Domain Archaea

Archaea are not like bacteria in many ways. Archaea differ in the makeup of their cell walls. They also differ in their membrane lipids and in their genetics and metabolism. For example, archaeal cell walls do not have peptidoglycan (PEP-ti-doh-GLIE-KAN), a protein-carbohydrate compound found in bacterial cell walls. Archaeal cell walls have different amino acids than bacterial cell walls do. Archaea also have different types of lipids in their cell membranes. Archaea are like eukaryotic cells in that archaeal genes have introns, portions of DNA that do not code for amino acids and that are transcribed into RNA but are removed before being translated into proteins.

Archaea were first discovered in extreme environments, such as swamps, salt lakes, and hot springs. Until recently, scientists believed that archaea lived only in these extreme environments. However, after finding archael genetic material in samples of surface waters from the North Pacific and Antarctic Oceans, scientists now think that archaea may be more common than they once thought.

Archaeal Groups

Genetic analysis of archaea has revealed at least three groups of archaea: methanogens, halophiles, and thermoacidophiles.

Methanogens (muh-THAN-uh-JENZ) are named for their unique way of getting energy: they convert hydrogen gas and carbon dioxide into methane gas. Oxygen is poisonous to them, so methanogens can live only in anaerobic environments (environments that lack oxygen), such as in deep fresh water, marine mud, swamp mud, and sewage. The methane that bubbles out at sites such as swamps is called marsh gas. Methanogens also thrive in the intestinal tracts of organisms such as cows and termites. A cow can belch between 200 and 400 liters of methane per day.

Halophiles (HAL-oh-FlELZ) are "salt-loving" archaea that live in environments that have very high salt concentrations, such as the Great Salt Lake and the Dead Sea. High salt concentrations would kill most bacteria but favor the growth of halophiles because halophiles have adapted to live in very salty water.

Thermoacidophiles (THUHR-moh-uh-SID-uh-FlELZ) live in very acidic environments that have very high temperatures, such as the hot springs of Yellowstone National Park. Some thermoacidophiles live at temperatures up to 110°C (230°F) and at a pH of less than 2. Thermoacidophiles also live near volcanic vents on land or near hydrothermal vents called black smokers. Black smokers are cracks in the ocean floor that leak very hot, dark-colored, acidic water, as shown in Figure 23-2. Scientists have found large communities of worms, clams, crabs, and mussels living near these thermal vents. These communities live at great depths and in total darkness, where photosynthesis cannot take place, so they depend on thermoacidophilic archaea as a primary source of food.

(a) Bacillus (rod-shaped) Escherichia coli

(b) Coccus (round-shaped) Micrococcus luteus

(c) Spirillum (spiral-shaped) Spirillum volutans figure 23-3

(a) Bacillus (rod-shaped) Escherichia coli

(b) Coccus (round-shaped) Micrococcus luteus

(c) Spirillum (spiral-shaped) Spirillum volutans

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