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There is an entire domain of life that is made up of single-celled organisms without nuclei.

We’re not referring to the Bacteria domain, however. In this lesson, we will examine the main characteristics of the least commonly known domain, the Archaea.

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The Tree of Life

Phylogenetic tree of life
Tree of Life

Take a look at this phylogenetic tree of life. This is a simplified version of the evolution of life on Earth. The black line at the bottom of the screen represents the universal ancestor of all organisms.

As you move up the trunk of the tree and into the branches, time moves forward. Each fork in the tree represents a point in evolutionary time where two groups of organisms became distinct. Branches that are close to one another are more closely related to one another than to more distant branches.Look at the left major branch labeled ‘Bacteria.’ I’m sure you’re familiar with bacteria and have at least heard of a couple of these purple branches. For example, the iconic E.

coli is part of the branch labeled ‘Proteobacteria.’ Now look at the major branch farthest to the right. ‘Eukarya’ might be a new addition to your vocabulary, but I’m sure you know what most of these branches represent. This side of the tree holds all protozoa, fungi, plants, and animals, including you.But what about that red group in the middle: the Archaea? The names with those branches are probably all foreign to you.

So let’s take a closer look and find out just what the Archaea are all about.

Definition of Archaea

If I had to succinctly define the Archaea, I would probably say that it is a diverse phylogenetic domain of prokaryotes, distinct from Bacteria and Eukarya, that includes many extremophiles.I’m sure that sounds complicated and a bit unhelpful but, by the end of this lesson, that definition will make more sense.

Let’s start breaking it down by looking at some of the characteristics of Archaea.Looking at the phylogenetic tree again, you can see that the Archaea domain is on the right, next to the Eukarya branch. This means that despite the Archaea being prokaryotic and unicellular, like bacteria, they are genetically closer to the Eukarya.

This statement can be a little deceptive, however. Scientists that examine the genomes of the Archaea have found that they are quite different than either Bacteria or Eukarya. In fact, there are many genes found only in Archaea that have no known function. There could be very novel biological and biochemical processes going on in this group that greatly expand the limits of what is considered biologically possible.Many Archaea are extremophiles, providing some clues to the function of some of those unique genes. An extremophile is an organism that has evolved to thrive under chemical or physical extremes.

So an extremophile is an organism that loves extreme chemical or physical conditions. These extremes can include very high or low temperatures, the ends of the pH spectrum, or solutions with high salt content.Undoubtedly, some of the novel genes found in Archaea are responsible for their ability to survive in crazy environments. Now, not all Archaea are extremophiles; some are perfectly happy at normal environmental conditions like those found in soil, lakes, and oceans. But let’s talk about a few of the extremophile groups.


Hyperthermophiles are organisms that grow best at temperatures above 80 degrees Celsius. The Archaea Pyrolobus fumarii thrives in incredibly deep water at the bottom of the ocean in hydrothermal vent chimneys that can be well over the boiling point of water. Pyrolobus can actively grow at 113 degrees Celsius and can survive in an autoclave, a device that uses heat and pressure to sterilize equipment, at 121 degrees Celsius for an hour. These are conditions that even the toughest bacterial endospores can’t survive.

Extreme Halophiles

Extreme halophiles are organisms that require salt for growth and can survive in high-salt environments. These Archaea grow best between 12 and 23% salt but can survive in a saturated salt solution of 32%.

As a comparison, seawater is only 3.5% salt and most bacteria are unable to grow in seawater because it is too salty. The Great Salt Lake, pools of evaporating seawater like the San Francisco Salt Ponds, and the surfaces of salted meats and fish all have very high salt concentrations that extreme halophiles can survive.In fact, the extreme pink color of some salt ponds, like those in San Francisco, is actually due to the pink pigments produced by the Archaea growing in the water. Extreme halophiles, like Halobacterium and Natronobacterium, are only able to thrive in these high-salt environments.One note here: Archaea in this group are able to survive salt-based food preservation methods like used with meats and fish.

But no pathogenic Archaea have been discovered yet, so you are not going to get sick from consuming these organisms. As is the case in science, this could change at any time. But it seems that Archaea did not evolve to cause disease in mammals.


There are also Archaea that are thermoacidophiles.

These organisms grow best at high temperatures and extremely low pH. For many Archaea, this means a pH less than 2, the same pH as stomach acid. One genus of Archaea, called Picrophilus, can even survive pH values below zero. Many geothermal hot springs have very low pH due to the high sulfuric acid content. Even the combination of harsh environmental conditions like heat and acid can’t seem to stop the Archaea from growing happily.

Diverse Metabolism

All Archaea are chemotrophic, meaning they use chemicals to obtain energy. Some use organic compounds, like sugars, while others use inorganic compounds, like iron.

One of the most common inorganic energy sources used by Archaea is hydrogen gas. An exception to the chemotrophic rule is Halobacterium. It can also capture energy from light, kind of like photosynthesis, but not the typical form we associate with bacteria, algae, and plants.Some Archaea are able to grow happily while respiring oxygen, while other Archaea will die instantly when exposed to oxygen and use a different chemical for respiration. Many of the thermophilic Archaea are able to metabolize sulfur in one way or another, with many using sulfur in place of oxygen. Some can also use sulfate or nitrate for respiration.

Deep-sea hydrothermal vents typically spew high quantities of sulfur that is exploited by the Archaea living in the vent environment.All organisms need both a source of energy and a source of carbon. Since all of the cell components are carbon-based molecules, cells need ready access to carbon atoms that can be used as building blocks within the cell. Many of the Archaea are autotrophs, which means they are able to chemically convert carbon dioxide into organic molecules that can be used by the cell.Archaea are famous for being able to grow using hydrogen and carbon dioxide while producing methane, also known as natural gas, as a waste product.

In fact, some of the natural gas deposits mined from the Earth consist of gas formed by Archaea over many years. These Archaea are referred to as methanogens, or in other words ‘methane-generators.’ These organisms play an important role in decomposing organic matter in nature.

Lesson Summary

Let’s review.The phylogenetic tree of life is divided into three domains: Bacteria, Eukarya, and Archaea. The Archaea are a diverse group of unicellular prokaryotes. Members of this group have evolved to exploit some very extreme environments.

Some are hyperthermophiles that grow best at temperatures above 80 degrees Celsius. Hot springs and hydrothermal vents can be teeming with this type of Archaea. Extreme halophiles are able to survive in areas with salt concentrations up to 10 times that of seawater.

The Great Salt Lake and salt-preserved meats can contain halophilic Archaea.Thermoacidophiles prefer environments with high temperatures and very low pH. Some species can survive below pH zero.

Archaea can grow and generate energy using a variety of different metabolic tactics. Many Archaea use inorganic substances, like hydrogen gas and sulfur, for growth.Scientists believe that the ability for Archaea to grow in extreme environments using inorganic substances, like sulfur and hydrogen, gives us clues to how life evolved on the planet. It is not hard to imagine ancient relatives of the Archaea thriving during the hot, harsh chemical conditions on early Earth.

Learning Outcomes

Once you have watched this lesson, you should be prepared to:

  • Define Archaea and understand what differentiates them from Bacteria and Eukarya
  • Identify and describe the different categories of extremophiles
  • Summarize the different metabolisms of archaea, including autotrophs and methanogens

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