ALGAE

singular alga members of a group of predominantly aquatic, photosynthetic organisms of the kingdom Protista. They range in size from the tiny flagellate Micromonas that is 1 micrometre (0.00004 inch) in diameter to giant 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 an original source of petroleum products, and they provide foods and industrial products for humans. The algae have many types of life cycles, from simple to complex. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. Some algae are ancient, while other groups have evolved more recently. The taxonomy of algae is changing rapidly because so much new information is being discovered. The study of algae is termed phycology, and one who studies algae is known as a phycologist. The algae as treated in this article do not include the prokaryotic (nucleus-lacking) blue-green algae (cyanobacteria) and prochlorophyte algae. Beginning in the 1970s, some scientists suggested that the study of the prokaryotic algae should be incorporated into the study of bacteria because of shared cellular features. Other scientists consider the oxygen-producing photosynthetic capability of blue-green and prochlorophyte algae to be as significant as cell structure, and they continue to classify them as algae. In this article the algae are defined as eukaryotic (nucleus-bearing) organisms that photosynthesize but lack the specialized reproductive structures of plants. Plants always have multicellular reproductive structures where the fertile, gamete-producing cells are surrounded by sterile cells; this never occurs in algae. Algae lack true roots, stems, and leaves, but they share this feature with the plant division Bryophyta (e.g., mosses and liverworts). Beginning in the 1830s, algae were classified into major groups based on colour. The red, brown, and green seaweeds are well known to those who have walked along a rocky seashore. The colours are a reflection of the chloroplast pigment moleculesi.e., the chlorophylls, carotenoids, and phycobiliproteins. Many more than three groups are now recognized, and each class of algae shares a common set of pigment types that is distinct from those of all other groups. The algae are not closely related in an evolutionary sense. Specific groups of algae share enough features with protozoa and fungi that it is difficult to distinguish them from certain protozoa or fungi without using the presence of chloroplasts and photosynthesis as delimiting features. Thus, some algae have a closer evolutionary relationship with the protozoa or fungi than they do with other algae, and the converse is also truesome protozoa or fungi are more closely related to algae than to other protozoa or fungi. In fact, if the algae are united into one evolutionary group with a common ancestor, then that evolutionary group will include animals, fungi, plants, and protozoa as well. Knowledge and use of algae are perhaps as old as mankind (Homo sapiens), and possibly earlier species such as H. erectus also knew of, and used, algae. Seaweeds are still eaten by coastal societies, and algae are considered acceptable foods in many restaurants. Anyone who has slipped on a slimy rock (covered with diatoms) while crossing a stream has had firsthand experience with algae. Others know algae as green sheens on pools and ponds. The algae are the base of the food chain for all marine organisms since most plants do not live in the oceans. Because the oceans occupy about 71 percent of the Earth's surface area, the role of algae in supporting aquatic life is essential. This article discusses the algae in general terms. 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. Additional reading General works include E. Yale Dawson, Marine Botany (1966); John McNeill Sieburth, Sea Microbes (1979); F.E. Round, The Ecology of Algae (1981); G. Robin South and Alan Whittick, Introduction to Phycology (1987); and Robert Edward Lee, Phycology, 2nd ed. (1989). Various groups 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); Jrgen Kristiansen and Robert A. Andersen (eds.), Chrysophytes: Aspects and Problems (1986); Jrgen Kristiansen, G. Cronberg, and U. Geissler (eds.), Chrysophytes: Developments and Perspectives (1989); Alan J. Brook, The Biology of Desmids (1981); Dietrich Werner (ed.), The Biology of Diatoms (1977); F.J.R. Taylor (ed.), The Biology of Dinoflagellates (1987); Donald M. Anderson, Alan W. White, and Daniel G. Baden (eds.), Toxic Dinoflagellates (1985); Elenor R. Cox (ed.), Phytoflagellates (1980); Christopher S. Lobban and Michael J. Wynne (eds.), The Biology of Seaweeds (1981); and Gilbert M. Smith, The Fresh-Water Algae of the United States, 2nd ed. (1950). The effects of algae on the environment are discussed in Daniel F. Jackson (ed.), Algae and Man (1964); M.B. Saffo, New Light on Seaweeds, BioScience 37(9):654664 (October 1987); and Arthur C. Mathieson, Seaweed Cultivation: A Review, pp. 2566 in C.J. Sindermann (ed.), Proceedings of the Sixth U.S.