Algae

Algae (singular: alga) are photosynthetic, eukaryotic organisms that do not develop multicellular sex organs. Algae can be unicellular, or they may be large, multicellular organisms. Algae can occur in salt or fresh waters, or on the surfaces of moist soil or rocks . The multicellular algae develop specialized tissues, but they lack the true stems, leaves, or roots of the more complex, higher plants.

The algae are not a uniform group of organisms. They actually consist of seven divisions of distantly related organisms. These are considered together more as a matter of human convenience, than as a reflection of their ordered, biological, or evolutionary relationships. Therefore, the term "algae" is a common one, rather than a word that connotes a specific, scientific meaning.

Algae and their characteristics

As considered here, all of the algae are eukaryotic organisms, meaning their cells have nuclear material of deoxyribonucleic acid (DNA) organized within a discrete, membrane-bounded organelle, known as the nucleus. In view of this definition, the so-called blue-green algae are not discussed in this article, because those organisms are prokaryotic (that is, without an organized nucleus) and are more appropriately referred to as blue-green bacteria , or as cyanobacteria. The cyanobacteria are also different from the true algae in that they do not contain the photosynthetic pigment known as chlorophyll a, they do not have cell walls made of cellulose , and they do not store energy as starch or related polysaccharides.

Virtually all species of algae are photosynthetic. They have a relatively simple anatomy , which can range in complexity from single-celled organisms to colonial filaments, plates, or spheres, to the large, multicellular structures of the brown algae, known as thalli. Algal cell walls are generally made of cellulose, but can contain pectin, a class of hemicellulose polysaccharides that give the algae a slimy feel. The larger, multicellular algae have relatively complex tissues, which can be organized into organ-like structures that serve particular functions.

Types of algae

The actual term "algae" is not very useful in formal biology , because a number of disparate groups of unrelated organisms are aggregated under this broad term. The seven divisions of organisms that are considered within the algae are the Euglenophyta, Chrysophyta, Pyrrophyta, Chlorophyta, Rhodophyta, Paeophyta, and Xanthophyta. These divisions are separated on the basis of various features including their morphology and the biochemistry of their pigments, cell walls, and energy-storage compounds. The colors of these various algae types differ according to their particular mixtures of photosynthetic pigments, which typically include a combination of one or more chlorophylls and various accessory pigments. The latter can include carotenoids, xanthophylls, and phycobilins (these mask the green of the primary chlorophylls in various ways). The major differences among the seven divisions of algae are briefly summarized below.

Euglenophyta (euglenoids)

The Euglenophyta or euglenoids are 800 species of unicellular, protozoan-like algae, most of which occur in fresh waters. The euglenoids lack a true cell wall, and are bounded by a proteinaceous cell covering known as a pellicle. Euglenophytes have one to three flagellae for locomotion, and they store carbohydrate reserves as paramylon. The primary photosynthetic pigments of euglenophytes are chlorophylls a and b, while their accessory pigments are carotenoids and xanthophylls.

Most euglenoids have chloroplasts, and are photosynthetic. Some species, however, are heterotrophic, and feed on organic material suspended in the water. Even the photosynthetic species, however, are capable of surviving for some time if kept in the dark, as long as they are "fed" with suitable organic materials.

Chrysophyta (golden-brown algae)

The Chrysophyta are the golden-brown algae and diatoms , which respectively account for 1,100 and 40,000-100,000 species of unicellular algae. These algae occur in both marine and fresh waters, although most species are marine. The cell walls of golden-brown algae and diatoms are made of cellulose and pectic materials, a type of hemicellulose. In the diatoms especially, the cell wall is heavily impregnated with silica and is therefore quite rigid and resistant to decay. These algae store energy as a carbohydrate called leucosin, and also in oil droplets. The golden-brown algae achieve locomotion using one to two flagellae. The photosynthetic pigments of these algae are chlorophylls a and c, and the accessory pigments are carotenoids and xanthophylls, including a specialized pigment known as fucoxanthin.

Communities of diatoms (class Bacillariophyceae) can be extremely diverse, with more than 500 species commonly recorded from the phytoplankton , periphyton, and surface muds of individual ponds and lakes. Diatoms have double shells, or frustules, that are largely constructed of silica (SiO2), the two halves of which (called valves) fit together like a pillbox. Diatom species are distinguished on the basis of the shape of their frustules, and the exquisite markings on the surface of these structures.

The golden-brown algae (class Chrysophyceae) are much less diverse than the diatoms. Some species of golden-brown algae lack cell walls, while others have pectin-rich walls. Golden-brown algae are especially important in open waters of the oceans, where they may dominate the productivity and biomass of the especially tiny size fractions of the phytoplankton. These are known as the nanoplankton, consisting of cells smaller than about 0.05 mm in diameter.

Pyrrophyta (fire algae)

The Pyrrophyta are the fire algae, including the dinoflagellates, which together account for 1,100 species of unicellular algae. Most of these species occur in marine ecosystems, but some are in fresh waters. The dinoflagellates have cell walls constructed of cellulose, and have two flagellae. These algae store energy as starch. The photosynthetic pigments of the Pyrrophyta are chlorophylls a and c, and the accessory pigments are carotenoids and xanthophyll, including fucoxanthin.

Some species of dinoflagellates can temporarily achieve a great abundance, as events that are commonly known as "red tides" because of the resulting color of the water. Red tides can be toxic to marine animals, because of the presence of poisonous chemicals that are synthesized by the dinoflagellates. Some species of dinoflagellates develop a bioluminescence , which can be clearly seen at night, and may cause the surface of the ocean to look as if it were aflame.

