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There 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.
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.
Some archaea look like little rods or tiny balls, and some even get around like bacteria, using long hair- or whip-like appendages called flagella that stick out of their cell walls and act like a microscopic outboard motor to get them where they are going.
Like bacteria, archaea lack a true nucleus. Both bacteria and archaea usually have one DNA molecule suspended in the cell's cytoplasm contained within a cell membrane. Most, but not all, have a tough, rigid outer cell wall.
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.
Archaeans dine on a variety of substances for energy, including hydrogen gas, carbon dioxide and sulfur. One type of salt-loving archaean uses sunlight to make energy, but not the way plants do it. This archaean has a light-harvesting pigment in the membrane surrounding its cell. This pigment, called bacteriorhodopsin (back-tear-ee-oh-row-dop-sin), reacts with light and enables the cell to make ATP, an energy molecule.
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.)
Archaea comes from the Greek word meaning “ancient.” An appropriate name, because many archaea thrive in conditions mimicking those found more than 3.5 billion years ago. Back then, the earth was still covered by oceans that regularly reached the boiling point — an extreme condition not unlike the hydrothermal vents and sulfuric waters where archaea are found today.
Some scientists consider archaea living fossils that may provide hints about what the earliest life forms on Earth were like, and how life evolved on our planet.
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.