Archaea is a relatively new classification of life initially proposed by Carl Woese, an American microbiologist, in 1977.

He found that bacteria, which are prokaryotic cells without a nucleus, could be divided into two distinct groups based on their genetic material. Both bacteria and archaea are single-cell organisms, but archaea have a completely different cell membrane structure that lets them survive in extreme environments.

Woese at first suggested that life be grouped into the three domains of Eukarya, Bacteria and Archaebacteria. (You may see these three names starting with lower-case letters, but when you talk about the specific domains, the terms are capitalized.)

When more research revealed that the cells of the domain Archaebacteria were actually quite different from bacteria, the old term was dropped. The new domain names are Bacteria, Archaea and Eukarya, where Eukarya consists of organisms whose cells have a nucleus.

On the tree of life, cells of the domain archaea are situated between the cells of bacteria and those of the eukarya, which include multicellular organisms and higher animals.

Archaea reproduce asexually through binary fission; the cells split in two like bacteria. In terms of their membrane and chemical structure, the archaea cells share features with eukaryotic cells. Unique archaea characteristics include their ability to live in extremely hot or chemically aggressive environments, and they can be found across the Earth, wherever bacteria survive.

Those archaea that live in extreme habitats such as hot springs and deep-sea vents are called extremophiles. Because of their fairly recent identification as a separate domain on the tree of life, fascinating information about archae, their evolution, their behavior and their structure is still being discovered.

Archaea are prokaryotes, which means that the cells don't have a nucleus or other membrane-bound organelles in their cells.

Like bacteria, the cells have a coiled ring of DNA, and the cell cytoplasm contains ribosomes for the production of cell proteins and other substances the cell needs. Unlike bacteria, the cell wall and membrane can be stiff and give the cell a specific shape such as flat, rod-shaped or cubic.

Archaea species share common characteristics such as shape and metabolism, and they can reproduce via binary fission just like bacteria. Horizontal gene transfer is common, however, and archaea cells may take up plasmids containing DNA from their environment or exchange DNA with other cells.

As a result, archaea species can evolve and change rapidly.

The basic structure of archaea cell walls is similar to that of bacteria in that the structure is based on carbohydrate chains.

Because archaea survive in more varied environments than other life forms, their cell wall and cell metabolism have to be equally varied and adapted to their surroundings.

As a result, some archaea cell walls contain carbohydrates that are different from those of bacteria cell walls, and some contain proteins and lipids to give them strength and resistance to chemicals.

Some of the unique characteristics of archaea cells are due to the special features of their cell membrane.

The cell membrane lies inside the cell wall and controls the exchange of substances between the cell and its environment. Like all other living cells, the archaea cell membrane is made up of phospholipids with fatty acid chains, but the bonds in the archaea phospholipids are unique.

All cells have a phospholipid bilayer, but in archaea cells, the bilayer has ether bonds while the cells of bacteria and eukaryotes have ester bonds.

Ether bonds are more resistant to chemical activity and allow archaea cells to survive in extreme environments that would kill other life forms. While the ether bond is a key differentiating characteristic of archaea cells, the cell membrane also differs from that of other cells in the details of its structure and its use of long isoprenoid chains to make its unique phospholipids with fatty acids.

The differences in cell membranes indicate an evolutionary relationship in which bacteria and eukaryotes developed subsequent to or separately from archaea.

Like all living cells, archaea rely on the replication of DNA to ensure that daughter cells are identical to the parent cell. The DNA structure of archaea is simpler than that of eukaryotes and similar to the bacterial gene structure. The DNA is found in single circular plasmids that are initially coiled and that straighten out prior to cell division.

While this process and the subsequent binary fission of the cells is like that of bacteria, the replication and translation of DNA sequences takes place as it does in eukaryotes.

Once the cell DNA is uncoiled, the RNA polymerase enzyme that is used to copy the genes is more similar to eukaryote RNA polymerase than it is to the corresponding bacterial enzyme. Creation of the DNA copy also differs from the bacterial process.

DNA replication and translation is one of the ways in which archaea are more like the cells of animals than those of bacteria.

As with bacteria, flagella allow the archaea to move.

Their structure and operating mechanism are similar in archaea and bacteria, but how they evolved and how they are built differ. These differences again suggest that archaea and bacteria evolved separately, with a point of differentiation early on in evolutionary terms.

Similarities among members of the two domains can be traced to later horizontal DNA exchange between cells.

The flagellum in archaea is a long stalk with a base that can develop a rotary action in conjunction with the cell membrane. The rotary action results in a whiplike motion that can propel the cell forward. In archaea, the stalk is constructed by adding material at the base, while in bacteria, the hollow stalk is built up by moving material up the hollow center and depositing it at the top.

Flagella are useful in moving cells toward food and in spreading out after cell division.

The main differentiating characteristic of archaea is their ability to survive in toxic environments and extreme habitats.

Depending on their surroundings, archaea are adapted with regard to their cell wall, cell membrane and metabolism. Archaea can use a variety of energy sources, including sunlight, alcohol, acetic acid, ammonia, sulfur and carbon fixation from carbon dioxide in the atmosphere.

Waste products include methane, and methanogenic archaea are the only cells able to produce this chemical.

The archaea cells able to live in extreme environments can be classified depending on their ability to live in specific conditions. Four such classifications are:

• Tolerance for high temperatures: hyperthermophilic.
  
• Able to survive acidic environments: acidophilic.
• Can survive in highly alkaline liquids: alkaliphilic.
• Tolerance for high salt content: halophilic.

Some of the most hostile environments on Earth are the deep-sea hydrothermal vents at the bottom of the Pacific Ocean and hot springs such as those found in Yellowstone National Park. High temperatures in combination with corrosive chemicals are usually hostile to life, but archaea such as ignicoccus have no trouble with those locations.

The resistance of archaea to such conditions has led scientists to investigate whether archaea or similar organisms could survive in space or on otherwise hostile planets such as Mars.

With their unique characteristics and comparatively recent emergence to prominence, the Archaea domain promises to reveal more interesting characteristics and capabilities of these cells, and it may offer surprising revelations in the future.