Chlorophyta (green algae)

The Chlorophyta or green algae consist of about 7,000 species, most of which occur in fresh water, although some others are marine. Most green algae are microscopic, but a few species, such as those in the genus Cladophora, are multicellular and macroscopic. The cell walls of green algae are mostly constructed of cellulose, with some incorporation of hemicellulose, and calcium carbonate in some species. The food reserves of green algae are starch, and their cells can have two or more organelles known as flagella , which are used in a whiplike fashion for locomotion. The photosynthetic pigments of green algae are chlorophylls a and b, and their accessory pigments are carotenoids and xanthophylls.

Some common examples of green algae include the unicellular genera Chlamydomonas and Chlorella, which have species dispersed in a wide range of habitats. More complex green algae include Gonium, which forms small, spherical colonies of four to 32 cells, and Volvox, which forms much larger, hollow-spherical colonies consisting of tens of thousands of cells. Some other colonial species are much larger, for example, Cladophora, a filamentous species that can be several meters long, and Codium magnum, which can be as long as 26 ft (8 m).

The stoneworts (class Charophyceae) are a very distinctive group of green algae that are sometimes treated as a separate division (the Charophyta). These algae can occur in fresh or brackish waters, and they have cell walls that contain large concentrations of calcium carbonate. Charophytes have relatively complex growth forms, with whorls of "branches" developing at their tissue nodes. Charophytes are also the only algae that develop multicellular sex organs, although these are not comparable to those of the higher plants.

Rhodophyta (red algae)

The Rhodophyta or red algae are 4,000 species of mostly marine algae, which are most diverse in tropical waters. Species of red algae range from microscopic to macroscopic in size. The larger species typically grow attached to a hard substrate, or they occur as epiphytes on other algae. The cell walls of red algae are constructed of cellulose and polysaccharides, such as agar and carrageenin. These algae lack flagellae, and they store energy as a specialized polysaccharide known as floridean starch. The photosynthetic pigments of red algae are chlorophylls a and d, and their accessory pigments are carotenoids, xanthophyll, and phycobilins.

Some examples of red algae include filamentous species such as Pleonosporum spp., so-called coralline algae such as Porolithon spp., which become heavily encrusted with calcium carbonate and contribute greatly to the building of tropical reefs, and thalloid species, such as the economically important Irish moss ( Chondrus crispus).

Paeophyta (brown algae)

The Paeophyta or brown algae number about 1,500 species, almost all of which occur in marine environments. These seaweeds are especially abundant in cool waters. Species of brown algae are macroscopic in size, including the giant kelps that can routinely achieve lengths of tens of meters. Brown algae have cell walls constructed of cellulose and polysaccharides known as alginic acids. Some brown algae have relatively complex, differentiated tissues, including a holdfast that secures the organism to its substrate, air bladders to aid with buoyancy, a supporting stalk or stipe, wide blades that provide the major surface for nutrient exchange and photosynthesis , and spore-producing, reproductive tissues. The specialized, reproductive cells of brown algae are shed into the water and are motile, using two flagella to achieve locomotion. The food reserves of these algae are carbohydrate polymers known as laminarin. Their photosynthetic pigments are chlorophylls a and c, while the accessory pigments are carotenoids and xanthophylls, including fucoxanthin, a brown-colored pigment that gives these algae their characteristic dark color.

Some examples of brown algae include the sargassum weed (Sargassum spp.), which dominates the extensive, floating ecosystem in the mid-Atlantic gyre known as the Sargasso Sea. Most brown seaweeds, however, occur on hard-bottom, coastal substrates, especially in cooler waters. Examples of these include the rockweeds (Fucus spp. and Ascophyllum spp.), the kelps (Laminaria spp.), and the giant kelps (Macrocystis spp. and Nereocystis spp.). The giant kelps are by far the largest of the algae, achieving a length as great as 328 ft (100 m).

Xanthophyta (yellow-green algae)

The Xanthophyta or yellow-green algae are 450 species that primarily occur in fresh waters. They are unicellular or small-colonial algae, with cell walls made of cellulose and pectic compounds, and sometimes containing silica. The yellow-green algae store carbohydrate as leucosin, and they can have two or more flagellae for locomotion. The primary photosynthetic pigment of yellow-green algae is chlorophyll a, and the accessory pigments are carotenoids and xanthophyll.

Ecological relationships

Many types of algae are microscopic, occurring in single cells or small colonies. The usual habitat of many of the microscopic algae is open waters, in which case they are known as phytoplankton. Many species, however, live on the surfaces of rocks and larger plants within shallow-water habitats, and these are known as periphyton. Other microscopic algae live on the moist surfaces of soil and rocks in terrestrial environments.

Microscopic algae are at the base of their ecological food web—these are the photosynthetic, primary producers that are fed upon by herbivores. In the open waters of ponds, lakes, and especially the vast oceans, the algal phytoplankton is the only means by which diffuse solar radiation can be fixed into biological compounds. In these open-water (or pelagic) habitats the phytoplankton are consumed by small, grazing animals known as zooplankton , most of which are crustaceans. The zooplankton are in turn fed upon by larger zooplankton or by small fish (these predators are known as planktivores), which may then be eaten by larger fish (or piscivores). At the top of the open-water food web may be fish-eating birds , seals , whales, very large fish such as sharks or bluefin tuna , or humans. Therefore, the possibility of all of the animals occurring higher in the food webs, including the largest of the top predators, are ultimately dependent on the productivity of the microscopic phytoplankton of the pelagic marine ecosystem.

