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7.2: Prokaryote Classification

Difficulty Level: At Grade Created by: CK-12
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With so many different bacteria, how are they all classified?

By shape? By size? By some other criteria? As you can imagine, classifying bacteria is probably not an easy task. Bacteria are classified by their traits, some of which have to do with their shape, others with the cell wall, and even additional traits.

The Prokaryotic Domains

Domain Bacteria

Bacteria are the most diverse and abundant group of organisms on Earth. They live in almost all environments. They are found in the ocean, the soil, and the intestines of animals. They are even found in rocks deep below Earth’s surface. Any surface that has not been sterilized is likely to be covered with bacteria. The total number of bacteria in the world is amazing. It’s estimated to be 5 × 1030, or five million trillion trillion. You have more bacteria in and on your body than you have body cells!

Bacteria called cyanobacteria are very important. They are bluish green in color (see Figure below) because they contain chlorophyll. They make food through photosynthesis and release oxygen into the air. These bacteria were probably responsible for adding oxygen to the air on early Earth. This changed the planet’s atmosphere. It also changed the direction of evolution. Ancient cyanobacteria also may have evolved into the chloroplasts of plant cells.

Cyanobacteria Bloom. The green streaks in this lake consist of trillions of cyanobacteria. Excessive nutrients in the water led to overgrowth of the bacteria.

Thousands of species of bacteria have been discovered, and many more are thought to exist. The known species can be classified on the basis of various traits. One classification is based on differences in their cell walls and outer membranes. It groups bacteria into Gram-positive and Gram-negative bacteria, as described in Figure below.

Classification of Bacteria. Different types of bacteria stain a different color when stained with Gram stain. This makes them easy to identify.

Domain Archaea

Scientists still know relatively little about Archaea. This is partly because they are hard to grow in the lab. Many live inside the bodies of animals, including humans. However, none are known for certain to cause disease.

Archaea were first discovered in extreme environments. For example, some were found in hot springs. Others were found around deep sea vents. Such Archaea are called extremophiles, or “lovers of extremes.” Figure below describes three different types of Archaean extremophiles. The places where some of them live are thought to be similar to the environment on ancient Earth. This suggests that they may have evolved very early in Earth’s history.

Extremophile Archaea. Many Archaea are specialized to live in extreme environments. Just three types are described here.

Archaea are now known to live just about everywhere on Earth. They are particularly numerous in the ocean. Archaea in plankton may be one of the most abundant types of organisms on the planet. Archaea are also thought to play important roles in the carbon and nitrogen cycles. For these reasons, Archaea are now recognized as a major aspect of life on Earth.


  • Bacteria live in virtually all environments on Earth.
  • Archaea live everywhere on Earth, including extreme environments.


Use this resource to answer the questions that follow.

  1. How do eubacteria and archaebacteria differ? Describe these differences.
  2. Describe the extreme environments of archaebacteria.


1. Distinguish between Gram-positive and Gram-negative bacteria, and give an example of each.

2. Summarize the evolutionary significance of cyanobacteria.

3. What are extremophiles? Name three types.

4. Compare and contrast Archaea and Bacteria.

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extremophile Any type of Archaea that lives in an extreme environment, such as a very salty, hot, or acidic environment.
Gram-negative Bacteria that stain red with Gram stain; have a thin cell wall with an outer membrane.
Gram-positive Bacteria that stain purple with Gram stain; have a thick cell wall without an outer membrane.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Nov 22, 2016
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