What Are Denitrifying Bacteria and How Do They Work
Let us know the meaning of denitrification before learning about the denitrifying bacteria. Denitrification is the process to remove nitrates or nitrites from the soil, water, and air by chemical reduction. Denitrifying bacteria are microorganisms whose action results in the conversion of nitrates in the soil to free atmospheric nitrogen. Some examples of denitrifying bacteria are Thiobacillus denitrificans, Micrococcus denitrificans, and some species of Serratia, Pseudomonas, and Achromobacter.
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Pseudomonas aeruginosa is a species of denitrifying bacteria, which carries out denitrification in the absence of oxygen (anaerobic conditions) like in swampy or waterlogged soils and reduces the amount of fixed nitrogen by up to 50 per cent.
Examples of the Nitrifying and Denitrifying Bacteria
Some examples of nitrifying bacteria are- Nitrosomonas, Nitrosococcus, Nitrosolobus, and Nitrosospira.
Some examples of denitrifying bacteria are- Pseudomonas, Thiobacillus denitrificans, Serratia, Achromobacter, and Micrococcus denitrificans.
Denitrification Mechanism
Denitrifying bacteria use denitrification to generate ATP, and the most common denitrification process is given below, with the nitrogen oxides being converted back to gaseous nitrogen:
2 NO3-+ 10 e- + 12 H+ → N2 + 6 H2O
It results in one molecule of nitrogen and six molecules of water.
Denitrifying bacteria are part of the nitrogen cycle, and they convert nitrates in the soil to free atmospheric nitrogen. The above reaction represents the half-reaction of the process of denitrification. This reaction can be further classified into two different half-reactions and each reaction requires a specific enzyme. The transformation from nitrate to nitrite is performed by the enzyme nitrate reductase (NAR).
NO3− + 2 H+ + 2 e− → NO2− + H2O.
Nitrite reductase (Nir) converts nitrite into nitric oxide.
2 NO2− + 4 H+ + 2 e− → 2 NO + 2 H2O.
Then, Nitric oxide reductase (Nor) converts nitric oxide into nitrous oxide.
2 NO + 2 H+ + 2 e− → N2O + H2O.
Further, Nitrous oxide reductase (Nos) terminates the reaction by converting nitrous oxide into dinitrogen.
N2O + 2 H+ + 2 e− → N2 + H2O.
It is very important to notice that any of the products produced at any step can be exchanged with the soil environment.
Role of Denitrifying Bacteria
Denitrifying bacteria play an important role in the oxidation of methane (where methane is converted into carbon dioxide, water, and energy) in deep freshwater bodies. This is very important because methane is the second most significant pollutant of the greenhouse, and it has a global warming potential of 25 times more than carbon dioxide. Freshwaters are the major contributors to methane emission globally.
To understand it, a study was conducted on Europe's lakes and they found that the anaerobic oxidation is coupled to denitrification. It is also referred to as nitrate or nitrite dependent anaerobic methane oxidation (n - damo) and it is a dominant sink of methane in deep lakes. For a long period, it was believed that the mitigation of methane emission was only due to aerobic methanotrophic bacteria. The oxidation of methane takes place in the anoxic or oxygen-depleted zones of freshwater bodies. In the case of this experiment, this is carried out by M. oxyfera-like bacteria. This bacteria is similar to Candidatus Methylomirabilis oxyfera, which acts as a denitrifying methanotroph.
The result came out from the study of Constance lake, that nitrate was depleted in the water at the same length as the methane. This suggests that methane oxidation was coupled with denitrification. In this experiment, Methylomirabilis oxyfera - like bacteria carried out the methane oxidation because their abundance peaked at the same depth. Where the methane and nitrite profile met. N - damo process is very significant because it helps in decreasing the emission of methane from deep freshwater bodies. It also helps in turning nitrates into nitrogen gas, reducing excess nitrates.
Role of Nitrifier Denitrification
Nitrifier denitrification serves as a path of nitrification in which ammonia (NH3) is oxidized to nitrite (NO2−) and further by the reduction of NO2− to nitric oxide (NO), nitrous oxide (N2O), and molecular nitrogen (N2). All these transformations are carried out by autotrophic nitrifiers.
The Function of Denitrifying Bacteria
The main function of denitrifying bacteria is to give out nitrogen gas by converting the nitrate and nitrite, nitrogen gas re-enters into the atmosphere with the help of this process. Nitrogen further enters the ocean through fertilizers, where it enters into the marine food web.
Use of Denitrifying Bacteria for Wastewater Treatment
Denitrification bacteria are one of the important components to treat wastewater. as wastewater mainly contains a large amount of nitrogen, which might be in the form of ammonium or nitrate. Nitrogen could be damaging to human health and the ecological process if it is not treated. There are many physical, chemical, and biological methods to remove nitrogenous compounds from polluted water. One example of the process is ammonia-oxidizing bacteria having a metabolic feature which in a combination with other nitrogen-cycling metabolic activities like nitrite oxidation and denitrification, to remove nitrogen from wastewater in activated sludge. Since denitrifying bacteria are heterotrophic in nature, an organic carbon source is supplied to the bacteria in an anoxic basin. When there is no oxygen available, denitrifying bacteria use the oxygen present in the nitrate to oxidize the carbon which leads to the formation of nitrogen gas from nitrate, and then nitrogen bubbles up out of the wastewater.
