January 11, 2000
The group of organisms collectively referred to as "Algae" were at one time included entirely in the kingdom of Plantae. However, with the present-day kingdom classification, all the eukaryotic algae (cells having distinct nuclei) are now in the kingdom Protista, with the exception of Blue-green algae, Cyanobacteria and Prochlorophyta, which are in the kingdom Monera with the other prokaryotic organisms (cells having no distinct nuclei). Nevertheless, some phycologists still consider algae to be plants because they contain chlorophyll and carry out photosynthesis. Before describing the specifics of green algae (Chlorophyta) it is important that one has a general understanding of all algae.
Algae are very important in the balance between food producing and consuming organisms. Algae are autotrophs (food producing) and provide food for countless species of water- dwelling animals. Algae also make it possible for animals to exist on land. As algae carry out photosynthesis, they release oxygen into the atmosphere. They are so plentiful that they produce 90% of the world's atmospheric oxygen.
Algae vary in size and shape from microscopic hard-shelled forms to rubbery kelps that grow as long as 230 ft. Most algal cells are supported by an inner wall of cellulose. Layers of cells are held together by a jelly like substance called pectin. Some algae are unicellular and move with flagella; others are multicellular and are nonmotile.
Algae are common in freshwater lakes, streams, oceans, as well as damp habitats such as damp rock faces, tree trunks, moss hammocks or damp soil. A few even grow within the pores of rocks in deserts, relying upon the night time dew for their source of moisture. Others grow on melting snow or attach to the under surface of floating ice. They grow on other plants, wood, turtles, water fleas, and even inside plants and animals.
Algae are classified into five groups according to the pigments they contain. These five groups are; golden algae; fire algae; green algae; brown algae and red algae. Regardless of their color all algae contain a green pigment called chlorophyll. Most also contain a second type of chlorophyll. Algae are also commonly classified by the form in which they store food and by their means of reproduction.
The shapes of algae are also used in classification. Even though many algae are only made up of one cell, they can have different shapes, such as stars, needles, pyramids, cubes, round balls, eggs, long threads, vases and worms. Colonies may be shaped as a hollow ball, a diamond, a cube, a star and a flat plate. Multicellular algae may be shaped like small brushes, palm trees, leaves, whips, tubes and flat ribbons.
There are three ways algae may form other plants like themselves: 1) Asexually, 2) sexually where the parent plant releases gametes. They join together and grow into new parent cells. A zygote is formed by two gametes joining together. 3) Another types of sexual reproduction some algae utilize, forms swimming cells called zoospores that move about on the water. These grow into two types of short threads or filaments which produce the gametes.One thread produces eggs and the other produces sperms. These join together to the bottom of the ocean and become a small leaf-shaped plant. In time, it grows into a large plant.
Now to the specifics of green algae. Chlorophyta, the green algae, is one of the largest algal phyla and one of the most diverse, from common pond scum to the bright green sea weeds. The 7000 species of green algae range from microscopic single cells, long strings and filaments, flat plants (the common sea lettuce) and even hollow tubes to some multicellular organisms reaching 25 ft long. Green algae, unlike some other groups of algae, contain the same three pigments found in land plants: Chlorophyll a, Chlorophyll b and a type of carotene. Like many land plants, green algae store food as starch. Some groups of green algae produce oil as well as starch. The similarities between plants and green algae fossils have led some evolutionists to suggest that plants evolved from green algae some 2 billion years ago.
Green algae may occur as single cells (either motile of nonmotile), in colonies (more often nonmotile) and as multicellular filaments. The unicellular forms assume an almost endless variety of shapes. Colonial forms may be loose aggregates of single cells or may have these cells arranged in a characteristic pattern. Some filamentous types bear a superficial resemblance to higher plants. The motile unicellular organisms are free swimming, moving by means of whip-like flagella (usually two in number). Even the nonmotile, species may produce motile reproductive cells (zoospores).
Unicellular Group: Chlamydomonas inhabit fresh water pools. They have two flagella which they lose in reproduction. Chlamydomonas reproduce asexually which involves the division into 2-8 daughter cells within the cell wall and membrane of the mother cell. The flagella are released and they form 2 flagella on each of the daughter cells just before they are released from the mother cell; Chlorella is a small unicellular algae that is used predominately in studies of the cellular processes and in the study of algae as a food source. They are known for living inside animals, and for being the fastest multiplying green algae that has been studied. Chlorella also contains vitamins, fats and starches but it has not been made to taste good; Desmids are often mistaken as diatoms but they are a plankton and a free-floating algae. They are usually unicellular but sometimes are joined to form a filament-like colony. They are often pinched in the middle so that they look like two cells that are attached, but they are two symmetrical halves. The cell walls have unusual patterns, which make desmids one of the most interesting freshwater algae; Protococcus is a unicellular green algae but it may form into clumps. It is most commonly found in damp forests, forming slippery film on rocks and green dust on tree trunks. Protococcus reproduces asexually. The cell divides by binary fission, which in doing so, produces two genetically identical daughter cells.
