Introduction to Algae
Algae, organisms that lack true roots, stems, and leaves; that usually contain chlorophyll; and whose reproductive organs are single-celled structures. Green algae and the few chlorophyll-containing bacteria were probably the first organisms capable of carrying on photosynthesis (the process of manufacturing their own food).
About 25,000 species of algae have been identified. They range in size from microscopic single-celled forms to multicelled (many-celled) forms more than 150 feet (45 m) long. They abound in both fresh and ocean water and in moist environments on land. Masses of microscopic freshwater algae that are often seen floating on the water surface are called pond scum. The larger marine algae are commonly called seaweed. Certain species of algae contaminate reservoirs and swimming pools, giving the water a fishy taste and a foul odor.
Algae are the major source of the oxygen needed by aquatic animal life. Single-celled algae known as phytoplankton provide the food for larger organisms that, in turn, provide food for yet larger organisms. The interrelationship of these organisms is part of what is called the food web.
Eudorina is a green algae.Description
Different types of algae contain different pigments. Most contain chlorophyll, a green pigment. The presence of other pigments, however, may cause algae to appear brown, blue-green, red, or golden brown.
Algae vary greatly in structure. The single-celled species can be round, oval, or elongate, and they can exist individually or in colonies (loose groupings of similar cells). The colonies have various shapes, such as hollow spheres or slender threads. Single-celled algae called diatoms have cell walls covered with hydrated silica, a glasslike substance that produces a great variety of geometric patterns. (S.) Multicelled algae, such as kelp, sea lettuce, and rock-weed, usually have irregular, complex shapes. color page titled Examples of Ocean Life.)Some multicelled algae have hard shells composed of calcium carbonate.
Many species of single-celled algae are capable of limited forms of self-propulsion. These algae move about by means of flagella, which are long, whiplike growths that project from the main part of the cell.
Single-celled algae typically reproduce asexually by fissionthe division of a cell into two or more new cells. Some single-celled algae reproduce sexually, through either the fusion of two cells or the exchange of genetic material between cells. Most multi-celled algae reproduce by a process called alternation of generations, in which one generation reproduces asexually and the next sexually. In one generation, a cell called a spore develops directly into a new organism. In the next, two cells called gametes, or sex cells, combine their genetic material to produce a new organism. Multicelled algae can also reproduce asexually by fragmentationthe formation of a new individual from a detached portion of another.
Distribution
Although most algae are found in bodies of water, many microscopic species, which require only a thin film of moisture, can exist on or in the soil, on damp rocks, and on trees. Some microscopic algae are found on aquatic animals and plants. A few species of algae that lack chlorophyll live in the digestive tracts of animals (including humans) or inside plants.
Most multicelled algae grow at the bottom of bodies of waterusually near the shore. However, multicelled marine algae have been collected from depths as great as 884 feet (269 m). Multicelled algae also grow attached to various submerged objects, such as twigs and rocks. A few species of multi-celled algae are free-floating; for example, those found in the Sargasso Sea.
Most single-celled algae are free-floating. Some species, however, grow attached to rocks or trees along the shoreline, or on submerged objects.
Algae are found in a variety of temperature ranges. Some grow on ice and snow; others thrive in hot springs where the water temperature exceeds 180 F. (82 C.). Some algae are adapted to live in turbulent waters, while others live in the intertidal zone (the area between the high-tide line and the low-tide line) of seacoasts, where they are exposed to dry conditions for at least a few hours each day. A few species of algae live in close association with organisms called fungi, forming lichens. (S
Economic Importance
Because of their production of oxygen and their role in the food web, algae are normally beneficial to aquatic life. However, a bloom (a large and sudden growth in the population of phytoplankton) can cause the death of many fish. In most cases fish die because the decomposition of large amounts of algae depletes the oxygen in the water. Phytoplankton that produce blooms called red tides produce toxins that kill fish directly. These toxins are also poisonous to humans; persons who eat shellfish contaminated with the toxins can become seriously ill. Most blooms occur in bodies of water that have been polluted with sewage or with runoff containing organic substances such as fertilizers.
Large forms of algae, primarily kelp, are a popular food in the Far East. They are dried and compressed into flat cakes. In Wales, a species of red alga is used to make a flat bread called laver bread. Algae are rich in vitamin C, thiamine, and potassium. They are used extensively as food for livestock and as fertilizers.
Agar, or agar-agar, is a gelatinous substance derived from marine algae. Agar has many uses. It is used in laxatives, in materials on which dental impressions are made, in textile sizing, and in additives that thicken such foods as cheese, soup, and bakery products. Agar is also used as a gelling agent in cooking and, in laboratory work, as a medium in which bacteria are grown.
Carrageenin, a jellylike substance derived from a marine alga commonly known as Irish moss, is also important in industry. Its most frequent use is in chocolate milk, where it holds the cocoa particles in suspension. In addition, carrageenin is used in sauces, syrups, toothpastes, and cosmetics. Algin, derived from giant kelp, is also widely used in industry. The shells of dead diatoms make up diatomite, which has many commercial uses, including heat insulation and the filtration of liquids. Certain species of single-celled algae have been tested for possible use in spacecraft as a source of oxygen and food for astronauts.
Classification
Algae, with the exception of blue-green algae, are classified in the kingdom Protista; blue-green algae are classified in the kingdom Monera. Some biologists, however, include the brown, the red, and the green algae in the plant kingdom.