Bacteria in The Nitrogen Cycle

Bacteria play an important role in the nitrogen cycle. Nitrogen enters the living world by way of bacteria, which convert nitrogen gas into reactive nitrogenous compounds. Some of the reactive compounds can be absorbed by plants, which they use for protein and DNA synthesis. As animals eat plants, and animals eat other animals, the nitrogen is passed through the food chain.

Nitrogen-fixing bacteria
Nitrogen-fixing bacteria convert nitrogen gas (N₂) into ammonia (NH₃) as part of fixation. Ammonia is converted to ammonium (NH₄^-) by reacting with molecules in the soil. Ammonium is absorbed by plants via their roots.

Nitrifying bacteria
Ammonia, or ammonium, is used by nitrifying bacteria which produce nitrites. A different kind of nitrifying bacteria are able to convert nitrites to nitrates. This process is known as nitrification. Nitrates are also able to be used by plants.

Denitrifying bacteria
As the nitrogen cycle is a cycle, the nitrogenous compounds must be returned to the atmosphere as N₂. Denitrifying bacteria are a type of anaerobic bacteria which convert nitrates to N₂ during respiration. This stage is known as denitrification

There are three main categories of bacteria. Nitrogen-fixing bacteria in the soil and legume nodules convert nitrogen gas to ammonia. Nitrifying bacteria convert ammonia to nitrites and then nitrates, which can be absorbed by plants. Denitrifying bacteria convert nitrates to nitrogen gas.
Figure 1: The main three types of bacteria involved in the nitrogen cycle