-Japan Meeting on Aquaculture (1982).The morphology and physiology of algae are examined in Harold C. Bold and Michael J. Wynne, Introduction to the Algae: Structure and Reproduction, 2nd ed. (1985); Christopher S. Lobban, Paul J. Harrison, and Mary Jo Duncan, The Physiological Ecology of Seaweeds (1985); W.D.P. Stewart (ed.), Algal Physiology and Biochemistry (1974); Jeremy D. Pickett-Heaps, Green Algae: Structure, Reproduction, and Evolution in Selected Genera (1975); Craig D. Sandgren (ed.), Growth and Reproductive Strategies of Freshwater Phytoplankton (1988); Ralph A. Lewin (ed.), The Genetics of Algae (1976); Greta A. Fryxell (ed.), Survival Strategies of the Algae (1983); Annette W. Coleman, Lynda J. Goff, and Janet R. Stein-Taylor (eds.), Algae as Experimental Systems (1989); Barry S.C. Leadbeater and Robert Riding (eds.), Biomineralization in Lower Plants and Animals (1986); E.G. Pringsheim, Pure Cultures of Algae: Their Preparation & Maintenance (1946, reprinted 1972); and Tracy L. Simpson and Benjamin E. Volcani (eds.), Silicon and Siliceous Structures in Biological Systems (1981). Analyses of algal evolution are found in Mark A. Ragan and David J. Chapman, A Biochemical Phylogeny of the Protists (1978); and Helen Tappan, The Paleobiology of Plant Protists (1980). For classification, see D.E.G. Irvine and D.M. John (eds.), Systematics of the Green Algae (1984). Robert A. Andersen Classification Diagnostic features The classification of algae into taxonomic groups is based upon the same rules as for the classification of plants. The organization of groups above the order level has changed substantially since 1960. Research using electron microscopes has demonstrated new and important features of the flagellar apparatus, cell division process, and organelle structure and function. Similarities and differences among algal, fungal, and protozoan groups have led scientists to propose taxonomic changes, and these changes are continuing. Molecular studies, especially comparative gene sequencing studies, have supported some of the changes that followed electron microscopic studies, but they have suggested additional change as well. Since 1960 the number of classes has nearly doubled and algal classification has changed constantly. Furthermore, the apparent evolutionary scattering of algal groups among protozoan and fungal groups implies that a natural classification of algae by themselves is impossible. 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 argue 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 plants, and the chromophyte algae should be grouped with the aquatic fungi and certain protozoa. The Euglenophyceae are most closely related to the trypanosome flagellates, the protozoa that cause sleeping sickness. It is unclear where the red algae or cryptomonads belong. In summary, the algae are not all closely related, and they do not form a single evolutionary lineage devoid of other organisms. Division-level classification, like kingdom-level classification, is tenuous for algae. For example, some phycologists place the classes Bacillariophyceae, Phaeophyceae, and Xanthophyceae in the division Chromophyta, while others place each class in separate divisions: Bacillariophyta, Phaeophyta, Xanthophyta. Yet, almost all phycologists agree on the definition of the 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 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 (scales, angle of insertion, microtubular roots, striated roots) of flagellate cells, the nuclear division process (mitosis), the cytoplasmic division process (cytokinesis), and the cell covering. Many scientists combine the Micromonadophyceae with the Pleurastrophyceae and name the group the Prasinophyceae. Because classes are better defined and more accepted than divisions, taxonomic discussions of algae are usually conducted at the class level. The divisions provided below are commonly used, but they 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 continues to be debated, and a standard outline of the kingdom has not been established. The differences between the classification presented below and that in the article protist (see protist: Annotated classification) reflect the taxonomic variations that arise from individual interpretations. Evolution and paleontology The evolutionary relationships of algae are not well understood. Modern ultrastructural and molecular studies have added so much new and important information that the evolution of algae is being reassessed. The poor fossil record for some groups of algae also hinders evolutionary studies. Finally, the realization that some algae are more closely related to protozoa or fungi 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 on an ancient lineage that includes some zooflagellate protozoa, which is supported by ultrastructural and molecular data. Most scientists consider the colourless euglenophytes to be an older group and believe that the chloroplasts were added more recently. Some scientists consider the red algae to be very primitive eukaryotes that evolved from the prokaryotic blue-green algae. Evidence in support of this view are the nearly identical photosynthetic pigments and the very similar starches. 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. It is difficult to imagine, however, the evolutionary selection of a nonflagellate stage for an aquatic organism. Still other scientists suggest that the red algae evolved from the fungi by obtaining a chloroplast. Evidence in support of this view are similarities in mitosis and in cell wall plugs, special structures inserted in a cell wall hole that interconnects two cells. Some evidence suggests that the plug regulates movement between the two cells. Ribosomal gene sequence data from studies in molecular biology suggest that the red algae arose suddenly along with the animal, fungal, and plant (as green algae) lineages. Whatever the origin of the red algae, they bear little resemblance to any other living group. The green algal classes are evolutionarily related, but their origin is unclear. Most consider the classes Micromonadophyceae to be the most ancient group, and fossil data support this view. The class Ulvophyceae is also ancient, whereas the classes Charophyceae and Chlorophyceae are more recent. The class Dinophyceae is also of uncertain origin. During the 1960s and '70s the unusual structure and chemical composition of the nuclear DNA was interpreted as a very primitive feature. Some scientists considered the Dinophyceae to be mesokaryotes (an intermediate