13.12: Archaea thrive in habitats too extreme for most other organisms.

Archaea are famous for their ability to live in places where life would seem to be impossible, such as in water around hydrothermal vents that emerge from the seafloor, which would be boiling if it were at sea level. At 2,000 meters below the surface, the water pressure reaches 200 atmospheres, or almost 3,000 pounds per square inch, and the 400° C water coming from the vent cannot boil. Archaea thrive there. Most organisms die at temperatures between 40° and 50° C, because their protein molecules are denatured, so the ability of archaea to survive at temperatures above 100° C is truly remarkable.

Equally impressive is the ability of archaea to live in water as acidic as pH 0 or as salty as a saturated NaCl solution, and to metabolize an extraordinary range of compounds—including sulfur, iron, and hydrogen gas—to obtain energy (FIGURE 13-17). Some bacteria also can tolerate extreme physical and chemical conditions, and organisms that can live in these conditions, both bacteria and archaea, are called extremophiles (“lovers of extreme conditions”).

The extremophile nature of so many archaea leads to a practical problem that limits our knowledge of them. How do you grow an organism in your lab if it requires 200 atmospheres of pressure and a water temperature of 100° C or higher, and eats a gas that is toxic to humans? It’s not easy, and consequently only about one-quarter of the identified archaeal species have been cultured in the lab. We know that the other species exist, because their transfer RNA has been identified in samples taken from the environment, and biologists are confident that thousands, perhaps millions, of species of archaea await discovery.

Extremophile archaea have important applications in bioengineering and environmental remediation. For example, Thermus aquaticus, an extremophile archaeon that lives in hot springs where the water is nearly boiling, produces an enzyme important in making PCR possible (see Chapter 5), and this enzyme is now sold for laboratory use under the name Taq polymerase. Far from being destroyed at high temperatures, Taq polymerase works best at 75° C, a temperature at which most human enzymes would simply fall apart.

Bioengineers and biotechnologists believe that extremophile archaea and bacteria that thrive at high temperatures and pressures and metabolize toxic substances have enormous potential for industries that carry out activities under such conditions. Recent experiments have demonstrated the ability of some archaea to efficiently degrade hydrocarbons, making it possible for them to be used in the removal of sludge that accumulates in oil refinery tanks and, potentially, in the cleanup of contaminated environments such as oil slicks (FIGURE 13-18). Naturally occurring archaea are helping to break down some of the more than 200 million gallons of oil released into the Gulf of Mexico following an oil rig explosion and subsequent massive leak in 2010. Other archaea show promise in clearing mineral deposits from pipes in the cooling systems of power plants.

But not all archaea are extremophiles. Archaea live everywhere that bacteria do, and many of them live in places you would find comfortable. In fact, you are home to many archaea and, on occasion, some of them make their presence all too obvious.

Beans are notorious for their tendency to produce gas in the intestine, but archaea are actually the culprits (FIGURE 13-19). Beans contain a couple of carbohydrates with chemical bonds that are not broken down very well by any human enzymes. Methane-producing archaea, on the other hand, have no such difficulties, producing an enzyme that targets those bonds. As a result, it is the archaea in your intestine that digest most of these carbohydrates. But in the process of breaking these bonds, they produce gases that, as they escape the digestive system, can cause considerable distress.