Prokaryotic Habitats

Prokaryotes live in certain habitats based on their biochemical abilities. Many bacteria are obligate anaerobes (AHB-luh-git AN-uhr-OHBS), organisms that cannot live where molecular oxygen, O2, is present. The obligate anaerobe Clostridium tetani causes the nerve disease tetanus. Facultative anaerobes (FAK-uhl-TAYT-iv AN-uhr-OHBS) can live with or without oxygen. Escherichia coli, which is common in the human digestive tract, is a facultative anaerobe. Prokaryotes that need oxygen to live are called obligate aerobes. The obligate aerobe Mycobacterium tuberculosis causes the disease tuberculosis (TB).

Prokaryotes have varying temperature requirements for growth. Psychrophilic (SIE-kroh-FIL-ik) (cold-loving) prokaryotes grow well at 0°C to 20°C (32°F to 68°F). Their cellular enzymes and cell membranes work well at cold temperatures. Some prokaryotes of the Antarctic survive very cold temperatures by living just under the surfaces of rocks. The weak sunlight warms the rocks just enough so that the psychrophiles have a slow-growing life cycle.

Prokaryotes that grow best at temperatures between 20°C (68°F) and 40°C (104°F) are called mesophiles. Humans are also classified as mesophiles. Prokaryotes that grow at very hot temperatures—between 45°C (113°F) and 110°C (230°F)—are called thermophiles (THUHR-moh-FIELZ). Thermophilic species have evolved proteins that resist being destroyed by heat. The development of biotechnology owes much to the purification of the heat-stable, DNA-replicating enzyme Taq polymerase from the thermophile Thermus aquaticus, shown in Figure 23-10. T. aquaticus was first isolated from a hot spring in Yellowstone National Park.

Most bacterial species grow best at a pH of 6.5 to 7.5 (7.0 is neutral). However, certain species, such as those that make yogurt and sour cream from milk, favor acidic environments that have a pH of 6.0 or lower. Acid-loving prokaryotes are called acidophiles.

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