Final answer:
In the nitrogen cycle, ammonia is first oxidized to nitrite by Nitrosomonas and then to nitrate by Nitrobacter, both processes collectively referred to as nitrification. Denitrification by bacteria such as Pseudomonas and Clostridium reduces nitrate to nitrogen gas, completing the cycle. The correct option is A.
Step-by-step explanation:
In the nitrogen cycle, different bacteria play distinct roles in converting nitrogen into forms usable by plants and other organisms. The organisms involved in this process are specialized and the process itself can be divided into several steps.
Bacteria of the genus Nitrosomonas are responsible for the oxidation of ammonia (NH3) to nitrite (NO2-). This process is known as nitrification. Then, bacteria of the genus Nitrobacter take over to oxidize the nitrite (NO2-) to nitrate (NO3-), also as part of nitrification. The production of nitrate is crucial because plants predominantly take up nitrogen in this form.
Conversely, the process of denitrification involves the reduction of nitrate (NO3-) to nitrogen gas (N2), which is performed by bacteria such as Pseudomonas and Clostridium. This step is essential for returning nitrogen to the atmosphere and maintaining the balance of the nitrogen cycle.
To sum up, in the NO2 oxidizing bacteria process, the steps involved are the conversion of nitrite to nitrate, which is then followed by certain bacteria converting nitrate to nitrogen gas through denitrification.