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Denitrifying Bacteria

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What are Denitrifying Bacteria?

MVSAT 2024

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. 

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FAQs on Denitrifying Bacteria

1. Where are denitrifying bacteria found?

It is found in almost 50 genres with over 125 different species and it is estimated to represent ten to fifteen per cent of the total bacterial population in water, soil, and sediments. Denitrifying includes several species, some among them are pseudomonas, Alkaligenes, Ballicus, and others. 

2. Is Pseudomonas denitrification bacteria?

Yes, pseudomonas is denitrification bacteria, P. stutzeri, which belongs to the genus Pseudomonas, is widely found in soil, freshwater, oceans, and animals. It is an aerobic Gram-negative bacterium and a type of denitrifying bacterium but recently it is now recognized to be highly adapted to anaerobic conditions.

3. Why do plants and animals need nitrogen?

Nitrogen is an essential thing to survive on the Eearth like plants and animals like oxygen. It is required by plants and animals to build amino acids in the proteins, which are the building block of life. Oxygen can be absorbed directly from the air by animals and plants but this thing is not possible in the case of nitrogen. 

4. How do denitrifying bacteria help in wastewater treatment?

Denitrification is an important step in wastewater management. A large amount of nitrogen present in wastewater needs to be treated. Denitrifying bacteria generally convert ammonium to nitrogen and use nitrates present in the waste as the source of oxygen. 

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