Other algae are macroscopic, meaning they can be readily observed without the aid of magnification. Some of these algae are enormous, with some species of kelps commonly reaching lengths greater than tens of meters long. Because they are primary producers, these macroscopic algae are also at the base of their ecological food webs. In most cases, however, relatively few herbivores can directly consume the biomass of macroscopic algae, and the major trophic interaction of these plants is through the decomposer, or detritivore part of the food web. In addition, because of their large size, macroscopic algae are critically important components of the physical structure of their ecosystems, providing habitat for a wide range of other organisms. The largest kelps develop a type of ecosystem that is appropriately referred to as a marine "forest" because of the scale and complexity of its habitat structure.

Some species of green algae occur as mutualistic symbionts with fungi , in an association of two organisms known as lichens . Lichens are common in many types of habitats. Other green algae occur in a mutualism with certain animals. In general, the host animal benefits from access to the photosynthetic products of the green alga, while the alga benefits from protection and access to inorganic nutrients . For example, species of unicellular Chlorella live inside of vacuoles within host cells of various species of freshwater protozoans, sponges , and hydra . Another species of green alga, Platymonas convolutae, occurs in cells of a marine flatworm, Convoluta roscoffensis. Various other green algae occur inside of marine mollusks known as nudibranchs. Similarly, various species of dinoflagellates occur as symbionts with marine corals.

Each species within an algal community has its particular ecological requirements and tolerances. Consequently, algal species tend to segregate along gradients in time and space, according to varying patterns of environmental resources, and of biological interactions, such as competition and predation. For example, during the growing season there is a time-series of varying abundances of phytoplankton species in open-water habitat. At certain times, particular species or closely related groups of species are abundant, but then these decline and other species of phytoplankton become dominant. This temporal dynamic is not totally predictable; it may vary significantly from year to year. The reasons for these patterns in the abundances and productivity of algal species are not understood, but they are likely associated with differences in their requirements for nutrients and other environmental factors, and perhaps with differing competitive abilities under resource-constrained conditions.

In a similar way, species of seaweeds tend to sort themselves out along stress-related environmental gradients associated with varying distances above and below the high-tide mark on rocky marine shores. The most important environmental stress for intertidal organisms is desiccation (drying), caused by exposure to the atmosphere at low tide, with the intensity of drying being related to the amount of time that is spent out of the water, and therefore to the distance above the high-tide line. For sub-tidal seaweeds the most important stress is the physical forces associated with waves, especially during storms. The various species of brown and red algae are arranged in rather predictable zonations along transects perpendicular to rocky shores. The largest kelps only occur in the sub-tidal habitats, because they are intolerant of desiccation. Within this near-shore habitat the species of algae are arranged in zones on the basis of their tolerance to the mechanical forces of wave action, as well as their competitive abilities in the least stressful, deeper-water habitats somewhat farther out to sea, where the tallest species grow and develop a kelp forest. In the intertidal, the various species of wracks and rockweeds dominate particular zones at various distances from the low-tide mark, with the most desiccation-tolerant species occurring closest to the high-tide mark. Competition, however, also plays an important role in the distributions of the intertidal seaweeds.

Factors limiting the productivity of algae

Some species of algae can occur in extreme environments. For example, species of green algae have been observed in hot-water springs at Yellowstone National Park, in highly acidic volcanic lakes, in the extremely saline Great Salt Lake and Dead Sea, and on the surfaces of glaciers and snow. Some algae even survive suspended in the atmosphere as spores or in droplets of moisture.

These are, however, extremely stressful environmental conditions. Most algae occur in less stressful habitats, where their productivity tends to be limited by the availability of nutrients (assuming that sufficient light is available to support the photosynthetic process). In general, the productivity of freshwater algae is primarily limited by the availability of the nutrient phosphate (PO4-3), while that of marine algae is limited by nitrate (NO3-) or ammonium (NH4+). Some algal species, however, may have unusual nutrient requirements, and their productivity may be limited by certain micronutrients, such as silica, in the case of diatoms.

The structure of algal communities may also be greatly influenced by ecological interactions, such as competition and herbivory. For example, when herbivorous sea urchins are abundant, they can sometimes over-graze species of kelps in subtidal ecosystems of the west coast of North America , degrading the kelp forests . However, where sea otters (Enhydra lutris) are abundant this does not happen because the otters are effective predators of the urchins.

Another example of a biological influence on the structure of an algal community concerns the zebra mussel (Dreissena polymorpha). This is a bivalve mollusk that has been accidentally introduced by ocean-going ships to the Great Lakes of North America, where it has become an important pest because it clogs water pipes with its prolific growths, and can displace native species by competitively appropriating hard-substrate habitats. More to the present point, however, the zebra mussel is such an effective filter-feeder on phytoplankton, that its large populations in parts of the Great Lakes are apparently responsible for some of the clarification of the water that has occurred in recent years. Grazing by the zebra mussel has actually resulted in decreased standing crops of phytoplankton, even in well-fertilized waters.

Economic products obtained from algae

The most important economic products obtained from algae are associated with brown and red seaweeds, which can be utilized as food for people, and as resources for the manufacturing of industrial products. These seaweeds are mostly harvested from the wild, although increasing attention is being paid to the cultivation of large algae.

