangial branches that bear structures, called spermatangia, which contain spermatia, the male gametes. The female gametophyte produces special carpogonial branches that bear carpogonia, the female gametes. Fertilization occurs when a male spermatium, carried by water currents, “bumps into” the extended portion of a female carpogonium and the two gametes fuse. The fertilized carpogonium (the zygote) and the female gametophyte tissue around it develop into a basketlike or pustulelike structure called a carposporophyte. The carposporophyte eventually produces and releases diploid carpospores that develop into tetrasporophytes. Certain cells of the tetrasporophyte undergo meiosis to produce tetraspores, and the cycle is repeated. In the life cycle of Polysiphonia, and many other red algae, there are separate male and female gametophytes, carposporophytes that develop on the female gametophytes, and separate tetrasporophytes.

      The life cycles of diatoms (diatom), which are diploid, are also unique. Diatom walls, or frustules, are composed of two overlapping parts (the valves). During cell division, two new valves form in the middle of the cell and partition the protoplasm into two parts. Consequently, the new valves are generally somewhat smaller than the originals, so after many successive generations, most of the cells in the growing population are smaller than their parents. When such diatoms reach a critically small size, sexual reproduction may be stimulated. The small diploid cells undergo meiosis, and among pennate (thin, elliptical) diatoms the resulting haploid gametes fuse into a zygote, which grows quite large and forms a special kind of cell called an auxospore. The auxospore divides, forming two large, vegetative cells, and in this manner the larger size is renewed. In centric diatoms there is marked differentiation between nonmotile female gametes, which act as egg cells, and motile (typically uniflagellate) male gametes.

evolution and paleontology of algae

 Modern ultrastructural and molecular studies have provided important information that has led to a reassessment of the evolution of algae. In addition, the fossil record for some groups of algae has hindered evolutionary studies, and the realization that some algae are more closely related to protozoa (protozoan) or fungi (fungus) than they are to other algae came late, producing confusion in evolutionary thought and delays in understanding the evolution of the algae.

      The Euglenophyceae are believed to be an ancient lineage of algae that includes some zooflagellate protozoa, which is supported by ultrastructural and molecular data. Some scientists consider the colourless euglenophytes to be an older group and believe that the chloroplasts were incorporated by symbiogenesis more recently. The order of algae with the best fossil record are the Dasycladales, which are calcified unicellar forms of elegant construction dating back at least to the Triassic Period.

 Some scientists consider the red algae, which bear little resemblance to any other group of organisms, to be very primitive eukaryotes that evolved from the prokaryotic blue-green algae. Evidence in support of this view includes the nearly identical photosynthetic pigments and the very similar starches among the red algae and the blue-green algae. Many scientists, however, attribute the similarity to an endosymbiotic origin of the red algal chloroplast from a blue-green algal symbiont. Other scientists suggest that the red algae evolved from the Cryptophyceae, with the loss of flagella, or from fungi by obtaining a chloroplast. In support of this view are similarities in mitosis and in cell wall plugs, special structures inserted into holes in the cell walls that interconnect cells. Some evidence suggests that such plugs regulate the intercellular movement of solutes. Ribosomal gene sequence data from studies in molecular biology suggest that the red algae arose along with animal, fungal, and green plant lineages.

      The green algal classes are evolutionarily related, but their origins are unclear. Most consider the class Micromonadophyceae to be the most ancient group, and some fossil data support this view. The class Ulvophyceae is also ancient, whereas the classes Charophyceae and Chlorophyceae are more recent.

      The class Dinophyceae is of uncertain origin. During the 1960s and '70s the unusual structure and chemical composition of the nuclear DNA of the Dinophyceae were interpreted as somewhat primitive features. Some scientists even considered the Dinophyceae to be mesokaryotes (intermediate between the prokaryotes and the eukaryotes); however, this view is no longer accepted. Their peculiar structure is considered as a result of evolutionary divergence, perhaps about 300 or 400 million years ago. The Dinophyceae may be distantly related to the chromophyte algae, but ribosomal gene sequence data suggest that their closest living relatives are the ciliated protozoa. It is likely that the Dinophyceae arose from nonphotosynthetic ancestors and that later some species of Dinophyceae adopted chloroplasts by symbiogenesis and thereby became capable of photosynthesis, although many of these organisms still retain the ability to ingest solid food, similar to protozoa.

