Some bacteria have the ability to 'fix' nitrogen. This means they

Options

A. convert ammonia into nitrites and nitrates
B. break down useful nitrogen-rich compounds and release ammonium ions
C. convert nitrates into nitrogen gas
D. convert atmospheric nitrogen gas into biologically useful forms of nitrogen.

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process through which atmospheric nitrogen (N₂) is converted into ammonia (NH₃), which can be utilized by living organisms, particularly plants.

2. **Role of Nitrogen-Fixing Bacteria**:
- Certain bacteria, known as nitrogen-fixing bacteria, have the ability to perform this conversion. Examples include Rhizobium, Azotobacter, and Nostoc.

3. **Biological Nitrogen Fixation**:
- When bacteria convert atmospheric nitrogen into ammonia, this process is referred to as biological nitrogen fixation. This is essential because atmospheric nitrogen is not directly usable by plants.

4. **Conversion Process**:
- The nitrogen-fixing bacteria take nitrogen gas from the atmosphere and convert it into ammonia. This ammonia can then be further processed by other soil bacteria into nitrites (NO₂⁻) and nitrates (NO₃⁻), which are forms of nitrogen that plants can absorb.

5. **Conclusion**:
- In summary, the ability of some bacteria to 'fix' nitrogen means they convert atmospheric nitrogen into biologically useful forms, primarily ammonia, which can then be utilized by plants and other organisms.