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Archaea and bacteria are both microorganisms. They are similar in size and shape, but differ greatly upon closer inspection of their genes and proteins. Find out more in this lesson.
Archaea, Bacteria, and Eukarya
All living things can be classified into a place on the Tree of Life. This phylogenetic tree has three major branches, called Archaea, Bacteria, and Eukarya.
Three Major Domains of Life
Tree of Life
A phylogenetic tree traces the evolutionary history of organisms, and indicates common ancestors. We already see a major difference between archaea and bacteria from this classification: they have a different evolutionary history as they occupy very different places on the Tree of Life.
How was this Tree of Life composed? The sequence of 16s ribosomal ribonucleic acid (16s rRNA), a fundamental unit of ribosomes, was compared across organisms. This showed that the underlying genetic code for a component of ribosomes differed greatly between archaea, bacteria, and eukarya. Thus, they form three distinct branches of the Tree of Life.
There was a time when archaea weren’t understood to be different from bacteria and, in fact, were erroneously named archaebacteria (and the Tree of Life was drawn wrong!). But they are definitely not bacteria, even though archaea and bacteria share some similarities. Archaea deserve their own branch on the Tree of Life.
Similarities Between Them
Archaea and bacteria are both prokaryotes, meaning they do not have a nucleus and lack membrane-bound organelles. They are tiny, single-cell organisms which cannot be seen by the naked human eye called microbes. When we look at them through a microscope, we find that archaea and bacteria resemble each other in shape and size. They exist as rods, cones, plates, and coils. Both archaea and bacteria have flagella, thread-like structures that allow organisms to move by propelling them through their environment.
Example of Archaea
Halobacteria
Differences Between Them
An archaea might be very insulted if you mistook it for a bacteria, however, and vice-versa. The differences between archaea and bacteria are profound. As mentioned, the genetic code of rRNA differs enough to place them in quite different branches of the Tree of Life, reflecting differing evolutionary paths. (This still needs to be confirmed by sequencing the 16s rRNA of more organisms.)
Remember how both move via flagella? Despite this functional similarity, and structural similarity (i.e. they look similar), they have very different genes encoding them and are comprised of different proteins. Genome sequencing of archaea also reveals genes that resemble eukaryotes more than bacteria. This is a big difference between archaea and bacteria.
Another distinction between these two prokaryotes is the composition of the cell wall. For example, all bacteria contain peptidoglycans (a molecule composed of both protein and sugar rings) in their cell walls. However, archaea do not have this compound in their cell walls.
Cell division in archaea undergoes distinct processes not found in bacteria. Also, bacteria can form spores that lie dormant for years, until a proper habitat is found in which they can grow. Archaea haven’t been found to do that.
Extreme Environments
Archaea were first identified in extreme environments, like hot deep sea vents, or cold Antarctic oceans. For example, Sulfolobus lives in hot springs and Halococcus is found in pools with high salt content.
For a time, archaea were thought to be extremophiles (organisms that are adapted to extreme environments), while bacteria were thought to favor moderate conditions. For example, bacteria like E.coli, Salmonella, and Lactobacillus can survive in moderate conditions in the human body, where they help or hinder human health.
 
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