Which of the following asymbiotic free-living cyanobacteria is concerned with nitrogen fixation?

Options

A. Tolypothrix
B. Anabaenopsis
C. Stigonema
D. All of these

Answer

To solve the question regarding which asymbiotic free-living cyanobacteria is concerned with nitrogen fixation, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Nitrogen Fixation**:
- Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds, making it available for plants. This process is crucial because plants cannot directly utilize atmospheric nitrogen.

2. **Identify Types of Nitrogen-Fixing Bacteria**:
- There are two main types of nitrogen-fixing bacteria:
- **Symbiotic bacteria**: These bacteria live in close association with plants (e.g., Rhizobium with legumes).
- **Non-symbiotic (asymbiotic) bacteria**: These bacteria live independently in the soil or water and can fix nitrogen without a direct relationship with plants.

3. **Examine the Given Options**:
- The options provided are:
- Option 1: Tolypothrix
- Option 2: Anabaena (likely meant to be Anabaenopsis)
- Option 3: Stegonema
- Option 4: All of these

4. **Research Each Option**:
- **Tolypothrix**: A genus of cyanobacteria that is known to fix nitrogen and is free-living.
- **Anabaena (or Anabenopsis)**: A filamentous cyanobacterium that can also fix nitrogen and is free-living.
- **Stegonema**: Another filamentous cyanobacterium that is involved in nitrogen fixation and is non-symbiotic.

5. **Conclusion**:
- Since all three options (Tolypothrix, Anabaena, and Stegonema) are free-living cyanobacteria that are capable of nitrogen fixation, the correct answer to the question is **Option 4: All of these**.

### Final Answer:
**All of these (Tolypothrix, Anabaena, Stegonema) are asymbiotic free-living cyanobacteria concerned with nitrogen fixation.**

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