Some species of algae can be directly eaten by humans, and in eastern Asia they can be especially popular, with various species used as foods. An especially common food is the red alga known as nori in Japan and as laver in western Europe (Porphyra spp.), which has long been eaten by coastal peoples of China and Japan. This alga is often used as a wrapper for other foods, such as rice or plums, or it may be cooked into a clear soup. Nori has been cultivated for centuries in eastern Asia. Another alga known as dulse or sea kale (Rhodymenia palmata) is consumed dried or cooked into various stews or soups. Other commonly eaten seaweeds include the sea lettuce (Ulva lactuca), and murlins or edible kelp (Alaria esculenta).

Potentially, seaweeds are quite nutritious foods, because about 50% of their weight occurs as carbohydrates, with smaller concentrations of proteins and fats, and diverse micronutrients, including iodine. In practice, however, seaweeds are not very nutritious foods for humans, because we do not have the enzymes necessary to metabolize the most abundant of the complex, algal carbohydrates.

In some places, coastal livestock such as sheep and cattle, and wild ungulates such as deer , will graze algal biomass from the intertidal zone at low tide. These animals can take better advantage of the algal carbohydrates than can humans, largely because of the digestive abilities of the symbiotic microorganisms in their rumens and guts. Sometimes, algal biomass is harvested by humans and added to the fodder of livestock as a source of micronutrients.

The major economic importance of brown seaweeds, however, is as a natural resource for the manufacturing of a class of industrial chemicals known as alginates. These chemicals are extracted from algal biomass, and are used as thickening agents and as stabilizers for emulsions in the industrial preparation of foods and pharmaceuticals, and for other purposes.

Agar is another seaweed product, prepared from the mucilaginous components of the cell walls of certain red algae. Agar is used in the manufacturing of pharmaceuticals and cosmetics, as a culture medium for laboratory microorganisms, and for other purposes, including the preparation of jellied desserts and soups. Carrageenin is another, agar-like compound obtained from red algae that is widely used to stabilize emulsions in paints, pharmaceuticals, ice cream, and other products. Irish moss (Chondrus crispus) is a purplish alga that is a major source of carrageenin.

Researchers are investigating methods for the economic cultivation of red and brown seaweeds for the production of alginates, agar, and carrageenin. In California, use has been made of rafts anchored to float about 13 yd (12 m) below the surface, in shallow, less than 328-ft (100-m) deep water, to grow the highly productive, giant kelp Macrocystis as an industrial feedstock. Seaweeds are also cultivated on floating devices in coastal China, and research is investigating whether growth rates in dense plantings can be economically increased by enriching the seawater with nitrogen-containing fertilizers .

In some places, large quantities of the biomass of brown and red algae wash ashore, especially after severe storms that detach these algae from their substrates. This material, known as wrack, is an excellent substrate for composting into an organic-rich material that can greatly improve soil qualities in terms of aeration and water- and nutrient-holding capacity.

Over extremely long periods of time, the frustules of diatoms can accumulate in large quantities. This material is known as diatomaceous earth, and its small reserves are mined for use as a fine polishing substrate, as a fine filtering material, and for other industrial purposes.

Algae as environmental problems

Red tides are events of great abundance (or "blooms") of red, brown, or yellow-colored dinoflagellates of various species. These algae synthesize biochemicals, such as saxitoxin and domoic acid, which are extremely poisonous and can kill a wide range of marine animals, as well as humans who eat shellfish containing the toxins. The toxic syndromes of humans associated with dinoflagellate toxins are known as paralytic, diarrhetic, and amnesic shellfish poisoning.

Scientists cannot yet predict the environmental conditions that cause red tides to develop, although it seems that they are related to the availability and ratio of nutrients to temperature . Red tides are natural phenomena, but some scientists believe that human interference may have increased the frequency of these phenomena in some regions.

The dinoflagellates involved with toxic dinoflagellate blooms are commonly of the genera Alexandrium, Dinophysis, Nitzchia, or Ptychodiscus. The algal toxins can be accumulated by filter-feeding shellfish such as clams and oysters. If these are eaten while they contain red-tide toxins, they can poison humans or animals in the local ecosystem. Even creatures the size of large whales can die from eating fish containing large concentrations of dinoflagellate toxins.

In addition, freshwater algae can cause problems when they are overly abundant. Algal blooms can cause foul tastes in the water stored in reservoirs, which may be required by nearby towns or cities as drinking water. This can be a significant problem in naturally productive, shallow-water lakes of the prairies in North America.

Eutrophication is another major problem that is associated with algal blooms in lakes that are receiving large inputs of nutrients through sewage disposal or the runoff of agricultural fertilizers. Eutrophication can result in severe degradation of the aquatic ecosystem when large quantities of algal biomass sink to deeper waters and consume most of the oxygen during their decomposition . The anoxic (deficiency in oxygen) conditions that develop are lethal to the animals that live in the sediment and deep waters, including most species of fish. Because the primary limiting nutrient in fresh waters is usually phosphate, inputs of this nutrient can be specifically controlled by sewage treatment , and by the banning of detergents containing phosphorus . This has been done in many areas in North America, and eutrophication is now less an environmental problem than it used to be.

See also Biological community; Eukaryotae; Food chain/web; Symbiosis.

Resources

books

Buchanan, B.B., W. Gruissem, and R.L. Jones. Biochemistry and Molecular Biology of Plants. Rockville, MD: American Society of Plant Physiologists, 2000.

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Pritchard, H.N. and P.T. Bradt. Biology of Nonvascular Plants. St. Louis: Mosby, Inc., 1984.

Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology ofPlants. 6th ed. New York: Worth Publishers Inc., 1998.

