Until the 1970s, these single-celled microbes were thought to be bacteria, belonging to the domain Eubacteria. But Archaeans are now known to have some unique characteristics that make them different from other prokaryotes and the eukaryotic cells of plants and animals.
How are Archaea Different from Bacteria?
Members of Archaea (say ARE-key-uh) differ from bacteria in that they do not have the polymer peptidoglycan as part of their cell wall structure. This tough organic molecule is found in nearly all bacterial cells walls, and is important in the differentiation between Gram-positive and Gram-negative bacteria.
In addition to lacking peptidoglycan, Archaeans have unique rRNA sequences. rRNA is the nucleic acid that is a key structural component of ribosomes, organelles that function as the cell’s protein-making machinery. A medically important distinguishing feature of Archaea is that they are not known to cause disease in human or animals.
Since this group of organisms was found to have unique rRNA sequences, and to lack peptidoglycan, they are categorized in their own domain. The three domains, or most basic taxonomic groupings of living things, include:* Eubacteria – Single-celled prokaryotic organisms that have peptidoglycan as part of their cell wall structure.
* Archaea – Single-celled prokaryotic organisms that do not have peptidoglycan in their cell wall and that have unique rRNA sequences.
Eukarya – Single and multi-celled organisms that have nucleated cells with membrane-bound organelles.
The Members of Archaea
Many Archaeans are found in extreme environments; high heat, high salt, high or low pH. Other members of this domain are part of the normal flora (beneficial microbes) of humans and other animals.
The extreme Archaea are grouped into sub-categories based on the type of environment they are found in. The descriptive name they are given ends in “-phile”, which means to love, like or be attracted to.
Those Archaeans referred to as Thermophiles live in environments of high heat, such as in hot springs or around thermal vents. Halophiles thrive in environment with a very high salt content, such as oceans or the Great Salt Lake. Acidophiles and Alkilinophiles prefer low and high pH levels, respectively.
These members of Archaea are strict anaeroebes, meaning that they require an environment free of oxygen. They produce methane gas as an end product of their metabolism. Methane is the stuff of intestinal gas, which humans and other mammals produce a good bit of, depending on their diet.
To learn more about the domain Archaea, see:* The Virtual Microbiology Classroom
* Berkeley’s Introduction to Archaea