      The origin of the chromophyte algae also remains unknown. Ultrastructural and molecular data suggest that they are in a protistan lineage that diverged from the protozoa and aquatic fungi about 300 to 400 million years ago. At that time, chloroplasts were incorporated, originally as endosymbionts, and since then the many chromophyte groups have been evolving. Fossil, ultrastructural, and ribosomal gene sequence data support this hypothesis.

      The Cryptophyceae (cryptomonad) are an evolutionary enigma. They have no fossil record, and phylogenetic (phylogenetic tree) data are conflicting. Although some workers align them near the red algae, because both groups possess phycobiliproteins in their chloroplasts, most scientists suggest that independent symbiotic origins for the red or blue colour of their chloroplasts could explain the similarity. Cryptophytes have flagellar hairs and other flagellar features that resemble those of the chromophyte algae; however, the mitochondrial structure and other ultrastructural features are distinct and argue against such a relationship.

      The fossil record for the algae is not nearly as complete as it is for land plants and animals. Red algal fossils are the oldest known algal fossils. Microscopic spherical algae (Eosphaera and Huroniospora) that resemble the living genus Porphyridium are known from the Gunflint Iron Formation (Gunflint microfossils) of North America (formed about 1.9 billion years ago). Fossils that resemble modern tetraspores are known from the Amelia Dolomites of Australia (formed some 1.5 billion years ago). The best characterized fossils are the coralline red algae represented in fossil beds since the Precambrian time.

      Some of the green algal classes are also very old. Organic cysts resembling modern Micromonadophyceae cysts date from about 1.2 billion years ago. Tasmanites formed the Permian “white coal,” or tasmanite, deposits of Tasmania and accumulated to a depth of several feet in deposits that extend for miles. Similar deposits in Alaska yield up to 150 gallons of oil per ton of sediment. Certain Ulvophyceae fossils that date from about one billion years ago are abundant in Paleozoic (Paleozoic Era) rocks. Some green algae deposit calcium carbonate on their cell walls, and these algae produced extensive limestone formations. The Charophyceae, as represented by the large stoneworts (stonewort) (order Charales), date from about 400 million years ago. The oospore, the fertilized female egg, has spirals on its surface that were imprinted by the spiraling protective cells that surrounded the oospore. Oospores from before about 225 million years ago had right-handed spirals, whereas those formed since that time have had left-handed spirals. The reason for the switch remains a mystery.

      Fossil Dinophyceae date from the Silurian Period (430 million years ago). Some workers consider at least a portion of the acritarchs, a group of cystlike fossils of unknown affinity, to be Dinophyceae, but few scientists agree with that view. The acritarchs occurred as early as 700 million years ago.

 The Chromophyta have the shortest fossil history among the major algal groups. Some scientists believe that the group is ancient, whereas others point out that there is a lack of data to support this view and suggest that the group evolved recently, as indicated by fossil and molecular data. The oldest chromophyte fossils, putative brown algae, are approximately 400 million years old. Coccolithophores, coccolith-bearing members of the Prymnesiophyceae, date from the Late Triassic Epoch (230 to 200 million years ago), with one reported from approximately 280 million years ago. Coccolithophores were extremely abundant during the Mesozoic Era, contributing to deep deposits such as those that constitute the white cliffs of southeast England. Most species became extinct at the end of the Cretaceous Period (65.5 million years ago), along with the dinosaurs (dinosaur), and indeed there are more extinct species of coccolithophores than there are living species. The Chrysophyceae, Bacillariophyceae, and Dictyochophyceae date from about 100 million years ago, and despite the mass extinctions 65.5 million years ago, many species still flourish. In Lompoc, Calif., U.S., their siliceous remains have formed deposits of diatomite almost 0.5 km (0.3 mile) in depth, while at Mývatn in Iceland the lake bottom bears significant amounts of diatomite in the form of diatomaceous (diatomaceous earth) ooze, many metres in depth.