Bill Freedman

KEY TERMS

Accessory pigments

—Pigments such as the carotenoids, xanthophylls, and phycobilins, which can absorb solar radiation, and pass some of the absorbed energy to chlorophyll pigments for use in photosynthesis.

Bloom

—An event of great abundance of phytoplankton, to the degree that the water is distinctly colored by the algal pigments.

Epiphyte

—A plant which relies upon another plant, such as a tree, for physical support, but does not harm the host plant.

Eukaryotic cell

—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Gyre

—A zone of spirally circulating oceanic water, that tends to retain floating materials, as in the Sargasso Sea of the Atlantic Ocean.

Macroscopic

—A size range that can be seen without magnification.

Microscopic

—A size range that cannot be seen without magnification.

Mutualism

—A symbiotic relationship between two species that is mutually beneficial.

Periphyton

—Unicellular algae that occur on the surfaces of the rocks and larger plants of aquatic ecosystems.

Phytoplankton

—Microscopic algae that occur suspended in the water column.

Primary photosynthetic pigment

—This refers to the green-colored, chlorophyll pigments of algae and higher plants. These absorb red and blue light, to make the energy available to drive photosynthesis.

Thallus

—A single plant body lacking distinct stem, leaves, and roots.

Zooplankton

—Minute animal life that lives in water.

Algae

Algal characteristics

Types of algae

Ecological relationships

Factors limiting the productivity of algae

Economic significance of algae

Algae as environmental problems

Resources

Algae (singular: alga) are photosynthetic, eukaryotic organisms that do not develop multicellular sex organs. Algae can be single-celled (unicellular), or they may be large and comprised of many cells. Algae can occur in salt or fresh waters, or on the surfaces of moist soil or rocks. The multicellular algae develop specialized tissues, but they lack the true stems, leaves, or roots of the more complex, higher plants.

The algae are not a uniform group of organisms, but rather consist of a number of divisions of distantly related phtosynthetic organisms. This grouping is thus not a reflection of their biological or evolutionary relatedness.

Algal characteristics

Algae are eukaryotic organisms, meaning their cells have nuclear material of deoxyribonucleic acid

(DNA) organized within a discrete, membrane-bounded organelle known as the nucleus.

Virtually all species of algae are photosynthetic. They have a relatively simple anatomy, which can range in complexity from single-celled organisms to colonial filaments, plates, or spheres, to the large, multicellular structures of the brown algae (thalli). Algal cell walls are generally made of cellulose, but can contain pectin (a class of hemicellulose polysaccharides) that give the algae a slimy feel. The larger, multicellular algae have relatively complex tissues, which can be organized into organ-like structures that serve particular functions.

Types of algae

The algal divisions are Euglenophyta, Chrysophyta, Pyrrophyta, Chlorophyta, Rhodophyta, Paeophyta, and Xanthophyta. These divisions are separated on the basis of various features including shape (morphology) and the biochemistry of their pigments, cell walls, and energy-storage compounds. The colors of these various algae types differ according to their particular mixtures of photosynthetic pigments, which typically include a combination of one or more chlorophylls and various accessory pigments. The latter can include carotenoids, xanthophylls, and phycobilins (these mask the green of the primary chlorophylls in various ways).

The Euglenophyta or euglenoids comprise approximately 800 species of unicellular, protozoan-like algae. Most live in fresh water. The euglenoids lack a true cell wall, and are bounded by a proteinaceous cell covering known as a pellicle. Euglenophytes have one to three flagellae for locomotion, and they store carbohydrate reserves as paramylon. The primary photosynthetic pigments of euglenophytes are chlorophylls a and b, while their accessory pigments are carotenoids and xanthophylls.

Most euglenoids have chloroplasts, and are photosynthetic. Some species, however, are heterotrophic and feed on organic material suspended in the water. Even the photosynthetic species, however, are capable of surviving for some time if kept in the dark, as long as they are “fed” with suitable organic materials.

Chrysophyta, or unicellular algae, are comprised of over 1, 000 species of golden-brown algae and upwards of 100, 000 known species of diatoms. These algae live in both marine and fresh waters, although most species are marine. The cell walls of golden-brown algae and diatoms are made of cellulose and pectin (a type of hemicellulose). They move using one to two flagellae. The photosynthetic pigments of these algae are chlorophylls a and c, and the accessory pigments are carotenoids and xanthophylls, including a specialized pigment known as fucoxanthin.

Diatoms have double shells, or frustules, that are largely constructed of silica (SiO₂), the two halves of which (called valves) fit together like a pillbox. Diatom species are distinguished on the basis of the shape of their frustules, and the exquisite markings on the surface of these structures.

The golden-brown algae (class Chrysophyceae) are much less diverse than the diatoms. Some species of golden-brown algae lack cell walls, while others have pectin-rich walls. Golden-brown algae are especially important in open waters of the oceans, where they may dominate the productivity and biomass of the especially tiny size fractions of the phytoplankton. These are known as the nanoplankton, consisting of cells smaller than about 0.05 mm in diameter.

Pyrrophyta (fire algae) are unicellular algae that include over 1, 000 species of dinoflagellates. Most of these species live in marine ecosystems, but some dwell in fresh waters. The dinoflagellates have cell walls constructed of cellulose, and have two flagellae. These algae store energy as starch. The photosynthetic pigments of the Pyrrophyta are chlorophylls a and c, and the accessory pigments are carotenoids and xanthophyll, including fucoxanthin.

