Difference Between Nitrogen-fixing Bacteria and Denitrifying Bacteria

The key difference between nitrogen-fixing bacteria and denitrifying bacteria is that nitrogen-fixing bacteria convert atmospheric nitrogen into ammonium or ammonia in the soil while denitrifying bacteria convert nitrates in the soil into atmospheric nitrogen.

The nitrogen cycle is one of the main biogeochemical cycles. There are several steps in the nitrogen cycle. Among them, nitrogen fixation and denitrification are two stages. In nitrogen fixation, nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia or ammonium ions in the soil. In denitrification, denitrifying bacteria convert nitrates in the soil into atmospheric nitrogen. Nitrogen-fixing bacteria increase soil fertility. In contrast, denitrifying bacteria reduce soil fertility.

CONTENTS

1. Overview and Key Difference
2. What is Nitrogen-fixing Bacteria 
3. What is Denitrifying Bacteria
4. Similarities Between Nitrogen-fixing Bacteria and Denitrifying Bacteria
5. Side by Side Comparison – Nitrogen-fixing Bacteria vs Denitrifying Bacteria in Tabular Form
6. Summary

What are Nitrogen-fixing Bacteria?

Nitrogen-fixing bacteria are a group of bacteria that convert atmospheric nitrogen or nitrogen gas into ammonia or ammonium ions in the soil. Azotobacter, Bacillus, Clostridium, and Klebsiella are several examples of nitrogen-fixing bacteria. N₂ gas accounts for about 78% of the atmosphere by volume. Atmospheric nitrogen enters the living world by the action of these nitrogen-fixing bacteria. Therefore, they convert N₂ gas into NH₃ and NH4⁺ ions. Then these NH₃ and NH4⁺ ions circulate via the nitrogen cycle, supplying nitrogen to all living organisms. Nitrogen-fixing bacteria use an enzyme called nitrogenase to convert nitrogen gas into ammonia. The nitrogenase enzyme catalyzes the conversion of atmospheric nitrogen into ammonia by the breaking of a triple covalent bond and the addition of three hydrogen atoms to each nitrogen atom. Nitrogen-fixing bacteria work more efficiently in an anaerobic environment.

Some nitrogen-fixing bacteria are free-living soil bacteria (Azotobacter) and free-living cyanobacteria, while some bacteria such as Rhizobium and Bradyrhizobium, etc. live in symbiotic relationships with leguminous plants. Nitrogen fixation is critical for plants growth, development and productivity. Therefore, nitrogen-fixing bacteria increase soil fertility and agricultural productivity.

What are Denitrifying Bacteria?

Denitrifying bacteria are a group of bacteria that converts nitrates in the soil into atmospheric nitrogen gas. This process is called denitrification, and it is one of the major steps of the nitrogen cycle. Denitrifying bacteria participate in releasing fixed nitrogen gas back into the atmosphere and completing the nitrogen cycle. These bacteria utilize several enzymes, including nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase. Bacteria such as Pseudomonas, Alkaligenes, Bacillus and Clostridium, etc. are several examples of denitrifying bacteria. They are mainly facultative anaerobic heterotrophic bacteria. They work under anaerobic or anoxic conditions like waterlogged soils. Moreover, these bacteria live in a wide range of habitats, including extreme environments that are highly saline and high in temperature.

Denitrifying bacteria use nitrate or oxidized nitrogen as their respiratory substrate in the absence of oxygen as a terminal electron acceptor. As a result, nitrate is released as gaseous nitrogen into the atmosphere. Nitrate is the plant accessible form of nitrogen in the soil. Since denitrifying bacteria remove nitrate from the soil, they are responsible for reducing soil fertility and agricultural productivity.

What are the Similarities Between Nitrogen-fixing Bacteria and Denitrifying Bacteria?

• Both nitrogen-fixing bacteria and denitrifying bacteria are an integral part of the nitrogen cycle.
• They are mainly soil microorganisms.
• They use enzymes to convert nitrogen forms.
• Both nitrogen-fixing bacteria and denitrifying bacteria prefer anaerobic environments.

What is the Difference Between Nitrogen-fixing Bacteria and Denitrifying Bacteria?

Nitrogen-fixing bacteria are bacteria that convert free atmospheric nitrogen into ammonia or ammonium in soil. Denitrifying bacteria are bacteria that convert nitrates in the soil to free atmospheric nitrogen. So, this is the key difference between nitrogen-fixing bacteria and denitrifying bacteria. Azotobacter, Bacillus, Clostridium, and Klebsiella are several types of nitrogen-fixing bacteria, while Pseudomonas, Alkaligenes, Bacillus, etc., are several types of denitrifying bacteria.

The below infographic lists more differences between nitrogen-fixing bacteria and denitrifying bacteria in tabular form.

Summary – Nitrogen-fixing Bacteria vs Denitrifying Bacteria

Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia in the soil. Denitrifying bacteria convert nitrates in the soil into free atmospheric nitrogen. Thus, this is the key difference between nitrogen-fixing bacteria and denitrifying bacteria. Since nitrogen-fixing bacteria add nitrogen into the soil, they help in increasing soil fertility and agricultural productivity. In contrast, denitrifying bacteria help in reducing soil fertility and agricultural productivity.

Reference:

1. “Biological Nitrogen Fixation.” Nature News, Nature Publishing Group, Available here.
2. Takaya, Naoki, et al. “Aerobic Denitrifying Bacteria That Produce Low Levels of Nitrous Oxide.” Applied and Environmental Microbiology, American Society for Microbiology, June 2003, Available here.

Image Courtesy:

1. “Root nodules on fava bean plant” By Jengod – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Pseudomonas stutzeri” By שועל – Own work (CC BY-SA 3.0) via Commons Wikimedia