Archaea – Definition, Examples, Vs Bacteria, Characteristics & Class

What is Archaea? What is its role in our ecosystem? How does it differ from bacteria?

Archaea is a domain of life consisting of single-celled organisms. They are prokaryotes (unlike eukaryotes) and they lack mitochondria. The name archaea comes from the Greek word archaios meaning ancient or

old. Archaea are found everywhere in nature, such as hot springs, oceans, lakes, soil, and even inside us.

Archaea – Definition, Examples, Vs Bacteria, Characteristics & Class

Archaea are very diverse and abundant. They play important roles in our environment. Some species are able to survive extreme temperatures and pressures. Others live in extremely acidic environments. Still others thrive in toxic environments.

Definition of Archaea

Archaea are prokaryotic microorganisms that have no cell nucleus. They are not photosynthetic, but some can use light energy for their metabolism. Archaeal cells do contain ribosomes, which are used to make proteins.

However, these ribosomes are different from those found in Eukarya (eukaryotic cells). There are three main types of archaea: methanogens, halophiles, and acidophiles.

Methanogens produce methane gas; halophiles grow in saltwater; and acidophiles grow at low pH levels.


Methanogens are a group of archaea that produce methane gas. Methane is a greenhouse gas produced by humans when we burn fossil fuels. It contributes to global warming.

Methanogens are also responsible for producing hydrogen sulfide, which smells like rotten eggs. Methanogens are found in the ocean, freshwater, soils, and sediments.

Most of them live in anaerobic conditions. Anaerobic means without oxygen. This includes places with little or no air.



Halophiles are archaea that live in salty water. They are called “halophiles” because they are so tolerant of high concentrations of salts. For example, they can live in seawater up to 10% salinity.

Salts cause problems for other living things. Salt kills plants and animals. But halophiles don’t seem to be affected by high salt concentrations.


Acidophiles are archaea capable of growing under low pH conditions. These include pH levels below 3.5. Low pH causes metals to precipitate out of solution. Metals are minerals that form compounds.

If you look at your fingernails, you’ll see that the outer layer is made of calcium carbonate. That’s a mineral formed from limestone. Limestone is made of calcium carbonates. Calcium carbonate is one type of metal.

When iron dissolves into the water, it forms ferrous ions. Ferrous ions combine with dissolved oxygen to form rust. Rust is another kind of metal. Iron is present in many rocks and soils.

So, if you dig down into the ground, you will find lots of iron. Iron is also present in the earth’s crust. 

In fact, most of the iron on Earth is concentrated in the core. As you go deeper, there is less iron. That’s why the bottom of the ocean has more iron than the top. Acidophiles can tolerate much lower pH levels than halophiles.


Characteristics of Archaea

Archaeans are prokaryotes. Prokaryotes are bacteria and viruses that don’t have a membrane-bound nucleus. Archaeans lack mitochondria, which are organelles found in eukaryotic cells.

Mitochondria convert food molecules into ATP, which is a molecule that provides energy for all life. Unlike mitochondria, archaeal cells don’t need oxygen to survive. Instead, they get energy from chemicals like ammonia, sulfur, and hydrogen.

Some archaeans even use sunlight as part of their metabolism. Like other organisms, archaeans reproduce through binary fission. During this process, two new cells are created. The old cells break apart and become nutrients for the new ones.

Classification of Archaea

The classification system for archaea is similar to that used for bacteria. The major difference is that archaeans do not contain cell walls. Archaeans are classified into three groups: Euryarchaeota, Crenarchaeota, and Thaumarchaeota.

Euryarchaeota contains Archaeoglobus fulgidus. Crenarchaeota contains Pyrobaculum aerophilum. And Thaumarchaeota contains Nitrosopumilus maritimus. There are several subgroups within each class.


Euryarchaeota archaea include methanogens, acidophiles, and haloalkaliphiles. Methanogens produce methane gas. Acidophiles grow best in acidic environments. Haloalkaliphiles can live in alkaline or neutral waters.

Most haloalkaliphile species are halotolerant. That means they can tolerate high salt concentrations.



Crenarchaeotal archaea are thermophilic. Thermophiles thrive in hot temperatures. Most crenarchaeotal archaeans are hyperthermophiles. Hyperthermophiles can withstand temperatures above 100 degrees Celsius (212 degrees Fahrenheit).

Some hyperthermophiles can survive at temperatures over 200 degrees Celsius (392 degrees Fahrenheit).