      The Xanthophyceae may be even more recent, with fossils dating from about 20 million years ago, while fossil records of the remaining groups of algae, notably the Euglenophyceae and the Cryptophyuceae, which lack mineralized walls, are negligible.

Classification of algae

Diagnostic features

 The classification of algae into taxonomic groups is based upon the same rules that are used for the classification of land plants, but the organization of groups of algae above the order level has changed substantially since 1960. Research using electron microscopes (electron microscope) has demonstrated differences in features, such as the flagellar apparatus, cell division process, and organelle structure and function, that are important in the classification of algae. Similarities and differences among algal, fungal (fungus), and protozoan groups have led scientists to propose major taxonomic changes, and these changes are continuing. Molecular studies, especially comparative gene sequencing, have supported some of the changes that followed electron microscopic studies, but they have suggested additional changes as well. Since 1960 the number of classes has nearly doubled. Furthermore, the apparent evolutionary scatter of some algae among protozoan and fungal groups implies that a natural classification of algae as a class is impracticable.

      Kingdoms are the most encompassing of the taxonomic groups, and scientists are actively debating which organisms belong in which kingdoms. Some scientists have suggested as many as 30 or more kingdoms, while others have argued that all eukaryotes should be combined into one large kingdom. Using cladistic analysis (a method for determining evolutionary relationships), the green algae should be grouped with the land plants, the chromophyte algae should be grouped with the aquatic fungi and certain protozoa, and the Euglenophyceae are most closely related to the trypanosome flagellates, including the protozoa that cause sleeping sickness. However, it is unclear where the red algae or cryptomonads (cryptomonad) belong, and the overall conclusion is that the algae are not all closely related, and they do not form a single evolutionary lineage devoid of other organisms.

      Division-level classification, as with kingdom-level classification, is tenuous for algae. For example, some phycologists place the classes Bacillariophyceae, Phaeophyceae, and Xanthophyceae in the division Chromophyta, whereas others place each class in separate divisions: Bacillariophyta, Phaeophyta, and Xanthophyta. Yet, almost all phycologists agree on the definition of the respective classes Bacillariophyceae, Phaeophyceae, and Xanthophyceae. In another example, the number of classes of green algae (Chlorophyta), and the algae placed in those classes, has varied greatly since 1960. The five classes of green algae given below are accepted by a large number of phycologists, but at least an equal number of phycologists would suggest one of many alternative classification schemes. The classes are distinguished by the structure of flagellate cells (e.g., scales, angle of flagellar insertion, microtubular roots, and striated roots), the nuclear division process ( mitosis), the cytoplasmic division process (cytokinesis), and the cell covering. Many scientists combine the Micromonadophyceae with the Pleurastrophyceae, naming the combined group the Prasinophyceae.

      Because classes are better defined and more accepted than divisions, taxonomic discussions of algae are usually constrained at the class level. The divisions provided below, though commonly used, are by no means accepted by all phycologists.

      “Phylum” and “division” represent the same level of organization; the former is the zoological term, the latter is the botanical term. The classification of protists (protist) continues to be debated, and a standard outline of the kingdom Protista has not been established. The differences between the classification presented below and the classification presented in the article on protists (see protist: Classification (protist)) reflect the taxonomic variations that arise from individual interpretations.

Annotated classification

Division Chlorophyta ( green algae)

 Chlorophylls a and b; starch stored inside chloroplast; mitochondria with flattened cristae; flagella, when present, lack tubular hairs (mastigonemes); unmineralized scales on cells or flagella of flagellates and zoospores; conservatively, between 9,000 and 12,000 species.

      Class Chlorophyceae

 Primarily freshwater; includes Chlamydomonas, Chlorella, and Oedogonium.

      Class Charophyceae

 Includes the macroscopic pondweed Chara, filamentous Spirogyra, and desmids (desmid).

      Class Micromonadophyceae

 Primarily marine; includes the smallest eukaryotic alga, Micromonas.

      Class Pleurastrophyceae

 Freshwater and marine; includes marine flagellate Tetraselmis.