Some species of dinoflagellates can temporarily achieve a great abundance, as events that are commonly known as “red tides” because of the resulting color of the water. Red tides can be toxic to marine animals, because of the presence of poisonous chemicals that are synthesized by the dinoflagellates.

Human health can also be affected. The toxin can also be released into the air; cases of respiratory illness in humans that occurred during a red tide outbreak in Florida were linked to the algal toxin.

The incidence of red tides has been increasing worldwide since the 1990s. The reasons for the increase are not clear; a suggestion favoured by many of the scientists involved is the warming of the ocean, particularly along the eastern coast of the United States.

Chlorophyta (green algae) comprise about 7, 000 species, most of which occur in fresh water, although some are marine. Most green algae are microscopic, but a few species, such as those in the genus Cladophora, are multicellular and can be seen with the un-aided eye. The cell walls of green algae are mostly constructed of cellulose, with some incorporation of hemicellulose, and calcium carbonate in some species. The food reserves of green algae are starch, and their cells can have two or more organelles known as flagella, which are used in a whip-like fashion for locomotion. The photosynthetic pigments of green algae are chlorophylls a and b, and their accessory pigments are carotenoids and xanthophylls.

Some common examples of green algae include the unicellular genera Chlamydomonas and Chlorella, which have species dispersed in a wide range of habitats. More complex green algae include Gonium, which forms small, spherical colonies of four to 32 cells, and Volvox, which forms much larger, hollow-spherical colonies consisting of tens of thousands of cells. Some other colonial species are much larger, for example, Cladophora, a filamentous species that can be several meters long, and Codium magnum, which can be as long as 26 ft (8 m).

The stoneworts are a very distinctive group of green algae. These live in fresh or brackish waters, and they have cell walls that contain large concentrations of calcium carbonate. Charophytes have relatively complex growth forms, with whorls of “branches” developing at their tissue nodes. Charophytes are also the only algae that develop multicellular sex organs, although these are not comparable to those of the higher plants.

Rhodophyta or red algae are comprised of approximately 4, 000 species of mostly marine algae, which are most diverse in tropical waters. Species of red algae range from microscopic to macroscopic in size. The larger species typically grow attached to a hard substrate or even on other algae. The cell walls of red algae are constructed of cellulose and polysaccharides such as agar and carrageenin. These algae lack flagellae, and they store energy as a specialized polysaccharide known as floridean starch. The photosynthetic pigments of red algae are chlorophylls a and d, and their accessory pigments are carotenoids, xanthophyll, and phycobilins.

Some examples of red algae include filamentous species such as Pleonosporum spp., so-called coralline algae such as Porolithon spp., which become heavily encrusted with calcium carbonate and contribute greatly to the building of tropical reefs, and thalloid species, such as the economically important Irish moss (Chondrus crispus ).

Paeophyta (brown algae) number about 1, 500 species. Almost all occur in marine environments. These seaweeds are especially abundant in cool waters. Species of brown algae are macroscopic in size, including the giant kelps that can routinely achieve lengths of tens of meters. Brown algae have cell walls constructed of cellulose and polysaccharides known as alginic acids. Some brown algae have relatively complex, differentiated tissues, including a holdfast that secures the organism to its substrate, air bladders to aid with buoyancy, a supporting stalk or stipe, wide blades that provide the major surface for nutrient exchange and photosynthesis, and spore-producing, reproductive tissues. The specialized, reproductive cells of brown algae are shed into the water and are motile, using two flagella to achieve locomotion. The food reserves of these algae are carbohydrate polymers known as laminarin. Their photosynthetic pigments are chlorophylls a and c, while the accessory pigments are carotenoids and xanthophylls, including fucoxanthin, a brown-colored pigment that gives these algae their characteristic dark color.

Some examples of brown algae include the sargassum weed (Sargassum spp.), which dominates the extensive, floating ecosystem in the mid-Atlantic gyre known as the Sargasso Sea. Most brown seaweeds, however, occur on hard-bottom, coastal substrates, especially in cooler waters. Examples of these include the rockweeds (Fucus spp. and Ascophyllum spp.), the kelps (Laminaria spp.), and the giant kelps (Macrocystis spp. and Nereocystis spp.). The giant kelps are by far the largest of the algae, achieving a length as great as 328 ft (100 m).

Xanthophyta (yellow-green algae) comprise about 450 species that primarily occur in fresh waters. They are unicellular or small-colonial algae, with cell walls made of cellulose and pectic compounds, and sometimes contain silica. The yellow-green algae store carbohydrate as leucosin, and they can have two or more flagellae for locomotion. The primary photosynthetic pigment of yellow-green algae is chlorophyll a, and the accessory pigments are carotenoids and xanthophyll.

Ecological relationships

Many types of algae are microscopic, occurring in single cells or small colonies. The usual habitat of many of the microscopic algae is open waters, in which case they are known as phytoplankton. Many species, however, live on the surfaces of rocks and larger plants within shallow-water habitats, and these are known as periphyton. Other microscopic algae live on the moist surfaces of soil and rocks in terrestrial environments.

Microscopic algae are at the base of their ecological food web—these are the photosynthetic, primary producers that are fed upon by herbivores. In the open waters of ponds, lakes, and especially the vast oceans, the algal phytoplankton is the only means by which diffuse solar radiation can be fixed into biological compounds. In these open-water (or pelagic) habitats the phytoplankton are consumed by small, grazing animals known as zooplankton, most of which are crustaceans. The zooplankton are in turn fed upon by larger zooplankton or by small fish (these predators are known as planktivores), which may then be eaten by larger fish (or piscivores). At the top of the open-water food web may be fish-eating birds, seals, whales, very large fish such as sharks or bluefin tuna, or humans. Therefore, the possibility of all of the animals occurring higher in the food webs, including the largest of the top predators, are ultimately dependent on the productivity of the microscopic phytoplankton of the pelagic marine ecosystem.