Do You Know?
What would happen if denitrification is stopped? It is a process of conversion of nitrates and nitrites into atmospheric nitrogen. If the denitrification process is stopped, nitrogen will not get recycled, so it won’t get returned to the atmosphere. All the nitrogen will bond up and no extra nitrogen will be available for use in the process.
FAQs on Denitrifying Bacteria in the Nitrogen Cycle
1. What are denitrifying bacteria?
Denitrifying bacteria are microorganisms that convert nitrate (NO₃⁻) into nitrogen gas (N₂) during the process of denitrification. This process occurs mainly under low-oxygen or anaerobic conditions and is a key step in the nitrogen cycle.
- They use nitrate as an alternative electron acceptor instead of oxygen.
- They help return nitrogen from soil and water back to the atmosphere.
- Common genera include Pseudomonas, Paracoccus, and Bacillus.
2. What is denitrification in the nitrogen cycle?
Denitrification is the biological process in which nitrate (NO₃⁻) is reduced stepwise to nitrogen gas (N₂), completing the nitrogen cycle. It is carried out mainly by denitrifying bacteria in oxygen-poor environments.
- Occurs in waterlogged soils, sediments, and wetlands.
- Converts nitrate → nitrite (NO₂⁻) → nitric oxide (NO) → nitrous oxide (N₂O) → nitrogen gas (N₂).
- Prevents excessive accumulation of nitrates in ecosystems.
3. How do denitrifying bacteria obtain energy?
Denitrifying bacteria obtain energy by using nitrate as a terminal electron acceptor during anaerobic respiration. Instead of oxygen, they reduce nitrate to nitrogen gas to generate ATP.
- Organic matter serves as the electron donor.
- Nitrate replaces oxygen in low-oxygen conditions.
- This process supports growth in anaerobic or oxygen-limited environments.
4. Where are denitrifying bacteria commonly found?
Denitrifying bacteria are commonly found in anaerobic environments such as waterlogged soils and aquatic sediments. They thrive where oxygen levels are low but nitrate is available.
- Agricultural soils with high fertilizer input.
- Wetlands, marshes, and lake sediments.
- Wastewater treatment plants.
5. What is the role of denitrifying bacteria in agriculture?
In agriculture, denitrifying bacteria reduce soil nitrate levels by converting them into atmospheric nitrogen gas. This can both benefit and harm farming systems.
- Prevents nitrate leaching into groundwater.
- Reduces soil fertility by removing available nitrogen.
- Influences fertilizer efficiency and crop yield.
6. What is the difference between nitrifying and denitrifying bacteria?
The main difference is that nitrifying bacteria convert ammonia into nitrate, while denitrifying bacteria convert nitrate into nitrogen gas. Both are essential but perform opposite steps in the nitrogen cycle.
- Nitrification is an aerobic process requiring oxygen.
- Denitrification is an anaerobic process occurring without oxygen.
- Nitrifiers increase nitrate levels; denitrifiers decrease them.
7. Can you give examples of denitrifying bacteria?
Examples of denitrifying bacteria include species of Pseudomonas, Paracoccus denitrificans, and Bacillus. These bacteria are widely studied for their role in the nitrogen cycle.
- Pseudomonas stutzeri – common soil denitrifier.
- Paracoccus denitrificans – model organism for denitrification studies.
- Some species of Thiobacillus also perform denitrification.
8. Why are denitrifying bacteria important for the environment?
Denitrifying bacteria are important because they regulate nitrogen levels and prevent nitrate pollution in ecosystems. By converting nitrate into nitrogen gas, they maintain balance in the nitrogen cycle.
- Reduce nitrate contamination in water bodies.
- Limit eutrophication caused by excess nutrients.
- Contribute to atmospheric nitrogen balance.
9. What conditions are required for denitrification to occur?
Denitrification requires low oxygen levels, available nitrate, and a source of organic carbon. These conditions favor anaerobic respiration by denitrifying bacteria.
- Oxygen-deficient or anaerobic environment.
- Presence of nitrate or nitrite.
- Organic matter as an energy source.
- Suitable temperature and neutral pH.
10. Does denitrification produce greenhouse gases?
Yes, denitrification can produce the greenhouse gas nitrous oxide (N₂O) as an intermediate product. Nitrous oxide is released when the reduction of nitrate to nitrogen gas is incomplete.
- N₂O has a high global warming potential.
- Commonly emitted from fertilized agricultural soils.
- Complete denitrification reduces N₂O to harmless nitrogen gas (N₂).
