algae Article Contents: Algal Classification Algal Reproduction Ecological Significance Economic Importance From: Encyclopedia of Life Science. Algae are a diverse group of aquatic photosynthetic eukaryoticorganisms that make up one of two subkingdoms in the traditional kingdom Protista. (The other subkingdom is Protozoa.) Newer classification schemes place some algae in their own kingdoms and others in the plant kingdom. Green Algae (Caulerpa racemosa) and Brown Algae (Lobophora variegata)
Enlarge Image Algae are characteristically phototrophic, meaning they obtain their energy from sunlight, but vary widely in terms of cell structure and organization, size, and habitats. Algal cells contain the structures andorganelles typical of eukaryotic cells, including a nucleus,mitochondria, an endomembrane system, and plastids (organelles that carry out photosynthesis). Some types of algae are unicellular, while others are filamentous or live in colonial arrangements. Algae are a main component of plankton, the mass of mostly microscopic organisms that float freely in aquatic
environments, but they also live in soil, on rocks, or even in symbiotic relationships with other organisms. Algae possess pigments that capture light of specific wavelengths, which the cells convert into chemical energy in the form of organic compounds. All algae contain chlorophyll a, the same pigment found in plants, but different types of algae contain different amounts of accessory pigments that absorb light of different wavelengths.
Algal Classification The familiar term algae refers to all photosynthetic organisms that are not plants or bacteria. Unlike bacteria, algae are eukaryotic, and unlike plants, algae lack roots, leaves, and flowers. The more than 22,000 distantly related algal species have traditionally been categorized into seven main divisions as a matter of convenience rather than biological or evolutionary significance: Chrysophyta, Pyrrophyta, Euglenophyta, Chlorophyta, Rhodophyta, Phaeophyta, and Xanthophyta. Considerations for categorizing the algae include the combination of photosynthetic pigments contained in the plastids, the cellular structure and organization, composition of the cell wall, the presence or absence of flagella, reproductive means, and motility. The field of modern systematics depends on evolutionary relationships to classify organisms, but comparison of the deoxyribonucleic acid (DNA)sequences of the many diverse groups of photosynthetic protists shows that they have deeply divergent ancestries. More modern classification schemes reflect the belief that eukaryoticalgae developed by three different endosymbioses. The first evolutionary line includes the algae with chloroplasts that possess two membranes, Glaucophyta, Rhodophyta, and Chlorophyta. The second line includes algae with chloroplasts that are surrounded by an additional membrane of chloroplast endoplasmic reticulum, Euglenophyta and Dinophyta. The third line includes algae that possess two membranes of chloroplast endoplasmic reticulum, continuous with the outer membrane of the nuclear envelope: Cryptophyta, Bacillariophyta, Chrysophyta, Prymnesiophyta, Xanthophyta, Eustigmatophyta, Rhaphidophyta, and Phaeophyta. A brief overview of some of the most common algal divisions follows. Members of the division Chrysophyta, commonly known as the goldenalgae, often contain the photosynthetic pigments chlorophyll c andcarotenoids such as fucoxanthin that give them a yellowish brown color in addition to the chlorophyll a that is common to all algae. Most golden algae are unicellular and free-swimming, but colonial and filamentous forms also exist. The cell walls of these mostly marine organisms contain silica compounds and pectin in addition to cellulose.
In the absence of light or in the abundance of food, chrysophytes can become facultatively heterotrophic and feed on bacteria and diatoms. Closely related to Chrysophyta are the diatoms, organisms with two protective overlapping shell halves, like a box with a lid, that form what is called diatomaceous earth over millions of years. The glassy shells contain pores that allow material to flow between the internal and external environments of the cells and give the organisms intricate decorative patterns. Chrysophytes store their food reserves as oil droplets, giving buoyancy to the structures positioning them close to the surface of a body of water so they can capture sunlight to fulfill their energy needs. With sufficient energy and nutrients, diatoms can reproduce asexually daily; however, this leads to progressively smaller progeny. Occasional sexual reproduction allows a full-sized organism to develop. The division Pyrrophyta (fire algae) consists of the dinoflagellates, unicellular algae that also possess chlorophyll c but that more closely resemble ciliated protists than other algae. As their name suggests, the dinoflagellates have a set of perpendicularly arranged flagella, one at the posterior end and another located within a groove and that spins as the cell swims. As diatoms do, these organisms produce oil droplets, but also store energy as starch. Some dinoflagellates are free-swimming, but some live in symbiotic relationships. For example, many corals have dinoflagellates living inside their tissues. The dinoflagellates undergo photosynthesis, releasing the organic products and oxygen to the corals, which, in turn, excrete waste products from which the dinoflagellates extract nutrients such as phosphates and nitrates in addition to carbon dioxide for their own metabolism. Dinoflagellates are known for their tendency to form algal blooms, a phenomenon that results from nutrient and light conditions that support overgrowth of the organisms.
