Which of the following can fix atmospheric nitrogen?

Options

A. Albugo
B. Cytopus
C. Saprolegnia
D. Anabeana

Explanation

To determine which of the following can fix atmospheric nitrogen, let's analyze the options provided: alveolus, cystopers, saproligna, and anabena.

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into ammonia (NH3), which can be utilized by plants. This process is crucial for agriculture as it enriches the soil with nitrogen.

2. **Identifying the Organisms**:
- **Alveolus**: This is not a nitrogen-fixing organism; it refers to a structure in the lungs or a small cavity in other biological contexts.
- **Cystopers**: This term does not refer to a known nitrogen-fixing organism.
- **Saproligna**: This is a genus of fungi and does not have the capability to fix atmospheric nitrogen.
- **Anabena**: This is a genus of cyanobacteria known for its ability to fix atmospheric nitrogen.

3. **Conclusion**:
- Among the options, **Anabena** is the only organism that can fix atmospheric nitrogen. It is often associated with ferns, particularly the mosquito fern (Azolla), which is used in paddy fields as a biofertilizer.

### Final Answer:
The organism that can fix atmospheric nitrogen is **Anabena**.

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