Bacteria involved in the two-step conversion ammonia into nitrate are
a. Azotobacter and Nitrosomonas
b. Pseudomonas and Nitrobacter
c. Azotobacter and Achromobacter
d. Nitrosomonas and Nitrobacter

Hint: The process of converting the ammonia into nitrates is called nitrification, in which process nitrites are formed which in turn convert into nitrates, and the bacteria which involve in this process are called nitrifying bacteria.

Complete answer:
- Nitrifying bacteria are the small group of bacteria, which depend on the inorganic substances for their energy source.
- These bacteria are very important for the conversion of ammonia in the irrigation soil of agricultural lands to nitrates.
- The process of converting the ammonia to nitrates involves, mainly two steps, which include the conversion of ammonia to nitrates, and these nitrites further converted to nitrates.
- There are some groups of bacteria involved in these steps.
- For the conversion of ammonia to nitrites, the bacteria that are involved are Nitrosomonas, nitrosospira, nitrosococcus and nitrosolobus.
- Whereas in order to convert the nitrites to nitrates, the bacteria involved are Nitrobacter and Nitrospira.
- During the first step of conversion of ammonia to nitrite, ammonia is oxidized by ammonia-oxidizing bacteria.
- Next in order to convert nitrite to nitrate, they require nitrite-oxidizing bacteria.
- The enzymes required are ammonia monooxygenase and hydroxylamine oxidoreductase.
- Nitrification is very important because it shows the availability of nitrogen as fertilizer.
- So for the above question, bacteria converting ammonia to nitrite is Nitrosomonas and from nitrite to nitrate is Nitrobacter.

Hence, the correct answer is option (B).

Note: In both terrestrial and aquatic systems, nitrifying bacteria release the greenhouse gases and nitrous oxide, probably through the reductive pathway and it is known as nitrifying denitrifier, and an environment, they are deducted by PH, temperature, etc.