Other algae are macroscopic, meaning they can be readily observed without the aid of magnification.

Some of these algae are enormous; for example, some species of kelps are tens of meters long. Because they are primary producers, these macroscopic algae are also at the base of their ecological food webs. In most cases, however, relatively few herbivores can directly consume the biomass of macroscopic algae, and the major trophic interaction of these plants is through the decomposer, or detritivore part of the food web. In addition, because of their large size, macroscopic algae are critically important components of the physical structure of their ecosystems, providing habitat for a wide range of other organisms. The largest kelps develop a type of ecosystem that is appropriately referred to as a marine “forest” because of the scale and complexity of its habitat structure.

Some species of green algae occur as mutualistic symbionts with fungi, in an association of two organisms known as lichens. Lichens are common in many types of habitats. Other green algae occur in a mutualism with certain animals. In general, the host animal benefits from access to the photosynthetic products of the green alga, while the alga benefits from protection and access to inorganic nutrients. For example, species of unicellular Chlorella live inside of vacuoles within host cells of various species of freshwater protozoans, sponges, and hydra. Another species of green alga, Platymonas convolutae, occurs in cells of a marine flatworm, Convoluta roscoffensis. Various other green algae occur inside of marine mollusks known as nudibranchs. Similarly, various species of dinoflagellates occur as symbionts with marine corals.

Each species within an algal community has its particular ecological requirements and tolerances. Consequently, algal species tend to segregate according to varying patterns of environmental resources, and of biological interactions such as competition and predation. For example, during the growing season there is a time-series of varying abundances of phytoplankton species in open-water habitat. At certain times, particular species or closely related groups of species are abundant, but then these decline and other species of phytoplankton become dominant. This temporal dynamic is not totally predictable; it may vary significantly from year to year. The reasons for these patterns in the abundances and productivity of algal species are not understood, but they are likely associated with differences in their requirements for nutrients and other environmental factors, and perhaps with differing competitive abilities under resource-constrained conditions.

In a similar way, species of seaweeds tend to sort themselves out along stress-related environmental gradients associated with varying distances above and below the high-tide mark on rocky marine shores. The most important environmental stress for intertidal organisms is desiccation (drying), caused by exposure to the atmosphere at low tide, with the intensity of drying being related to the amount of time that is spent out of the water, and therefore to the distance above the high-tide line. For sub-tidal seaweeds the most important stress is the physical forces associated with waves, especially during storms. The various species of brown and red algae are arranged in rather predictable zonations along transects perpendicular to rocky shores. The largest kelps only occur in the sub-tidal habitats, because they are intolerant of desiccation. Within this near-shore habitat the species of algae are arranged in zones on the basis of their tolerance to the mechanical forces of wave action, as well as their competitive abilities in the least stressful, deeper-water habitats somewhat farther out to sea, where the tallest species grow and develop a kelp forest. In the intertidal, the various species of wracks and rockweeds dominate particular zones at various distances from the low-tide mark, with the most desiccation-tolerant species occurring closest to the high-tide mark. Competition, however, also plays an important role in the distributions of the intertidal seaweeds.

Factors limiting the productivity of algae

Some species of algae can occur in extreme environments. For example, species of green algae have been observed in hot-water springs at Yellowstone National Park, in highly acidic volcanic lakes, in the extremely saline Great Salt Lake and Dead Sea, and on the surfaces of glaciers and snow. Some algae even survive suspended in the atmosphere as spores or in droplets of moisture.

These are, however, extremely stressful environmental conditions. Most algae occur in less stressful habitats, where their productivity tends to be limited by the availability of nutrients (assuming that sufficient light is available to support the photosynthetic process). In general, the productivity of freshwater algae is primarily limited by the availability of the nutrient phosphate (PO₄^-3), while that of marine algae is limited by nitrate (NO₃^-) or ammonium (NH₄⁺). Some algal species, however, may have unusual nutrient requirements, and their productivity may be limited by certain micronutrients, such as silica, in the case of diatoms.

The structure of algal communities may also be greatly influenced by ecological interactions, such as competition and herbivory. For example, when herbivorous sea urchins are abundant, they can sometimes overgraze species of kelps in subtidal ecosystems of the west coast of North America, degrading the kelp forests. However, where sea otters (Enhydra lutris ) are abundant this does not happen because the otters are effective predators of the urchins.

Economic significance of algae

The most important economic products obtained from algae are associated with brown and red seaweeds, which can be utilized as food for people, and as resources for the manufacturing of industrial products. These seaweeds are mostly harvested from the wild, although increasing attention is being paid to the cultivation of large algae.

Some species of algae can be directly eaten by humans, and in eastern Asia they can be especially popular, with various species used as foods. An especially common food is the red alga known as nori in Japan and as laver in western Europe (Porphyra spp.), which has long been eaten by coastal peoples of China and Japan. This alga is often used as a wrapper for other foods, such as rice or plums, or it may be cooked into a clear soup. Nori has been cultivated for centuries in eastern Asia. Another alga known as dulse or sea kale (Rhodymenia palmata ) is consumed dried or cooked into various stews or soups. Other commonly eaten seaweeds include the sea lettuce (Ulva lactuca ), and murlins or edible kelp (Alaria esculenta ).