between the prokaryotes and the eukaryotes). That view is no longer accepted by most scientists, and the peculiar structure is considered simply an evolutionary divergence. Some scientists consider the Dinophyceae to be distantly related to the chromophyte algae. Ribosomal gene sequence data suggest that their closest living relatives are the ciliates, a large, complex group of protozoa. As in the case of the other algae, the origin of the chromophyte algae is unknown. Ultrastructural and molecular data suggest that they are on a protistan lineage which diverged a long time ago. That lineage, however, apparently remained one of protozoa and later aquatic fungi until about 300 to 400 million years ago. At that time a chloroplast was added (originally as a symbiont), and since then the many chromophyte groups have been evolving. Fossil, ultrastructural, and ribosomal gene sequence data support this hypothesis. The Cryptophyceae are truly an enigma. They have no fossil record, and other 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 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. Much like the platypus, the cryptophytes appear as though they were constructed by an administrative committee. The fossil record for the algae is not nearly as complete as it is for plants and animals. Red algal fossils are the oldest known algal fossils. Microscopic spherical algae (Eosphaera and Huroniospora) resembling the living genus Porphyridium are known from the Gunflint Iron Formation of North America (1.9 billion years ago). Fossils that resemble modern tetraspores are known from the Amelia Dolomites of Australia (1.5 billion years ago). The best fossils are the coralline red algae that are represented in fossil beds since the Precambrian era. 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 produce up to 150 gallons of oil per ton of sediment. The Ulvophyceae fossils date from about 1 billion years ago and are abundant in Paleozoic rocks. Some deposit calcium carbonate along their cell walls, and these algae produced some limestone formations. The Charophyceae, as represented by the large stoneworts (order Charales), date from about 400 million years ago. The oospore, the fertilized female egg, has spirals on its surface, a result of pressing against the spiraling protective cells that surround the oospore. Oospores before about 225 million years ago had right-handed spirals, while 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 most scientists do not agree with that view. The acritarchs occurred as early as 700 million years ago. The Chromophyta have the shortest fossil history of the major algal groups. Some believe that the group is ancient, but there are no fossils. 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, a putative brown alga, are approximately 400 million years old. Coccolithophores, coccolith-bearing members of the Prymnesiophyceae, date from the late Triassic epoch (230 to 208 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 the White Cliffs of Dover. Most species became extinct at the end of the Cretaceous period (66.4 million years ago), along with the dinosaurs, and 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 following the mass extinctions 66.4 million years ago, these algae flourished. Their siliceous remains form deposits of diatomite almost 0.5 kilometre (0.3 mile) in depth. The enormous deposits and the siliceous nature of the fossils strongly suggest that these organisms evolved very recently. In fact, mammals and birds evolved before these algae. The Xanthophyceae may be even more recent, with fossils dating from about 20 million years ago. The remaining groups of algae, especially the Euglenophyceae and the Cryptophyceae, lack a good fossil record. These groups are without hard parts, which may explain the lack of fossils. Form and function The algal cell Eukaryotic algal cells contain three types of double-membrane-bound organelles: the nucleus, the chloroplast, and the mitochondrion. The nucleus contains most of the genetic material of the cell, and the DNA (deoxyribonucleic acid) molecules exist as linear strands. The DNA is condensed into obvious chromosomes only at the time of nuclear division (mitosis) in most algae; however, the nuclear DNA of the classes Dinophyceae and Euglenophyceae is always condensed. The two membranes surrounding the nucleus are referred to as the nuclear envelope, which typically has specialized nuclear pores that regulate the movement of molecules into and out of the nucleus. The chloroplast is the site of photosynthesis, the complex set of biochemical reactions that convert light energy, carbon dioxide, and water into sugars. The chloroplast contains flattened, membranous sacs, called thylakoids, that contain the photosynthetic light-harvesting pigments, the chlorophylls, carotenoids, or phycobiliproteins (see below Photosynthesis). The mitochondrion is the site 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). 