Unit Four. The Evolution and Diversity of Life
15. How We Name Living Things
15.8. Domain Archaea
The domain Archaea contains one kingdom by the same name, the Archaea. The term archaea (Greek, archaio, ancient) refers to the ancient origin of this group of prokaryotes, which most likely diverged very early from the bacteria. Notice in figure 15.10 that the Archaea, in red, branched off from a line of prokaryotic ancestors that led to the evolution of eukaryotes. Today, archaea inhabit some of the most extreme environments on earth. Though a diverse group, all archaea share certain key characteristics. Their cell walls lack the peptidoglycan characteristic of the cell walls of bacteria. They possess very unusual lipids and characteristic ribosomal RNA (rRNA) sequences. Also, some of their genes possess introns, unlike those of bacteria.
Figure 15.10. A tree of life.
This phytogeny, prepared from rRNA analyses, shows the evolutionary relationships among the three domains. The base of the tree was determined by examining genes that are duplicated in all three domains, the duplication presumably having occurred in the common ancestor. When one of the duplicates is used to construct the tree, the other can be used to root it. This approach clearly indicates that the root of the tree is within the bacterial domain. Archaea and eukaryotes diverged later and are more closely related to each other than either is to bacteria.
Archaea are grouped into three general categories: methanogens, extremophiles, and nonextreme archaea.
Methanogens (such as Methanococcus) obtain their energy by using hydrogen gas (H2) to reduce carbon dioxide (CO2) to methane gas (CH4). They are strict anaerobes, poisoned by even traces of oxygen. They live in swamps, marshes, and the intestines of mammals. Methanogens release about 2 billion tons of methane gas into the atmosphere each year.
Extremophiles are able to grow under conditions that seem extreme to us.
Thermophiles (“heat lovers”) live in very hot places, typically from 60° to 80°C. Many thermophiles have metabolisms based on sulfur. Thus, the Sulfolobus inhabiting the hot sulfur springs of Yellowstone National Park at 70° to 75°C obtain their energy by oxidizing elemental sulfur to sulfuric acid. The recently described Pyrolobus fumarii holds the current record for heat stability, temperature optimum (106°C), and temperature maximum (113°C). These extreme temperatures are characteristic of the deep- sea hydrothermal vents where this organism was discovered. P. fumarii is so heat-tolerant that it is not killed by a one-hour treatment in an autoclave (121°C)!
Halophiles (“salt lovers”) live in very salty places like the Great Salt Lake in Utah, Mono Lake in California, and the Dead Sea in Israel. Whereas the salinity of seawater is around 3%, these prokaryotes thrive in, and indeed require, water with a salinity of 15% to 20%.
pH-tolerant archaea grow in highly acidic (pH = 0.7) and very basic (pH = 11) environments.
Pressure-tolerant archaea have been isolated from ocean depths that require at least 300 atmospheres of pressure to survive, and tolerate up to 800 atmospheres!
Nonextreme archaea grow in the same environments bacteria do. As the genomes of archaea have become better known, microbiologists have been able to identify signature sequences of DNA present in all archaea and in no other organisms. When samples from soil or seawater are tested for genes matching these signature sequences, many of the prokaryotes living there prove to be archaea. Clearly, archaea are not restricted to extreme habitats, as microbiologists used to think.
Key Learning Outcome 15.8. Archaea are unique prokaryotes that inhabit diverse environments, some of them extreme.