Bacteria which directly convert atmospheric nitrogen into nitrogen compounds are called
(a) Nitrogen fixing bacteria
(b) Nitrifying bacteria
(c) Putrefying bacteria
(d) Denitrifying bacteria

Options

A. Nitrogen fixing bacteria
B. Nitrifying bacteria
C. Putrefying bacteria
D. Denitrifying bacteria.

Answer

To answer the question, "Bacteria which directly convert atmospheric nitrogen into nitrogen compounds are called," we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the type of bacteria that can convert atmospheric nitrogen (N2) into nitrogen compounds. This process is known as nitrogen fixation.

2. **Evaluating Option (a) Nitrogen Fixing Bacteria**:
- Nitrogen-fixing bacteria are specifically known for their ability to convert inert atmospheric nitrogen into usable nitrogen compounds (like ammonia or nitrates) that can be utilized by plants.
- Examples include Rhizobium, Azotobacter, and Anabaena.
- Therefore, this option is likely correct.

3. **Evaluating Option (b) Nitrifying Bacteria**:
- Nitrifying bacteria are involved in the nitrification process, which converts ammonia into nitrites and then into nitrates.
- They do not convert atmospheric nitrogen into nitrogen compounds.
- Hence, this option is incorrect.

4. **Evaluating Option (c) Putrefying Bacteria**:
- Putrefying bacteria are responsible for the decomposition of organic matter, particularly under anaerobic conditions.
- They do not play a role in nitrogen fixation.
- Therefore, this option is also incorrect.

5. **Evaluating Option (d) Denitrifying Bacteria**:
- Denitrifying bacteria convert nitrates back into nitrogen gas, thus releasing it into the atmosphere.
- This process is the opposite of nitrogen fixation and does not involve the conversion of atmospheric nitrogen into compounds.
- Thus, this option is incorrect.

6. **Conclusion**: Based on the analysis, the correct answer is **(a) Nitrogen Fixing Bacteria**.

### Final Answer:
(a) Nitrogen fixing bacteria