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. Plants and animals, by comparison, have only flattened cristae. Chloroplasts and mitochondria also have their own DNA. This DNA is not like nuclear DNA, however, because it is circular rather than linear, and therefore it resembles the DNA of prokaryotes. The similarity of chloroplastic and mitochondrial DNA to prokaryotic DNA has led many scientists to accept the hypothesis that these organelles resulted from a long and successful symbiosis of prokaryote cells inside eukaryote host cells. These symbioses have been used in defining the endosymbiosis hypothesis, which states that eukaryotic cells are formed in part by incorporating prokaryotes as specialized organelles. Algae have several single-membrane-bound organelles, including the endoplasmic reticulum, Golgi apparatus, lysosome, peroxisome, vacuole, contractile vacuole, and ejectile organelles. The endoplasmic recticulum 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 is a series of membranous sacs that are stacked like pancakes. The Golgi apparatus performs four distinct functions: it sorts many molecules synthesized elsewhere in the cell; it produces carbohydrates like cellulose or sugars, and sometimes it 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 used to break down old organelles, cells or cellular components during certain developmental stages, and particulate matter that is ingested by species that engulf food. Peroxisomes specialize in metabolically breaking down certain organic molecules and in destroying dangerous peroxide compounds, such as hydrogen peroxide, that are produced during some biochemical reactions. Vacuoles are membranous sacs that store many different substances depending upon the organism and its metabolic state. The contractile vacuole is not involved in long-term storage; rather, it is a highly specialized organelle that regulates the water content of cells. When too much water enters the cells, the contractile vacuole squirts it out. Some algae have special ejectile organelles that apparently act as protective structures. The Dinophyceae has trichocystsharpoonlike structures that lie beneath the cell surface and explode from the disturbed or irritated cell. Ejectosomes, of analogous structure, are found in the class Cryptophyceae. Several classes of algae in the division Chromophyta have mucous organelles. Gonyostomum semen, a freshwater member of the class Raphidophyceae, has numerous mucocysts, and when it is collected in a plankton net the mucocysts discharge, giving the net and sample a mucous consistency. The nonmembrane-bound organelles include the ribosomes, pyrenoids, microtubules, and microfilaments. The ribosome serves as the workbench during protein synthesis. It provides the site where genetic information, as messenger RNA, is translated into proteins. The ribosome carefully interprets the genetic code of the DNA so that the protein is made exactly to the genetic specifications. The pyrenoid, a dense structure that occurs within or beside chloroplasts of algae, has a concentration of ribulose biphosphate carboxylase, the enzyme necessary in photosynthesis for carbon fixation and thus sugar formation (see below Photosynthesis). Starch, the storage form of sugar, is often found around pyrenoids. The microtubules are tubelike structures formed from tubulin proteins. Some microtubules are almost always present in the cell, but others appear suddenly when needed and then disassemble after use. Microtubules provide a rigid structure, or cytoskeleton, in the cell that helps determine the shape of the cell, and in species without cell walls the cytoskeleton maintains the cell shape. Microtubules also provide a rail system along which vesicles are transported. The spindle apparatus, which separates the chromosomes when the nucleus divides, consists of microtubules. Finally, microtubules also form the basic structure, or axoneme, of the flagellum, and they are a major component of the flagellar root system that anchors the flagellum in the cell. Microfilaments are formed by the polymerization of proteins such as actin. Actin microfilaments contract and relax, and they function as tiny muscles inside the cell. Flagella A flagellum, when present, is structurally complex and contains 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 whole cylinder is surrounded by a membrane. Each of the nine pairs of microtubules has an a tubule and a b tubule. The a tubule has numerous molecules of the protein dynein attached along its length. Dynein is involved in converting the chemical energy of adenosine triphosphate (ATP) into the mechanical energy that permits flagellar movement. The scales and hairs apparently aid in swimming. The swellings and para-axonemal structures (crystalline rods and noncrystalline rods and sheets) are often involved in photoreception, providing the swimming cell with a means for detecting light, toward or from which it may swim. The flagellum bends as the dynein arms on one side of the axoneme move up the microtubules during the power stroke. These dynein molecules are then inactivated, and those on the opposite side slide up, causing the flagellum to bend in the opposite direction during the recovery stroke. The result is the whiplike movement characteristic of eukaryotic flagella. The flagellum membrane is also complex. It contains special chemoreceptors that aid the algal cell in recognizing cues ranging from environmental changes to mating partners. Often, scales, hairs, swellings, and para-axonemal structures cover the flagellum surface. The scales and hairs apparently aid in swimming. The swellings and para-axonemal structures (crystalline rods and noncrystalline rods and sheets) are often involved in photoreception, providing the swimming cell with a means for detecting light, toward or from which it may swim. The flagellum membrane flows into the plasma (cell) membrane, where the nine pairs of axonemal microtubules enter the main body of the cell. Each pair is joined by an additional microtubule, forming nine triplets. The cylinder of nine triplets, called the basal body, anchors the flagellum. Musclelike fibres and special microtubules (called microtubular roots) extend from the basal body and provide a greater anchorage base. Most flagellate cells have two flagella, and therefore two basal bodies. Typically, each basal body gives rise to two sets of microtubular roots. The orientation of the flagella and the arrangement of the microtubular roots and musclelike fibres are used to classify algae.