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.
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.
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.
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 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.
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 ]