      Class Ulvophyceae

 Primarily marine; includes sea lettuce Ulva.

Division Chromophyta

 Most with chlorophyll a; one or two with chlorophyllide c; carotenoids present; storage product beta-1,3-linked polysaccharide outside chloroplast; mitochondria with tubular cristae; biflagellate cells and zoospores usually with tubular hairs on one flagellum; mucous organelles common.

      Class Bacillariophyceae (diatoms (diatom))

 Silica cell walls, or frustules; centric diatoms commonly planktonic and valves radially symmetrical; pennate diatoms, usually attached or gliding over solid substrates, with valves bilaterally symmetrical; primarily in freshwater, marine, and soil environments; at least 12,000 to 15,000 living species; tens of thousands more species described from fossil diatomite deposits; Cyclotella and Thalassiosira (centrics) and Navicula and Nitzschia (pennates).

      Class Bicosoecophyceae

 May be included in the Chrysophyceae or in the protozoan group Zoomastigophora; colourless flagellate cells in vase-shaped loricas (lorica) (wall-like coverings); cell attached to lorica using flagellum as a stalk; lorica attaches to plants, algae, animals, or water surface; freshwater and marine; fewer than 50 species described; Bicosoeca.

      Class Chrysophyceae ( golden algae)

 Many unicellular or colonial flagellates; also capsoid, coccoid, amoeboid, filamentous, parenchymatous, or plasmodial; many produce silica cysts (statospores); predominantly freshwater; approximately 1,200 species; Chrysamoeba, Chrysocapsa, and Ochromonas.

      Class Dictyochophyceae

 Predominantly marine flagellates, including silicoflagellates that form skeletons common in diatomite deposits; fewer than 25 described species.

      Order Pedinellales

 When pigmented, has 6 chloroplasts in a radial arrangement; flagella bases attached almost directly to nucleus.

      Order Dictyochales (silicoflagellates)

 Typically with siliceous skeletons like spiny baskets enclosing the cells; flagella bases attach almost directly to nucleus; silicoflagellate skeletons common in diatomite deposits; Dictyocha, Pedinella, and Pseudopedinella.

      Class Eustigmatophyceae

 Newly described, with probably more to be discovered; mostly small, pale green, and spherical; fewer than 15 species; Eustigmatos and Nannochloropsis.

      Class Phaeophyceae ( brown algae or brown seaweeds)

 Range from microscopic forms to large kelps more than 20 metres long; at least 1,500 species, almost all marine; Ectocarpus, Macrocystis, and Sargassum.

      Class Prymnesiophyceae (Haptophyceae)

 Many with haptonema, a hairlike appendage between two flagella; no tubular hairs; many with organic scales; some deposit calcium carbonate on scales to form coccoliths; (coccolith) coccolithophorids may play a role in global warming because they can remove large amounts of carbon from the ocean water; predominantly marine and planktonic; approximately 300 species; more fossil coccolithophores known; Chrysochromulina, Emiliania, and Prymnesium.

      Class Raphidophyceae (Chloromonadophyceae)

 Flagellates with mucocysts (mucilage-releasing bodies) occasionally found in freshwater or marine environments; fewer than 50 species; Heterosigma, Vacuolaria, and Olisthodiscus.

      Class Synurophyceae

 Previously placed in Chrysophyceae; silica-scaled; unicellular or colonial flagellates sometimes alternating with capsoid benthic stage; cells covered with elaborately structured silica scales; approximately 250 species; Mallomonas, Synura, and Tesselaria.

      Class Xanthophyceae ( yellow-green algae)

 Primarily coccoid, capsoid, or filamentous; mostly in freshwater environments; about 600 species; Bumilleriopsis, Tribonema, and Vaucheria.

Division Cryptophyta

 Unicellular flagellates.

      Class Cryptophyceae (cryptomonad)

 Chlorophyll a, chlorophyllide c₂, and phycobiliproteins; starch stored outside of chloroplast; mitochondria with flattened cristae; tubular hairs on one or both flagella; special ejectosomes in a furrow or gullet near base of flagella; cell covered with periplast, often elaborately decorated sheet or scale covering; nucleomorph may represent reduced nucleus of symbiotic organism; approximately 200 described species; Chilomonas, Cryptomonas, Falcomonas, and Rhinomonas.