Thaumarchaeotal archaea contain ammonia-oxidizing archaea. Ammonia oxidizers can use ammonia as an electron acceptor. They create nitrate from ammonium. This reaction produces energy.

Other types of archaea use hydrogen sulfide instead of ammonia. Hydrogen sulfide is toxic to most living things. But some archaeans can use it as an energy source.

Thaumarchaeotal archaea live in extreme environments. They’re called extremophiles because they can survive in very harsh conditions. 

For example, some thaumarchaeotal archaeans can live in boiling water. Others can live without oxygen. One group of thaumarchaeotal archaebacteria lives in hydrothermal vents.

Hydrothermal vents are places where deep sea water comes up to the surface. It gets heated by volcanic activity. At these sites, archaea may be able to use geothermal heat to power their metabolism.

Examples of Archaea

Acidophilic Archaea

1. Acidianus manzaensis

This archaean lives in volcanic vents near Lake Manza in Japan. It grows best between pH 4.0 and 5.0. At these low pH levels, arsenic is soluble. Arsenic is a poisonous element. It’s usually found in soil and rock. It’s also found in coal.

Coal is fossilized plant material. The ash left behind after burning coal is called coke. Coke is used to make steel. Steel is a very important industrial product. It’s used to build bridges, buildings, cars, ships, and airplanes.

2. Halorhabdus utahensis

This archaean lives in salt lakes in Utah. It thrives in salty water with a high concentration of chloride ions. Chlorides are salts containing chlorine atoms.

Saltwater has higher concentrations of chlorides than freshwater does. In addition to being good for growing archaeans, saltwater is great for swimming!

3. Natronococcus occultus

Natronococcus occultus grows best in highly saline solutions. These solutions are made up of sodium and magnesium. Sodium and magnesium are both minerals. Magnesium is often combined with calcium.

Calcium is another mineral. Calcium is present in bones, teeth, shells, and seashells. It’s also found inside many foods. For example, milk is mostly made up of calcium.

4. Sulfolobus islandicus

Sulfolobus Islandicus lives in hot springs on the Pacific Ocean floor off the coast of Hawaii. It grows best at temperatures around 80 degrees Celsius (176 degrees Fahrenheit).

Many other archaeans prefer even hotter temperatures. But Sulfolobus prefers cooler conditions.

5. Thermoproteus tenax

Thermoproteus tenax lives in deep, hot ocean trenches near hydrothermal vents. Hydrothermal vents are places where hot fluids come out of the ground. Sometimes they’re called geysers.

Geysers are not always located next to volcanoes. Sometimes geysers form when magma rises through cracks in the Earth’s crust. When this happens, hot liquid flows out of the ground.

Hot liquids have different colors depending on what elements they contain. Red means iron. Yellow means sulfur. Orange means copper.

Black means carbon. Green means oxygen. Brown means nitrogen. Purple means phosphorus. Gray means silica. White means hydrogen. And black means helium.

6. Woesearchaeota euryarchaeota

Woesearchaeota Euryarchaeota live in hot springs. Some of them thrive in extremely acidic environments. Others grow best in alkaline conditions. All archaeans need a lot of energy to survive.

So they must find ways to get more energy. One way they do that is by using heat or chemicals. Another way is by eating other organisms. Still others eat sunlight.

7. Methanosarcina barkeri

Methanosarcina Barkeri lives in marine sediments. It can use methane gas as its source of energy. It gets this energy from organic matter decaying in the mud. Organic matter contains lots of carbon compounds. That’s why it gives off methane gas.

8. Methanopyrus kandleri

Methanopyrus Kandleri lives in marine sediments on the bottom of the ocean. Scientists aren’t sure how fast it metabolizes methane because it hasn’t been studied much.

However, scientists think that it may be able to metabolize methane faster than Methanosarcina barkerii.

9. Nanohaloarchaeum shimadai

Nanohaloarchaeum Shimadai lives in hypersaline ponds. Hypersalinity refers to having too much salt in your body. Your blood usually has about the same amount of salt as pure water.

The only reason hypersaline ponds exist is because there are areas of the world with very salty groundwater. Groundwater moves toward the lowest spots in the ground. This means that hypersaline lakes can sometimes occur unexpectedly.

10. Halorubrum lacusprofundi

Halorubrum Lacusprofundi lives in hypersaline lakes around the world. In these bodies of water, there is so much salt that all the other chemical elements disappear. Hydrogen, oxygen, nitrogen, carbon, and halogen remain.

