Types of ArchaeaThere are three main types of archaea: the crenarchaeota (kren-are-key-oh-ta), which are characterized by their ability to tolerate extremes in temperature and acidity. The euryarchaeota (you-ree-are-key-oh-ta), which include methane-producers and salt-lovers; and the korarchaeota (core-are-key-oh-ta), a catch-all group for archaeans about which very little is known. Among these three main types of archaea are some subtypes, which include: Methanogens (meth-an-oh-jins) — archaeans that produce methane gas as a waste product of their "digestion," or process of making energy. Halophiles (hal-oh-files) — those archaeans that live in salty environments. Thermophiles (ther-mo-files) — the archaeans that live at extremely hot temperatures. Psychrophiles (sigh-crow-files) — those that live at unusually cold temperatures. |
Archaeoglobus
Courtesy Karl Stetter
You can see more images of archaea at the website of Karl Stetter, a German scientist who has found and identified many archaeans (click on the word "Bilder" in lefthand column).
Pyrococcus furiosus grows optimally at 100 degrees Celsisus. Courtesy of K.O. Stetter and R. Rachel, University of Regensburg.
Archaea look and act a lot like bacteria. So much so that until the late 1970s, scientists assumed they were a kind of “weird” bacteria.
Then microbiologist Carl Woese devised an ingenious method of comparing genetic information showing that they could not rightly be called bacteria at all. Their genetic recipe is too different.
So different Woese decided they deserved their own special branch on the great family tree of life, a branch he dubbed the Archaea.
Although many archaea have tough outer cell walls, these walls contain different kinds of amino acids and sugars than those found in bacteria. Archaeal cell membranes also are chemically distinct from bacterial membranes with differing lipid structures and chemical links. This means that drugs that slow or kill bacteria by interfering with their ability to produce certain key proteins have no effect on archaea.
Methanopyrus
Courtesy Karl Stetter
The archaea very much resemble bacteria, so much so that they were once thought to be a weird group of bacteria. However, by studying archaeal cells on a molecular level, scientists have now come to think that these "weird bacteria" actually are a separate category of life altogether. In fact, in some ways, archaea are more like you than they are like bacteria!
Archaeans are single-celled creatures that join bacteria to make up a category of life called the Prokaryotes (pro-carry-oats). Prokaryotes' genetic material, or DNA, is not enclosed in a central cellular compartment called the nucleus. Bacteria and archaea are the only prokaryotes. All other life forms are Eukaryotes (you-carry-oats), creatures whose cells have nuclei. (Note: viruses are not considered true cells, so they don't fit into either of these categories.)
Methanothermus
Courtesy Karl Stetter
However, while archaeans resemble bacteria and have some genes that are similar to bacterial genes, they also contain other genes that are more like what you'd find in eukaryotes. Furthermore, they have some genes that aren't like any found in anything else.
For more details about the differences between bacteria and archaea, see this website. (Note: this site contains some pretty technical information and language.)
Archaeans are among the earliest forms of life that appeared on Earth billions of years ago. It’s now generally believed that the archaea and bacteria developed separately from a common ancestor nearly 4 billion years ago. Millions of years later, the ancestors of today's eukaryotes split off from the archaea. So historically, archaeans are more closely related to us than they are to bacteria.
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