10.17: The archaea domain includes many species living in extreme environments.

In a bubbling hot spring in Yellowstone National Park, the temperature ranges from boiling water (212° F, or 100° C) down to a relatively cool 165° F (74° C) at the surface. It would seem a most inhospitable place for life. Yet researchers have found 38 different species of archaea thriving there. Perhaps even more surprising, the genetic differences among these species are more than double the genetic differences between plants and animals.

In the freezing waters of Antarctica, too, archaea abound. More than a third of the organisms in the Antarctic surface waters are archaea. In swamps, completely devoid of oxygen, and in the extremely salty water of the Dead Sea, the story is the same: where once it was assumed that no life could survive, the archaea not only exist but thrive and diversify (FIGURE 10-31).

If biologists ever find themselves thinking that we have a handle on the breadth of life that is possible on earth, they would be wise to remember the archaea. Until relatively recently, our perception of life on earth was that there were bacteria and there were eukaryotes, such as the plants and animals. But in the past several decades, as researchers have explored some of the most unlikely of habitats, they have found archaea thriving. There are entire worlds of life on earth that we never even imagined.

Analyses of genetic sequences indicate that the archaea and the bacteria diverged about 3 billion years ago and that the eukarya split off from the archaea approximately 2.5 billion years ago. The archaea are grouped in one kingdom within the domain archaea, but we have no idea how many species exist. Given that they are the dominant microbes in the deep seas, it may very well be that archaea—of which we were completely ignorant until recent decades—are the most common organisms on earth. It is still too early to tell.

We do know that, like bacteria, all archaea are single-celled prokaryotes. For that reason, under a microscope they look very similar to bacteria. Several physical features distinguish them from the bacteria, however. Specifically, the archaeal cell walls contain polysaccharides not found in either bacteria or eukaryotes. The archaea also have cell membranes, ribosomes, and some enzymes similar to those found in the eukarya.

The archaea exhibit tremendous diversity and are often divided into five groups based on their physiological features.

• 1. Thermophiles (“heat lovers”), which live in very hot places
• 2. Halophiles (“salt lovers”), which live in very salty places
• 3. High- and low-pH-tolerant archaea
• 4. High-pressure-tolerant archaea, found as deep as 4,000 meters (about 2.5 miles) below the ocean surface, where the pressure is almost 6,000 pounds per square inch (compared with an air pressure of less than 15 pounds per square inch at sea level)
• 5. Methanogens, which are anaerobic and produce methane

There also seem to be large numbers of archaea living in relatively moderate environments that are also commonly home to bacteria. We explore the great diversity of the domain archaea in more detail in Chapter 13.