Denitrification i.e. conversion of fixed nitrates back into the atmospheric nitrogen is carried out by denitrifying bacteria e.g., thiobacillus. Nitrobacter,  nitrosomonas, and rhizobium  cause nitrification. So, the correct answer is option D.

Denitrification is a process in which nitrate (NO3-) is converted into nitrogen gas (N2) by microorganisms. This process is carried out by a group of bacteria known as denitrifiers. Among them, Thiobacillus is one of the most efficient denitrifiers.

Thiobacillus:

Thiobacillus is a group of gram-negative, aerobic bacteria that can grow in a wide range of environmental conditions. They are known for their ability to oxidize sulfur compounds, producing sulfuric acid in the process. They are also capable of denitrification, which makes them important in the cycling of nitrogen in the environment.

Denitrification:

Denitrification is a microbial process that occurs under anaerobic conditions. It is carried out by bacteria that use nitrate as an electron acceptor in the absence of oxygen. The process involves the reduction of nitrate to nitrogen gas, which is released into the atmosphere.

Denitrifiers:

Denitrifiers are a group of bacteria that can carry out denitrification. They are facultative anaerobes, which means that they can grow in the presence or absence of oxygen. They are typically found in soil, sediments, and water, where they play an important role in the nitrogen cycle.

Mechanism of denitrification:

The process of denitrification involves several steps, which are carried out by different groups of bacteria. The overall reaction can be summarized as follows:

2NO3- + 10e- + 12H+ → N2 + 6H2O

The reaction involves the reduction of nitrate to nitrogen gas, which is catalyzed by a series of enzymes. The process can be divided into four main steps:

1. Nitrate reduction: Nitrate is reduced to nitrite (NO2-) by nitrate reductase.

2. Nitrite reduction: Nitrite is reduced to nitric oxide (NO) by nitrite reductase.

3. Nitric oxide reduction: Nitric oxide is reduced to nitrous oxide (N2O) by nitric oxide reductase.

4. Nitrous oxide reduction: Nitrous oxide is reduced to nitrogen gas (N2) by nitrous oxide reductase.

Thiobacillus in denitrification:

Thiobacillus is one of the most efficient denitrifiers, capable of carrying out all four steps of denitrification. They are able to grow under a wide range of environmental conditions, including low pH and high sulfur concentrations. They are found in soil and sediments, where they play an important role in the nitrogen cycle.

Conclusion:

In conclusion, denitrification is carried out by a group of bacteria called denitrifiers, among which Thiobacillus is one of the most efficient. They are able to carry out all four steps of denitrification, making them important in the cycling of nitrogen in the environment.