Multicellular Groups: Oedogonium reproduces both sexually and asexually. Sexually, the Oedogonium produces an egg within another egg which is called oogonium. The antherida produces a sperm which enters the antheridia and fertilizes the egg. It results in a zygote, which forms a hard protective wall and can remain inactive for several months. Before the zygote's wall breaks open meiosis occurs and four flagellate zoospores are formed. Asexually, it forms a single cell, multiflagellate zoospore within a cell; Spirogyra is a multicellular green alga that grows in freshwater pools. Its cells form a slender filament that look transparent. Each of the chloroplasts, within the filament, contain a small protein body called a pyrenoid, which stores starch. Spirogyra can reproduce asexually in two ways. The cells can go through binary fusion which causes the filament to grow lengthwise. If the filament is broken it grows on its own. This process helps disperse the algae. The sexual reproduction of Spirogyra involves the process of conjugation. Two filaments form connecting tubes and the content of one cell flows into the other. The wall thickens around the zygote forming a zygospore that can survive harsh conditions; Ulothrix is a filament that can reproduce sexually and asexually. When it reproduces sexually, it produces 8-64 isogametes inside a cell. Each of the gametes have two flagella, which help them swim together and unite when released from the mother cell. When the cells unite, they form a zygote which later becomes a zygospore. Asexually, The Ulothrix reproduces by forming 4-8 zoospores in a cell. The zoospore contains 4 flagella which help them swim away to form new colonies; Ulva is most commonly known as sea lettuce. It has a life cycle that involves two distinct forms of the organism. The two forms may look alike, but they are genetically different. One of the forms is haploid (meaning they have [n] chromosomes). The haploid form of the organism is called gametophyte because it produces gametes. When two gametes fuse they form a diploid zygote. All of the cells that are developed form that zygote are diploid. The resulting diploid form is called a sporophyte because its cells undergo meiosis and therefore will produce spores. Each haploid spore will develop into a haploid gametophyte. The alteration between the sporophyte and the gametophyte stages in the life cycle is called alteration of generations. All plants and many types of algae go through this process of alteration of generations. This life cycle is widespread because it has great survival value. The species benefit from the recombination of parents' traits through the fusion of gametes and from the opportunity to reproduce by the less risky process of forming spores. The gametophytes and the sporophytes look identical in this specie, but in other plants, the two forms may look very different.
Colonial Group: Volvox organisms are made up of individual cells held together by strands of cytoplasm. Colonial algae are different from multicellular organisms because their cells do not have specialized functions. Cells in a colony can reproduce more rapidly and readily than single cells because the mating cells are always nearby. The size of the colony protects the members from the organisms that feed on a single cell. Volvox is one of the most beautiful colonies. The colony is a hollow ball formed by hundreds of thousands of bright green cells. The whole colony spins slowly through the water by the synchronized beating of the cells' flagella.
Since algae give off large accounts of oxygen and are a source of food for marine animals and some land animals, extensive research has been conducted in the suitability of green algae for providing oxygen and food in the area of space exploration as well as its use in atomic submarines. People could breathe the oxygen the plants give off. In turn the plants could use the carbon dioxide the people exhale. The algae would combine the carbon dioxide with the nitrogen gas to make their plant food. Chlorella has been found to be over half protein and has all vitamins but vitamin c as well as fats and starches, and can reproduce in 2 ½ hours to double its weight. One strain of Chlorella would take only 3 to 5 cubic feet per person in a spacecraft and provide enough oxygen to keep one person alive and feed him the exact amount of food needed to live. Research has also provided information regarding food manufacture, vitamin production, oxygen yields and growth rates under various conditions. Algae have also been found to eat human wastes.
In addition to space and submarine research, algae has been used in laboratories to study poisons, to determine nutritional or food requirements; to learn more about living processes and the causes of death. It has also been important in biofiltration, the use of microscopic plants to remove chemicals from polluted water. Iodine, calcium, and phosphorus are chemicals that can be removed from polluted water by algae, which concentrate them in or on our bodies.
Another area of focused research has been as a major food supply for the starving people of the world. Crops of algae need less space than any other crop. There is no waste. It reaches maturity in a few hours, so the harvest is very short and can be year round as long as there is sunlight for the algae to grow. The major drawback is that the countries that need it most do not have the money necessary to purchase the special equipment for algal farming. They also lack the scientists and engineers to get the system under way. Countries like the United states could do it very easily because the resources are available here. However, it is not done because we do not need food. We are able to grow conventional crops to supply not only our country but many other countries with the food needed. Smaller countries such as Japan, China and Israel have started producing algae on a small scale. When food becomes scarce, the research and production of algae will intensify.
Although algae is important as a source of food and oxygen, they can have negative effects, as when large populations produce an unpleasant taste and odor in drinking water or clog filtration equipment. In freshwater lakes and ponds polluted by nitrates and phosphates, algae populations sometimes increase suddenly in an "algal bloom", forming a dense, smelly scum and drastically decreasing the oxygen supply available to other life forms. However, it is apparent the positives outweigh the negatives.
"Algae". Microsoft Encarta 1996 Encyclopedia.
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