All archaea are single-celled organisms. They have prokaryotic cells but are thought to be more closely related to eukaryotes than they are to bacteria. Archaea have many characteristics that they share with both bacteria and eukaryotes. They also have many unique features.
Structure of archaea
Archaea are structurally very diverse and there are exceptions to most of the general cell features that I describe here.
As archaea are prokaryotic organisms, they are made from only one cell which lacks a true nucleus and organelles. They are generally of similar size and shape to bacteria cells. Other physical similarities they share with bacteria include a single ring of DNA, a cell wall (almost always) and often the presence of flagella.
Unlike bacteria, archaea are unaffected by antibiotics. Their cell walls are structurally different to those of bacteria and are not vulnerable to attack from antibiotics.
Archaea cells have unique membranes. The membranes of bacteria and eukaryotic cells are made from compounds called phospholipids. These phospholipids have non-branching tails. Archaeal membranes are made of branching lipids. The presence of branching lipids greatly alters the structure of the membranes of archaeal cells.
Where are archaea found?
Archaea were originally only found in extreme environments which is where they are most commonly studied. They are now known to live in many environments that we would consider hospitable such as lakes, soil, wetlands, and oceans.
Many archaea are extremophiles i.e lovers of extreme conditions. Different groups thrive in different extreme conditions such as hot springs, salt lakes or highly acidic environments.
Archaea that live in extremely salty conditions are known as extreme halophiles – lovers of salt. Extreme halophiles are found in places such as the Dead Sea, the Great Salt Lake and Lake Assal which have salt concentrations much higher than ocean water.
Other organisms die in extremely salty conditions. High concentrations of salt draw the water out of cells and cause them to die of dehydration. Extreme halophiles have evolved adaptations to prevent their cells from losing too much water.
Archaea that are found in extremely hot environments are known as extreme thermophiles. Most organisms die in extremely hot conditions because the heat damages the shape and structure of the DNA and proteins found in their cells. Extreme thermophiles struggle to grow and survive in moderate temperatures but are known to live in environments hotter than 100 ℃.
Acidophiles are organisms that love highly acidic conditions such as our stomachs and sulfuric pools. Acidophiles have various methods for protecting themselves from the highly acidic conditions. Structural changes to the cellular membranes can prevent acid entering their cell. Channels in the membrane of their cell can be used to pump hydrogen ions out of the cell to maintain the pH inside the cell.
Methanogens are a group of archaea that produce methane gas as a part of their metabolism. They are anaerobic microorganisms that use carbon dioxide and hydrogen to produce energy. Methane is produced as a byproduct.
Methanogens are anaerobic archaea and are poisoned by oxygen. They are commonly found in the soil of wetlands where all the oxygen has been depleted by other microorganisms. They are also found in the guts of some animals such as sheep and cattle. Methanogens found in the guts of animals help with the digestion of food. Methanogens are also used to treat sewage.
Each year, methanogens release around two billion tonnes of methane into the atmosphere. Methane is a greenhouse gas involved in global warming and climate change. The concentrations of methane are increasing around 1% each year partly due to human activities that involve methanogenesis such as cattle farming and rice production.
Different groups of archaea
Very little is known about the evolutionary tree of the Domain Archaea. Currently, it is separated into four evolutionary groups which are likely to change as we discover more about these microscopic organisms. The four current clades of archaea are Korarchaeotes, Euryarchaeotes, Crenarchaeotes, and Nanoarchaeotes.
Euryarchaeotes are one of the best-known groups of archaea. It includes a range of extreme halophiles (lovers of salt) and all methanogens. Some of these extreme halophiles are used in commercial salt production to help speed up the evaporation of saltwater ponds.
Some euryarchaeotes have a unique way of using light energy to produce food. Instead of using the well-known pigments, such as chlorophyll a, some euryarchaeotes use a combination of a protein and a pigment called retinal to trap light energy. Retinal is also a key molecule involved in vision for animals.
Crenarchaeotes and euryarchaeotes are the two best-known groups of archaea. This group includes the majority of the known thermophiles (lovers of heat). They most commonly live in hot or acidic environments. For example, they can be found in highly acidic, hot sulfur springs in temperatures over 75 ℃.
The first korarchaeotes were discovered in a hot spring in 1996 in Yellowstone National Park. They have since been discovered around the world but so far only in hot springs and deep sea hydrothermal vents.
Nanoarchaeotes are the most recently discovered archaea. They were first discovered in 2002 in Iceland. The nanoarchaeotes are some of the world’s smallest organisms. They are parasites that grow attached to a crenarchaeote cell. Since 2002, they have been discovered in various places around the world including Siberia, Yellowstone National Park and deep in the Pacific Ocean.
Last edited: 19 March 2018
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