Scientists classify life into three groups, called domains. They are: Eukarya (which includes the plants and animals we usually think of), Bacteria, and Archaea. Eukarya is made up entirely of eukaryotes. It includes unicellular organisms like diatoms, as well as multicellular organisms like sponges, crabs, and turtles. Bacteria and Archaea are prokaryotes. You can learn about the difference between prokaryotic and eukaryotic cells here.
So what types of life does the domain Archaea encompass?
Archaea are tiny prokaryotic organisms that can live in extreme environments. They can be found in deep sea vents and hot springs. They thrive in extremely salty, acidic and hot environments, and anaerobic (without oxygen) environments. They can even be found inside of humans!
Archaea for the most part cannot be classified into species because they are very hard to identify with a microscope. Most scientists sort them into groups based on function and/or structure. The classification of archaea is a relatively new field with a lot of contention, but the two main groups are:
Euryarchaeota- This includes halophiles (archaea that can only survive in very high salt concentrations), thermophiles (archaea that thrive at high temperatures), acidophiles that survive in extreme acid environments, and methanogens that produce methane and aid in human digestion.
Crenarchaeota- A smaller group than Euryarchaeota, made up of organisms that thrive in very hot environments (volcanoes, deep sea vents) and very cold environments (the Antarctic).
How do they get energy?
There are three types of archaea with different metabolism types.
- Phototrophs get energy from sunlight and organic compounds.
- Lithotrophs get energy from inorganic compounds and organic compounds or carbon fixation.
- Organotrophs get energy from organic compounds and sometimes carbon fixation.
Some archaea are autotrophs, which means they produce their own energy. Photoautotrophs produce energy from sunlight, and chemoautotrophs produce energy from organic or inorganic molecules. The counterparts to autotrophs are heterotrophs; they consume energy produced by autotrophs.
Pushing the limits of life
Here are some of the incredibly extreme environments archaea are known to survive in:
- Geysers, hydrothermal vents, hot springs above 212 F/100 C. Some can reproduce in temperatures as high as 252 F/122 C.
- Habitats with salinities (salt concentrations) as high as 20 to 25% (for comparison: the ocean has a salinity of 3.5%!)
- Acidic habitats with a pH as low as 0 (pH of some battery acid or hydrochloric acid).