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Nitrogen Fixing Bacteria: Role, Definition & Examples

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  • 0:00 Nitrogen Fixing Bacteria
  • 0:20 Fertilizing the World's Plants
  • 1:00 From Air to Soil
  • 1:45 Two Forms of Bacteria
  • 2:15 Examples
  • 3:35 Lesson Summary
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Lesson Transcript
Instructor: Adrienne Brundage
Bacteria that change nitrogen gas from the atmosphere into solid nitrogen usable by plants are called nitrogen-fixing bacteria. These bacteria are found both in the soil and in symbiotic relationships with plants.

What Are Nitrogen-Fixing Bacteria?

Nitrogen-fixing bacteria are microorganisms present in the soil or in plant roots that change nitrogen gases from the atmosphere into solid nitrogen compounds that plants can use in the soil. That's a mouthful! Let's break this concept down.

Who Fertilizes the World's Plants?

It's spring again, and you find yourself standing in the gardening section of your local home improvement store, looking for the very best fertilizers. You know those flowers in your front yard need a good source of nitrogen to grow big and strong, so you open your wallet for a big bag of Miracle Grow each and every spring.

But wait! Who buys the fertilizer for those endless fields of wild flowers, those rolling hills, or those vast expanses of rain forest? Surely all the plants in the world need a good source of nitrogen, right? So where do they get it? Since the Miracle Grow company hasn't been around since plants began to evolve, something else had to take up the slack. Enter bacteria.

From Air to Soil

Nitrogen is commonly found as an inert, or nonreactive, gas in the earth's atmosphere. In fact, 78% of the earth's atmosphere is nitrogen in its gaseous form. This element is so plentiful that it has been incorporated into all living things on Earth. In order for this to happen, though, that nitrogen gas needs to turn into something solid that can be used by living things.

A group of bacteria called nitrogen-fixing bacteria take on this challenge. These bacteria take nitrogen from the air and make it react with other compounds and make it solid in the process. This process of taking away the gaseous form of nitrogen and turning it solid is called nitrogen fixation.

Two Forms of Bacteria

Have you ever worked at a large company? If so, you know that different jobs are done by different people. The same thing goes for the nitrogen-fixing bacteria. There are two major forms: free-living bacteria, which live throughout the soil, and mutualistic bacteria, which live in nodules in the roots of certain plants like beans and peas. These two types of bacteria are responsible for fixing 90% of the nitrogen on Earth.

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