How Do Denitrifying Bacteria Help Maintain Ecological Balance?
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: Role, Function & Significance
1. What are denitrifying bacteria?
Denitrifying bacteria are a group of microorganisms, primarily facultative anaerobes, that facilitate the process of denitrification. Their primary role in the ecosystem is to convert nitrates (NO₃⁻) and nitrites (NO₂⁻) from the soil and water back into gaseous nitrogen (N₂), which is then released into the atmosphere, completing the nitrogen cycle.
2. What is the main function of denitrifying bacteria in the nitrogen cycle?
The main function of denitrifying bacteria is to return nitrogen to the atmosphere. While nitrogen-fixing and nitrifying bacteria make nitrogen available to plants in the form of ammonia and nitrates, denitrifying bacteria carry out the reverse process. They use nitrates as an electron acceptor in the absence of oxygen, converting them back into inert nitrogen gas (N₂), thus balancing the global nitrogen reservoir.
3. What are some common examples of denitrifying bacteria?
Several genera of bacteria are capable of denitrification. Some of the most common examples include:
Pseudomonas (e.g., Pseudomonas denitrificans)
Thiobacillus denitrificans
Micrococcus denitrificans
Species of Bacillus and Alcaligenes
4. Are denitrifying bacteria aerobic or anaerobic?
Denitrifying bacteria are typically facultative anaerobes. This means they can survive in both the presence and absence of oxygen. They prefer to use oxygen for respiration (aerobic respiration). However, in anaerobic conditions (like waterlogged soil or wastewater treatment tanks), they switch their metabolism to use nitrate (NO₃⁻) as the final electron acceptor instead of oxygen, which results in denitrification.
5. How are denitrifying bacteria used in wastewater treatment?
In wastewater treatment, denitrifying bacteria play a crucial role in removing excess nitrogen compounds like nitrates. High levels of nitrates can cause eutrophication (algal blooms) in natural water bodies. By creating controlled anaerobic conditions, treatment plants use these bacteria to convert harmful nitrates in the wastewater into harmless nitrogen gas, which is then safely released into the atmosphere.
6. What is the key difference between nitrifying and denitrifying bacteria?
The key difference lies in their opposite roles in the nitrogen cycle. Nitrifying bacteria (like Nitrosomonas and Nitrobacter) perform oxidation; they convert ammonia into nitrites and then into nitrates, making nitrogen available for plants. In contrast, denitrifying bacteria perform reduction; they convert these nitrates back into gaseous nitrogen, removing it from the soil and returning it to the atmosphere.
7. How are denitrifying bacteria different from nitrogen-fixing bacteria like Azotobacter?
Denitrifying bacteria and nitrogen-fixing bacteria perform opposite functions. Nitrogen-fixing bacteria (e.g., Azotobacter, Rhizobium) take inert nitrogen gas (N₂) from the atmosphere and convert it into ammonia (NH₃), a process called nitrogen fixation. This 'fixes' nitrogen into a usable form for plants. Conversely, denitrifying bacteria take usable nitrates (NO₃⁻) from the soil and convert them back into atmospheric nitrogen gas (N₂).
8. What would happen to an ecosystem if all denitrifying bacteria were removed?
If denitrifying bacteria were absent, the nitrogen cycle would be incomplete. Nitrogen would be continuously fixed from the atmosphere and converted into nitrates, but there would be no process to return it. This would lead to a massive accumulation of nitrates in soil and water bodies, causing severe water pollution (eutrophication) and depleting the vast reservoir of nitrogen in the atmosphere over geological time, ultimately disrupting the entire global ecosystem.
9. Why is denitrification considered both beneficial for wastewater treatment but potentially harmful for agriculture?
This dual nature depends on the context. It is beneficial in wastewater treatment because it removes excess nitrate pollutants. However, in agriculture, it is often harmful. Farmers add expensive nitrogen-based fertilisers (nitrates) to the soil to boost crop growth. Denitrifying bacteria in the soil can convert these valuable nitrates back into unusable nitrogen gas, leading to a significant loss of soil fertility and wasted fertiliser.