These five elements create over 20,000 known substances.

Acidophilic Archaea

Halophilic Archaea

1. Haloquadratum walsbyi

Haloquadratum Walsbyi lives inside rocks throughout the world. Rocks are mostly made of minerals that were formed many millions of years ago. Over time, tiny creatures such as bacteria grew into massive structures that we now call stones.

If you look at most pieces of stone, you will see holes and pits. There are some types of haloarchaea that live in these holes and pits. They help break down rock even further.

2. Natronomonas pharaonis

Natronomonas Pharaonis lives in hot springs across the world. Most people don’t realize that hot springs are actually natural pools filled with mineral rich water.

At one point, hot spring water was thought to heal burns and sores. Now we know that it makes great swimming pool water!

3. Halobacterium salinarum

Halobacterium Salinarum lives in high-salt concentrations. It uses sodium chloride (table salt) for part of its metabolism. When it grows, it combines hydrogen ions and sodium ions together to produce electricity.

A simple little reaction but it needs an extreme environment to work.

4. Haloferax volcanii

Haloferax Volcanii lives in high-concentration salt solutions. Its cells are full of small bubbles. Each bubble acts like a miniature power plant.

Inside each bubble is another DNA molecule that controls the function of the cell. What allows H. volcanii to survive in such a place is still unknown.

5. Halococcus dombrowskii

Halococcus Dombrowskii lives in extremely high-salt environments. Some species of halococci have been shown to use magnesium instead of sodium.

Magnesium produces less chlorine when it reacts with water. This allows them to live in more concentrated salt environments.

6. Natrinema pellirubrum

Natrinema Pellirubrum lives in alkaline vents near deep undersea volcanoes. Their proteins contain lots of different amino acids, which allow them to build large amounts of protein. This allows them to thrive in this harsh environment.

7. Haloterrigena turkmenica

Haloterrigena Turkmenica lives in high-saltsphere hydrothermal vents. They use iron sulfate to store energy from their surroundings. This helps them get through long winters where they would not be able to find food otherwise.

8. Natronocaldarachnion sodhrogyii

Natronocaldarachnio Sodhrogyii lives in high-sodium habitats. Sodium is used by some organisms to make enzymes that can break apart complex molecules.

9. Halomicrobium mukohataei

Halomicrobium Mukohataei lives in very salty mudflats. The mud contains a lot of organic material that is decaying. The microorganisms living in the mud use the organic matter as nutrients.

These microbes include Halomicrobium, Halorhabdus, and Nitrosopumilus.

10. Natronofaba magna

Natronofaba Magna lives in hypersaline lakes around the globe. There are 40 species of Natronofaba known to science. They all live within the same genus (Natronofaba), and there are no other known genera of haloarchaea.

All Nattronofaba species have flagella that enable them to move around on solid surfaces.

Halophilic Archaea

Methanogenic Archaea

1. Methanosarcina barkeri

Methanosarcina Barkeris lives in marine sediments. It has a special ability to create methane gas.

2. Methanothermobacter thermautotrophicus

Methanothermobacter Thermautotrophicus lives in hot vent areas. It gets its name because it metabolizes without oxygen.

3. Methanospirillum hungatei

Methanospirillum Hungatei lives in marine sediments and eats hydrogen. Hydrogen is produced naturally as a waste product of photosynthesis.

4. Methanococcoides burtonii

Methanococcoides Burtonii live in marine sediments in places like Hawaii. The bacteria live off hydrogen and carbon dioxide.

5. Methanobrevibacter smithii

Methanobrevibacter Smithii lives in the human gut. It uses hydrogen to produce methane.

6. Methanogenium frigidum

Methanogenium Frigidum lives in polar regions. It gets its name for being cold-adapted.

7. Methanosaeta concilii

Methanosaeta Concilii lives in anaerobic digesters. Anaerobic digestion is the process of breaking down organic wastes into biogas.

8. Methanothrix soehngenia

Methanothrix Sohngenia lives in marine sediments, eating hydrogen.

9. Methanolinea tardae

Methanolina Tardae lives in marine sediments eating hydrogen.

10. Methanometabolon trichophorum

Methanometabolon Trichophorum lives in anoxic environments. Anoxia is when there is no oxygen present.