Potentially, seaweeds are quite nutritious foods, because about 50% of their weight occurs as carbohydrates, with smaller concentrations of proteins and fats, and diverse micronutrients, including iodine. In practice, however, seaweeds are not very nutritious foods for humans, because we do not have the enzymes necessary to metabolize the most abundant of the complex, algal carbohydrates.

The major economic importance of brown seaweeds is as a natural resource for the manufacturing of a class of industrial chemicals known as alginates. These chemicals are extracted from algal biomass, and are used as thickening agents and as stabilizers for emulsions in the industrial preparation of foods and pharmaceuticals, and for other purposes.

Agar is another seaweed product, prepared from the mucilaginous components of the cell walls of certain red algae. Agar is used in the manufacturing of pharmaceuticals and cosmetics, as a culture medium for laboratory microorganisms, and for other purposes, including the preparation of jellied desserts and soups. Carrageenin is another, agar-like compound obtained from red algae that is widely used to stabilize emulsions in paints, pharmaceuticals, ice cream, and other products. Irish moss (Chondrus crispus ) is a purplish alga that is a major source of carrageenin.

Researchers are investigating methods for the economic cultivation of red and brown seaweeds for the production of alginates, agar, and carrageenin. In California, use has been made of rafts anchored to float about 13 yd (12 m) below the surface, in shallow, less than 328-ft (100-m) deep water, to grow the highly productive, giant kelp Macrocystis as an industrial feedstock. Seaweeds are also cultivated on floating devices in coastal China, and research is investigating whether growth rates in dense plantings can be economically increased by enriching the seawater with nitrogen-containing fertilizers.

In some places, large quantities of the biomass of brown and red algae wash ashore, especially after severe storms that detach these algae from their substrates. This material, known as wrack, is an excellent substrate for composting into an organic-rich material that can greatly improve soil qualities in terms of aeration and water- and nutrient-holding capacity.

Over extremely long periods of time, the frustules of diatoms can accumulate in large quantities. This material is known as diatomaceous earth, and its small reserves are mined for use as a fine polishing substrate, as a fine filtering material, and for other industrial purposes.

Algae as environmental problems

Red tides are events of great abundance (or “blooms”) of red, brown, or yellow-colored dinoflagellates of various species. These algae synthesize biochemicals, such as saxitoxin and domoic acid, which are extremely poisonous and can kill a wide range of marine animals, as well as humans who eat shellfish containing the toxins. The toxic syndromes of humans associated with dinoflagellate toxins are known as paralytic, diarrhetic, and amnesic shellfish poisoning.

Scientists cannot yet predict the environmental conditions that cause red tides to develop, although data from modeling studies points to the availability of nutrients and a water temperature that supports explosive growth of the algae.

The dinoflagellates involved with toxic dinoflagellate blooms are commonly of the genera Alexandrium, Dinophysis, Nitzchia, or Ptychodiscus. The algal toxins can be accumulated by filter-feeding shellfish such as clams and oysters. If these are eaten while they contain red-tide toxins, they can poison humans or animals in the local ecosystem. Even creatures the size of large whales can die from eating fish containing large concentrations of dinoflagellate toxins.

In addition, freshwater algae can cause problems when they are overly abundant. Algal blooms can cause foul tastes in the water stored in reservoirs, which may be required by nearby towns or cities as drinking water. This can be a significant problem in naturally productive, shallow-water lakes of the prairies in North America.

Eutrophication is another major problem that is associated with algal blooms in lakes that are receiving large inputs of nutrients through sewage disposal or the runoff of agricultural fertilizers. Eutrophication can result in severe degradation of the aquatic ecosystem when large quantities of algal biomass sink to deeper waters and consume most of the oxygen during their decomposition. The anoxic (deficiency in oxygen) conditions that develop are lethal to the animals that live in the sediment and deep waters, including most species of fish. Because the primary limiting nutrient in fresh waters is usually phosphate, inputs of this nutrient can be specifically controlled by sewage treatment, and by the banning of detergents containing phosphorus. This has been done in many areas in North America, and eutrophication is now less an environmental problem than it used to be.

Resources

BOOKS

Barsanti, Laura, and Paolo Gualtieri. Algae. Boca Raton, FL: CRC, 2005.

Meinesz, Alexandre, and Daniel Simberloff. Killer Algae. Chicago: University of Chicago Press, 2001.

Sprung, Julian. Algae: A Problem Solver Guide. Coconut Grove, FL: Ricordea Publishing, 2002.

Bill Freedman

Algae

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

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

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

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

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

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

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

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

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

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

Divisions of Algae

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

Glaucophyta.

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

Cryptophyta.

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

Haptophyta.

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

Dinophyta (or Pyrrhophyta).

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

Euglenophyta.

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

Heterokontophyta (or Ochrophyta).

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

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

Rhodophyta.

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

Chlorophyta.

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

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

Russell L. Chapman

Debra A. Waters

Bibliography

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

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

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

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

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

Cambridge: Cambridge University Press, 1995.

Algae

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

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

Types of algae

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

Words to Know

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

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

Phytoplankton: Microscopic algae that live suspended in the water.

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

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

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

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

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

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

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

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

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

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

Ecological importance of algae

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

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

Algal Blooms

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

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

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

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

Economic products obtained from algae

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

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

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

[See also Food web and food chain ]

Algae

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

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

Classification

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

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

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

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

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

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

Life Cycles

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

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

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

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

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

Economic and Ecological Importance

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

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

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

Virginia Card

Bibliography

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

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