Enlarge Image Dinoflagellates live in both marine water and freshwater and mostly reproduce by asexual reproduction. Members of Euglenophyta, the third strictly unicellular division ofalgae, are mostly freshwater and contain chlorophylls a and b and carotenoids. This branch of life surprised early biologists, who believed that all life-forms belonged in one of two categories, plants or animals. These organisms have a flagella, a light-sensitive eyespotthat helps the organism seek an environment conducive to photosynthesis, and numerous chloroplasts. They store their food as starch. Individual Euglenid cells have been known to survive after losing their chloroplasts. They absorb nutrients from their environments and produce nonphotosynthetic, colorless progeny. The division Chlorophyta contains the most diverse members and is perhaps the most widely known of the algae. As do plants, these
green algae have both chlorophylls a and b and β-carotene as their photosynthetic pigments. Most of the green algae are unicellular, freshwater organisms, but some are multicellular or marine, and others live in moist terrestrial environments. This form of algae is the type that coats the surfaces of ponds. Filamentous green algae form mats that float on top of water. One such organism, Spirogyra, undergoes sexual reproduction by conjugation in order to prevent death from dehydration or freezing temperatures. Strands of filaments line alongside one another, form connections, and produce zygotes that can withstand the unfavorable conditions until the next season. Green Algae (Anadyomene lacerata)
Enlarge Image Red algae, belonging to the phylum Rhodophyta, contain phycobilins, photosynthetic pigments that capture green and blue light that penetrates deep into the water, allowing the red algae to survive at greater depths than other algae. Some red algae have cell walls made of calcium carbonate and play a role in building coral reefs. Their life cycles are complex; mature sporophytes undergo meiosis to producehaploid spores that grow into haploid gametophytes, which produce either eggs or sperm. Fertilization results in a diploid sporophyte body, completing the cycle.
Red Algae
Enlarge Image Phaeophyta, the brown algae, contain chlorophylls a and c andfucoxanthin and have structures similar to those of plants. Holdfasts anchor the brown algae to seabeds or rocks, stemlike stipes bend with the waves, and blades, also called fronds, resemble leaves and contain the photosynthetic organelles. Gas-filled floats provide buoyancy to keep the blades nearer to the sunlight. Brown and redalgae together constitute seaweed. Some brown algae can reach up to 100 feet (30 m) in length.
Brown Algae (Ascophyllum nodosum)
Enlarge Image The Xanthophyta, or yellow-green algae, primarily inhabit freshwater. They lack chlorophyll b and the brown pigment fucoxanthin but do have chlorophyll c. The composition of the cell wall in Xanthophytes is unclear, but the shape often consists of two overlapping cylindrical halves. Many yellow-green algae are unicellular and possess flagella, but colonial and filamentous forms also exist.
Algal Reproduction
Algae exhibit a variety of reproductive strategies. Asexually, some employ mitosis, while others produce new organisms by fragmentation of cells from colonies or from multicellular aggregates or by producing spores that develop into mature organisms. Flagellated motile spores are common to algae that live in aquatic environments, and nonmotile spores are characteristic of terrestrial algae. Sexual reproduction involves meiosis, the production of haploid gametes that combine to form diploid zygotes, a process that results in greater genetic variation. Isogamous unions occur when identical gametes combine during fertilization. During heterogamous fertilization, distinct male and female gamete types combine. Some algae undergo a process
called alternation of generations, during which haploid generations, gametophytes, alternate with diploid generations, sporophytes.
Ecological Significance As primary producers, algae fill a vital niche in many ecosystems. Not only do they play an important role in aquatic food webs and serve to produce most of the planet's oxygen, they also provide a significant source of iodine and protein for many human societies. Given certain conditions, the beneficial role played by algae can turn threatening when a sudden increase in the availability of nutrients supports their overgrowth and upsets the delicate ecological balance of marine organisms in the community by disturbing the food webs. As the algaedie, other aquatic microorganisms decompose the organic matter, absorbing much of the available oxygen in the process. The increased oxygen demand by the microbial decomposers decreases the availability of oxygen for the other inhabitants, resulting in the death of other community members. The organism Gymnodinium breve, a dinoflagellate, causes outbreaks of a phenomenon known as red tide. During the spring and fall, the waters of the shores of the Pacific, Gulf Coast, and New England states churn, carrying an abundance of nutrients to the surface. These conditions allow Gymnodinium breve to thrive, giving the water a reddish appearance. The algae secrete products that are toxic to fish and other marine organisms and that concentrate in shellfish. Though the toxin is harmless to shellfish, it can be harmful to humans who ingest it, causing neuromuscular problems such as numbness.
Economic Importance Algae are economically important in a number of ways. Many Eastern cultures use red and brown algae as a food source. In Asia, nori, a red alga, is used in sushi and also as a component of soups. Some algaeare eaten directly as a vegetable or added to sweetened jellies. The brown alga commonly known as kelp is a good source of iodine, and other seaweeds provide necessary minerals and vitamins.
Kelp
Enlarge Image Rhodophyta such as Gelidium and Gracilaria are an important source of agar, a substance that microbiologists use to thicken culture media and also to produce the gelatin capsules that contain drugs or vitamins. Extracts from a purple seaweed, Carrageen, are used in the food production, cosmetics, and pharmaceutical industries. Kelps also provide algin, a gelling agent used in many foods such as ice cream and in substances such as toothpaste.
Further Information Jackson, John, ed. Encyclopedia of the Aquatic World. Vol. 1. New York: Marshall Cavendish, 2004. Phycological Society of America home page. Available online. URL: http://www.psaalgae.org/. Accessed January 13, 2008. Sigee, David C. Freshwater Microbiology: Biodiversity and Dynamic Interactions of Microorganisms in the Aquatic Environment. Hoboken, N.J.: John Wiley & Sons, 2005.
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Citation Information Cullen, Katherine. "algae." Science Online. Facts On File, Inc. Web. 13 Jan. 2011. <http://www.fofweb.com/activelink2.asp? ItemID=WE40&SID=5&iPin=ELS0005&SingleRecord=True>.