Denitrifying bacteria, microorganisms whose action results in the conversion of nitrates in soil to free atmospheric nitrogen, thus depleting soil fertility and reducing agricultural productivity. Thiobacillus denitrificans, Micrococcus denitrificans, and some species of Serratia, Pseudomonas, and Achromobacter are implicated as denitrifiers. Pseudomonas aeruginosa can, under anaerobic conditions (as in swampy or water-logged soils), reduce the amount of fixed nitrogen (as fertilizer) by up to 50 percent. Without denitrification, however, the Earth’s supply of nitrogen would eventually accumulate in the oceans, since nitrates are highly soluble and are continuously leached from the soil into nearby bodies of water. See also nitrogen cycle.

Psuedomonas and Thiobacillus convert ammonia back into atmospheric nitrogen.

Denitrifying bacteria play a crucial role in the nitrogen cycle by converting nitrates (NO3-) back into nitrogen gas (N2), thus completing the process of denitrification. Among the given options, Pseudomonas is a denitrifying bacterium.

Explanation:
Denitrifying Bacteria
Denitrifying bacteria are a group of microorganisms that possess the ability to convert nitrate (NO3-) to nitrogen gas (N2) in the process of denitrification. This is a vital step in the nitrogen cycle as it helps in the removal of excess nitrogen from the environment.

Pseudomonas
Pseudomonas is a genus of bacteria that comprises various species. Many species of Pseudomonas are known for their ability to carry out denitrification. They possess the necessary enzymes to convert nitrate to nitrogen gas.

Escherichia coli
Escherichia coli, commonly known as E. coli, is a bacterium that is a part of the normal gut flora of humans and animals. It is not primarily known for denitrification.

Nitrobacter
Nitrobacter is a genus of bacteria that plays a crucial role in the process of nitrification. They convert nitrite (NO2-) to nitrate (NO3-), which is the opposite process of denitrification.

Nitrosomonas
Nitrosomonas is another genus of bacteria involved in nitrification. They convert ammonia (NH3) to nitrite (NO2-), the first step in the nitrification process.

Conclusion
Among the given options, Pseudomonas is the denitrifying bacterium. It possesses the necessary enzymes to convert nitrate to nitrogen gas, thus completing the process of denitrification. Escherichia coli is not primarily known for denitrification but rather for its role in the gut flora. Nitrobacter and Nitrosomonas are involved in the nitrification process, which is different from denitrification.