Which one of the following is free living, nitrogen fixing cyanobacterium ?

Options

A. Nitrosomonas
B. Rhizobium
C. Stigonema
D. Nitrobacter

Answer

To solve the question "Which one of the following is a free living, nitrogen fixing cyanobacterium?", we can follow these steps:

### Step 1: Understand the Question
The question asks for a specific type of cyanobacterium that is both free-living and capable of fixing nitrogen.

### Step 2: Identify the Options
The options provided are:
1. Nitrosomonas
2. Rhizobium
3. Stegonema
4. Nitrobacter

### Step 3: Define Key Terms
- **Free-living nitrogen-fixing bacteria**: These are bacteria that can fix atmospheric nitrogen into a form usable by plants and do not require a host plant to survive.
- **Cyanobacteria**: These are photosynthetic bacteria that can perform nitrogen fixation.

### Step 4: Analyze Each Option
1. **Nitrosomonas**: This is a nitrifying bacterium that converts ammonia into nitrite. It does not fix nitrogen.
2. **Rhizobium**: This is a symbiotic nitrogen-fixing bacterium that forms relationships with leguminous plants. It is not free-living.
3. **Stegonema**: This is a cyanobacterium that is known to be free-living and capable of nitrogen fixation.
4. **Nitrobacter**: This is also a nitrifying bacterium that converts nitrite into nitrate. It does not fix nitrogen.

### Step 5: Conclusion
Based on the analysis, the only option that is a free-living nitrogen-fixing cyanobacterium is **Stegonema**.

### Final Answer
**Stegonema**

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