Division Pyrrophyta (Dinoflagellata (dinoflagellate))

 Predominantly unicellular flagellates; approximately half of the species are heterotrophic rather than photosynthetic; photosynthetic forms with chlorophyll a, one or more chlorophyllide c types, and peridinin or fucoxanthin; mitochondria with tubular cristae and flagella without tubular hairs; ejectile trichocysts below surface in many members; many with cellulosic plates that form a so-called armour around cell; some bioluminescent (bioluminescence), some containing symbionts; resting (interphase) nucleus contains permanently condensed chromosomes; several produce toxins that either kill fish or accumulate in shellfish and cause sickness or death in humans when ingested; more than 1,200 species described, most in the class Dinophyceae; Alexandrium, Dinophysis, Gonyaulax, Peridinium, and Polykrikos.Division Euglenophyta

 Primarily unicellular flagellates; both photosynthetic and heterotrophic.

      Class Euglenophyceae

 Chlorophylls a and b; paramylon stored outside chloroplasts; mitochondria with paddle-shaped cristae; flagella lack tubular hairs, but some with hairlike scales; pellicle covering of sliding sheets allows cells to change shape; approximately 1,000 described species; Colacium, Euglena, and Eutreptiella.

Division Rhodophyta ( red algae or red seaweeds)

 Predominantly filamentous; mostly photosynthetic, a few parasitic; photosynthetic species with chlorophyll a; chlorophyll d present in some species; phycobiliproteins (phycocyanin and phycoerythrin) in discrete structures (phycobilisomes); starch stored outside chloroplast; mitochondria with flattened cristae; flagella completely absent; coralline red algae contribute to coral reefs and coral sands; predominantly marine; approximately 4,100 described species; Bangia, Palmaria, Polysiphonia, Porphyra (laver), and Rhodymenia. Robert A. Andersen Ralph A. Lewin

Additional Reading

Ecology and phycology

Works that provide an introduction to algae include F.E. Round, The Ecology of Algae (1981); Christopher S. Lobban and Paul J. Harrison, Seaweed Ecology and Physiology (1994); Robert Edward Lee, Phycology, 3rd ed. (1999); Linda E. Graham and Lee Warren Wilcox, Algae (2000); Laura Barsanti and Paolo Gualtieri, Algae: Anatomy, Biochemistry, and Biotechnology (2006); and Colin S. Reynolds, The Ecology of Phytoplankton (2006).

Biology

Various groups of algae are studied in greater detail in J.C. Green, B.S.C. Leadbeater, and W.L. Diver (eds.), The Chromophyte Algae: Problems and Perspectives (1989); Carmelo R. Tomas and Grethe R. Hasle (eds.), Identifying Marine Phytoplankton (1997); F.E. Round, R.M. Crawford, and D.G. Mann, The Diatoms: Biology and Morphology of the Genera (1990); Hilda Canter-Lund and John W.G. Lund, Freshwater Algae: Their Microscopic World Explored (1995); B.S.C. Leadbeater and J.C. Green (eds.), The Flagellates: Unity, Diversity, and Evolution (2000); John D. Wehr and Robert G. Sheath (eds.), Freshwater Algae of North America: Ecology and Classification (2003); Terumitsu Hori, An Illustrated Atlas of the Life History of Algae, 3 vol. (1994); Edna Granéli and Jefferson T. Turner (eds.), Ecology of Harmful Algae (2006); and Joseph Seckbach (ed.), Algae and Cyanobacteria in Extreme Environments (2007).

Genetics and evolution

Analyses of the genetics and evolution of algae are found in Ralph A. Lewin (ed.), The Genetics of Algae (1976); Helen Tappan, The Paleobiology of Plant Protists (1980); Juliet Brodie and Jane Lewis (eds.), Unravelling the Algae: The Past, Present, and Future of Algae Systematics (2007); and Feng Chen and Yue Jiang (eds.), Algae and Their Biotechnological Potential (2001).Robert A. Andersen Ralph A. Lewin

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