Main Characteristics of Archaea (Vs. Bacteria)

1. Archaea have cell walls made of lipids instead of peptidoglycan.

2. They don’t divide into two new cells during reproduction. Instead, one parent cell gives rise to two identical daughter cells which remain attached.

3. Archaea usually lack mitochondria because their ancestor lost it after evolving anaerobic respiration.

4. Most archaeal species cannot produce ATP via oxidative phosphorylation.

5. The genetic code used by most archaea differs from the standard code used by bacteria and eukaryotes.

6. Archaea possess a unique type of DNA polymerase.

7. Their ribosomes differ significantly from those of bacteria and eukaryotic organisms.

8. Archaea are classified in three domains: Crenarchaeota, Euryarchaeota, and Thaumarchaeota.

9. Archaea account for 1-2% of all living organisms.

10. Archaea inhabit extreme habitats such as hot water, acid lakes, volcanic areas, and salt mines.

11. Archaea may be the earliest life forms to exist on earth.

12. Archaea are thought to have evolved about 2 billion years ago.

13. Archaea were first discovered in 1877 by German scientist Ferdinand Cohn.

14. In 1977, Carl Woese and Stanley Miller proposed that Archaea should be placed in a separate domain of life known as the “Euryarchaeota.”

15. Archaea are sometimes referred to as the third domain of life.

16. Archaea are also called extremophiles because they can tolerate very harsh conditions.

17. Archaea are often found in extreme environments like hot springs, acid lakes, and oil wells.

18. Archaea are also found in human intestines.

19. Archaea are found in many kinds of food including meat, dairy products, eggs, and vegetables.

20. Archaea are considered to play important roles in the global carbon cycle.

21. Archaea help decompose dead plants and animals. This helps release nutrients back into the soil so plants can grow.

22. Archaea are responsible for producing some medicines. For example, penicillin was produced by a strain of Penicillium mold.

23. Archaea are involved in making plastics.

24. Archaea are used in industrial processes. For instance, methanol is used in the production of synthetic fibers and polyester textiles.

25. Archaea are also used in the treatment of wastewater.

26. Archaea are used to make fuel. For example, hydrogen is produced when ammonia is heated.

The main difference between Archaea and Bacteria is that Archaea have cell walls composed of lipids rather than peptidoglycan. Also, Archaea do not reproduce by dividing into two new cells but instead reproduce by budding off of one parent cell.

Another major difference between these two groups of organisms is that Archaea do not contain mitochondria. These organelles are present in almost every other organism on Earth except Archaea.

Because of this, Archaea are sometimes referred to as the “ancient” or “primitive” organisms.

Main Characteristics of Archaea (Vs. Bacteria)

Archaea are divided into three different phyla based on how their cells look under a microscope. These three phyla are the Crenarchaeota (also called thermoacidiphilic), Euryarchaeota (or eurythermophilic) and Thaumarchaeotal.

The Crenarchaeota are anaerobic organisms with cell membranes made up of lipids. They live at temperatures ranging from 50 °C to 90 °C and pH levels ranging from 3 to 9.

The Euryarchaeota are aerobic organisms with cell membranes made of peptidoglycan. They live at temperatures of 65 °C to 95 °C and pH levels between 1 and 8.

Finally, the Thaumarchaeota are anoxic archaea. Their cell membrane contains no proteins and is made up of lipids only. They live at temperatures between 75 °C and 100 °C and pH levels below 2.5.

Final Thoughts

There are over 10,000 species of Archaea. Most of them are bacteria. However, there are about 40 species of Archaea that are classified as extremophiles.

Extremophiles are organisms that thrive in freezing, hot, acidic, alkaline, high pressure, toxic, radioactive, etc.

There are several types of extremophiles including psychrophiles, thermophiles, halophiles, barophiles, piezophiles, hyperthermophiles, acidophiles, alkaliphiles, and haloalkaliphiles.

Many of these organisms are found in extreme environments such as deep sea vents, volcanoes, glaciers, deserts, arctic regions, and even inside our bodies!

A few examples of extremophile organisms include:

1. Deep-sea vent worms that survive in hydrothermal vents.

2. Hot springs in Yellowstone National Park where geysers erupt at temperatures of 130 °F.

3. Halophiles which live in saltwater.

In conclusion, Archaea are fascinating organisms that we know little about. We don’t even know what most of them eat! However, it’s clear that they play an important role in our environment. I hope you enjoyed learning more about Archaea today!

Jennifer Dawkins