Which of the following is/are diazotrophs ?

Options

A. Rhizobium and Azotobacter
B. Frankia and Nostoc
C. Anabaena and Nostoc
D. All of these

Answer

To determine which of the following are diazotrophs, we need to understand what diazotrophs are and then evaluate the options provided.

### Step-by-Step Solution:

1. **Define Diazotrophs**:
- Diazotrophs are organisms, primarily bacteria and archaea, that have the ability to fix atmospheric nitrogen (N2) into a more usable form, such as ammonia (NH3). This process is essential for making nitrogen available to plants, as they can utilize nitrogen primarily in the form of ammonia.

2. **Identify Examples of Diazotrophs**:
- Common examples of diazotrophs include:
- **Rhizobium**: A genus of bacteria that forms symbiotic relationships with legumes and fixes nitrogen.
- **Azotobacter**: A free-living nitrogen-fixing bacterium found in soil.
- **Francia**: A genus of bacteria that also participates in nitrogen fixation.
- **Nostoc**: A genus of cyanobacteria that can fix nitrogen.
- **Anabaena**: Another genus of cyanobacteria known for its nitrogen-fixing capabilities.

3. **Evaluate the Options**:
- **Option A**: Rhizobium and Azotobacter - Both are diazotrophs.
- **Option B**: Francia and Nostoc - Both are diazotrophs.
- **Option C**: Anabaena and Nostoc - Both are diazotrophs.
- **Option D**: All of these - Since all the previous options contain organisms that can fix nitrogen, this option is correct.

4. **Conclusion**:
- Since all the options provided (A, B, and C) contain organisms that are classified as diazotrophs, the correct answer is **Option D: All of these**.