Nitrogen fixing Cyanobacterium is

Options

A. Rhizobium
B. Nostoc
C. Chlorella
D. Methanogens

Answer

To answer the question "Nitrogen fixing Cyanobacterium is?", we will analyze the options provided and identify which one correctly represents a nitrogen-fixing cyanobacterium.

### Step-by-Step Solution:

1. **Understanding Cyanobacteria**:
- Cyanobacteria, also known as blue-green algae, are photosynthetic bacteria that can fix atmospheric nitrogen.

2. **Evaluating the Options**:
- **Option 1: Rhizobium**:
- Rhizobium is a gram-negative soil bacterium known for its nitrogen-fixing ability. However, it is not a cyanobacterium; it forms a symbiotic relationship with the roots of certain plants (like legumes). Therefore, this option is incorrect.

- **Option 2: Nostoc**:
- Nostoc is a genus of cyanobacteria that can fix nitrogen. It possesses a special enzyme called nitrogenase, which allows it to convert atmospheric nitrogen into a form usable by plants. This makes Nostoc a correct example of a nitrogen-fixing cyanobacterium.

- **Option 3: Chlorella**:
- Chlorella is a type of green algae that performs photosynthesis but does not fix nitrogen. Thus, this option is incorrect.

- **Option 4: Methanogens**:
- Methanogens are archaebacteria that thrive in extreme environments and are not involved in nitrogen fixation. Therefore, this option is also incorrect.

3. **Conclusion**:
- After evaluating all the options, the only correct answer that identifies a nitrogen-fixing cyanobacterium is **Option 2: Nostoc**.

### Final Answer:
The nitrogen-fixing cyanobacterium is **Nostoc**.

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