Nitrogen is the necessary component of life, which is essential to all life-forms. The naturally available environmental nitrogen is inorganic, which convert into organic compounds like ammonia by the presence of nitrogen-fixing organisms. These natural nitrogenous products are taken up by plants and animals. The excess amounts of these products are removed in the form of nitrogenous waste. When nitrogenous waste is not broken down, it leads to depletion of the nitrogen source. In this case, denitrifying bacteria prevent this situation and convert the organic nitrate compounds into its inorganic form. In this way, nitrogen levels in the atmosphere are refilled, and the nitrogen cycle is completed.
There is a diverse group of bacteria that are capable of performing the denitrification process. Over 50 genera with over 125 different species of denitrifying bacteria have been identified, which represent 10-15% of the bacteria population in water, soil, and sediment. Denitrifying bacteria are also known as NRB or nitrate-reducing bacteria, that can convert nitrates in the soil to free atmospheric nitrogen and deplete soil fertility and finally reduce the productivity in the agricultural sector. By using various enzymes, these bacteria can metabolize nitrogenous compounds and turn nitrogen oxides back to nitrogen gas or nitrous oxide (N2O).
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Some notable denitrifying bacteria such as Micrococcus denitrificans, Thiobacillus denitrificans, some species of Serratia, Alkaligenes, Bacillus, Achromobacter, Hyphomicrobium, Paracoccus, and several species of Pseudomonas, and others are involved in denitrification process. In this case, Pseudomonas aeruginosa can reduce the amount of fixed nitrogen in the soil by up to 50% under anaerobic conditions.
Broad ranges of inorganic and organic compounds are used by denitrifying bacteria as a source of carbon and energy. These bacteria can be detrimental to agricultural soil and can also produce greenhouse gas, such as a nitrous oxide (N2O). In many cases, denitrifying bacteria are used for removing fixed nitrogen pollutants from ecosystems such as sewage sludge. Denitrifying bacteria can carry out the process of denitrification, which can occur in soil, in freshwater and marine environments.
Some Notable Denitrifying Bacteria
Thiobacillus denitrificans is a gram-negative, short rod-shaped bacterium that belongs to the family Hydrogenophilaceae under the phylum Proteobacteria. It is an obligately chemolithoautotrophic bacterium that grows as a facultatively anaerobic chemolithotroph.
This species is widely distributed in both soil and water habitats. Generally, it is found in mud, marine and freshwater sediments, sewage, digestion tanks, sewage lagoons, industrial waste treatment ponds, and even abandoned mines. It is considered as an easily cultivable bacterium, and in artificial habitats, it was first cultured by Beijerinck in 1904. The optimal temperature and pH for growth of this bacterium is 28 - 32 0C and 6.8 - 7.4, respectively.
This bacterium can help the denitrification process under the optimum conditions with a pH of 6.85 and a temperature of 32.8°C. It is one of the best-studied obligately chemolithoautotrophic bacterium that can combine inorganic sulfur-compound oxidation with the process of denitrification. Thiobacillus denitrificans plays a great role in the biogeochemical cycles on a global scale. It also takes part in the nitrate dependent oxidation of FeSx of subsurface ecosystems. Generally, it is often found in the communities of nitrate-rich habitats as a dominant organism.
2. Paracoccus denitrificans
Paracoccus denitrificans is a gram-negative, non-motile, and well-known nitrate-reducing coccoid bacterium. It belongs to the family Rhodobacteraceae under order Rhodobacterales of class Alphaproteobacteria. It has a typically rod-shaped body and becomes spherical shape during the stationary phase. The cell has double membranes with a cell wall. It was first isolated by a Dutch microbiologist, Martinus Beijerinckin, in 1910. Formerly, it was known as Micrococcus denitrificans, but D.H. Davis reclassified it to Paracoccus denitrificans in 1969.
This bacterium is found in the soil under both aerobic or anaerobic environments. It is known to be an extremophile because it has the ability to live in many different kinds of media and environments. It is very flexible and obtains energy both from organic (such as methanol and methylamine) and inorganic compounds (such as hydrogen and sulfur).
This bacterium takes part in the denitrification process and reduces the nitrogen fertilizers in agricultural soil. In this case, nitrogen is converted to dinitrogen and produce nitric oxide and nitrous oxide that cause damage to the atmosphere.
3. Pseudomonas denitrificans
Pseudomonas denitrificans is a gram-negative, aerobic, heterotrophic bacterium that takes part in the denitrification process of the nitrogen cycle by which nitrate is reduced into nitrogen gas (N2). It has a rod-shaped body with a polar flagellated cell which can produce vitamin B12. The size of the cell ranges around 1.05 x 0.8 µm, and chemically, it is composed of up to 48% lipids. The suitable growth temperature for Pseudomonas denitrificans is 25°C. It was first separated from garden soil in Vienna, Austria. It has medical and environmental significance, and it has industrial use to synthesize vitamin B12. It also has potential nitrate toxicity or wastewater treatment applications.
This bacterium is also used to produce commercial compounds such as 3-hydroxypropionic acid. It also takes part in the process of denitrification. In many cases, its denitrification capabilities help in wastewater management. It may cause diseases like meningitis in humans, and it may also inhabit the intestines of fish. It is found in a variety of habitats, including soils, surface waters, wastewaters, and also bottom lake sediment. It can also be able to live at low and high levels of oxygen. Generally, it is considered a decomposer, which also acts as an antagonist against the plant fungus (Vertilcillum lateritum ) through the production of anti-fungal metabolites.
Environmental nitrogen is a fundamental natural particle that is found in proteins and nucleic acid. They are a significant piece of every living thing. Denitrification is a natural procedure where nitrate (NO3-) is changed over to nitrogen (N) gases by the activity of denitrifying microscopic organisms. Thus, the decrease of nitrogen happens in the climatic condition. For the most part, denitrification happens when denitrifying microorganisms use nitrate (NO3-) for their breath in the place of oxygen in the air. This procedure happens most quickly in warm, wet soils with plenty of nitrates (NO3-).