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Learning goals

  1. What are archaea?

  2. Where are they on the tree of life?

  3. Environments: Where are the found?

  4. Families of archaea

What are archaea?

Very small, single celled organisms. Can only be seen through a microscope.

They have no nucleus, nor most other organelles.

People had first thought that they were bacteria. But when we analyzed their genes we discovered that their DNA is more similar to plants and animals than to bacteria.

They don’t belong in the bacteria category, and they’re definitely not animal, plant or fungus. So they are in their own domain.

The name archaea comes from the ancient Greek ἀρχαῖα, meaning “ancient things”.

Locating them on the tree of life

Domains in Tree of Life

from Life in Extreme Environments, http://learn.genetics.utah.edu, Univ. of Utah

Here’s another way to show the same information.

Phylogenetic Tree of Life by Ciccarelli in March 2006 Science

Image by Madeleine Price Ball. Simplified universal phylogenetic tree, made using information from the Interactive Tree of Life. Ciccarelli, et al., Mar 3 2006, Science Vol. 311


Most live in extreme environments, places where we once thought no life could exist.

The environments in which they live are much like Earth’s environment billions of years ago.

The three families of archaea

1. Methanogens: “methane-makers”

Use only CO2 (carbon dioxide), H (Hydrogen) and N (Nitrogen) to produce energy. They give off methane gas.

They live in swamps, marshes, gut of cattle, termites, etc.  They can’t breathe normal air, because they are poisoned by oxygen!

These are decomposers; and can be used in sewage treatment. They may someday be used to produce methane as fuel.

Methane from cows

Image from Dyson College Inst. for Sustainability & the Environment

2. Halophiles: “salt lovers”

Require extremely salty water.  Even the ocean is not salty enough! Some prefer up to 30% salt concentrations! Some live in the Dead Sea (Israel), the Great Salt Lake (Utah), and salt evaporation ponds all over the world.

Some get energy from photosynthesis, like plants. But they do not have chlorophyll like plants. Instead, they have a special colored molecule, a pigment, in their cells called bacteriorhodopsin. It absorbs energy from light, and transfers the energy into molecules in the cell.

Dead Sea Israel

Pic from The Ohio State Univ, Commodity chain project.

3. Thermophiles: “heat / cold lovers”

Some prefer temperatures above 60°C (up to 110°C for hyperthermophiles)


From a PowerPoint by Anjali Malik, Extremophiles

4. Psychrophiles prefer to live near or below freezing.

(Many will die at room temperature).


Thermophiles may live in hot sulfur springs, Yellowstone Park, near volcano vents.

Here we see deep sea hydrothermal vents, “black smokers”

Black smoker GIF

GIF from gfycat.com

Practical uses

Some archaea are part of the nitrogen cycle.

Some archaea are part of the carbon cycle. Especially important as decomposers in anaerobic ecosystems, such as marshes and sewage-treatment works.

Archaea-causing diseases?

There are no known examples of archaea as pathogens.

A few species engage in mutualism: Organisms of different species working together, each benefiting from the relationship.

Their discovery

In 1977, Carl Woese overturned one of the major dogmas of biology. Until that time, biologists had taken for granted that all life on Earth belonged to one of two primary lineages:

* the eukaryotes (which include animals, plants, fungi and certain unicellular organisms such as paramecium)

* and the prokaryotes (all remaining microscopic organisms).

Woese discovered that there were actually three primary lineages. Within what had previously been called prokaryotes, there exist two distinct groups of organisms no more related to one another than they were to eukaryotes.

Because of Woese’s work, it is now widely agreed that there are three primary divisions of living systems – the Eukarya, Bacteria, and Archaea, a classification scheme that Woese proposed in 1990.

“The Archaea are related to us, to the eukaryotes; they are descendants of the microorganisms that gave rise to the eukaryotic cell billions of years ago,” Woese said at the time.


Learning Standards

Massachusetts Science and Technology/Engineering Curriculum FrameworkLife

Science (Biology), Grades 6–8.
Classify organisms into the currently recognized kingdoms according to characteristics that they share. Be familiar with organisms from each kingdom.

Biology, High School
5.2 Describe species as reproductively distinct groups of organisms. Recognize that species are further classified into a hierarchical taxonomic system (kingdom, phylum, class, order, family, genus, species) based on morphological, behavioral, and molecular similarities.

Benchmarks for Science Literacy, American Association for the Advancement of Science

Students should begin to extend their attention from external anatomy to internal structures and functions. Patterns of development may be brought in to further illustrate similarities and differences among organisms. Also, they should move from their invented classification systems to those used in modern biology… A classification system is a framework created by scientists for describing the vast diversity of organisms, indicating the degree of relatedness between organisms, and framing research questions.

SAT Biology Subject Area Test

Evolution and diversity: Origin of life, evidence of evolution, patterns of evolution, natural selection, speciation, classification and diversity of organisms.

Teaching About Evolution and the Nature of Science, National Academy Press (1998)

Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification.

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