The free living , aerobic and saprotrophic `N_2 ` - fixing microbe is

Options

A. Azotobacter
B. Rhodospirillum
C. Rhizobium
D. Anabaena

Answer

To answer the question "The free-living, aerobic, and saprotrophic N2-fixing microbe is," we will follow these steps:

### Step-by-Step Solution:

1. **Understanding the Terms:**
- **Free-living:** This means the microbe does not form a symbiotic relationship with plants or other organisms.
- **Aerobic:** This indicates that the microbe requires oxygen for its metabolic processes.
- **Saprotrophic:** This means the microbe derives its nutrients from decomposing organic matter.

2. **Identifying Nitrogen-Fixing Microbes:**
- Nitrogen-fixing microbes convert atmospheric nitrogen (N2) into ammonia (NH3), which can be utilized by plants.
- Common nitrogen-fixing microbes include Azotobacter, Rhodospirillum, Rhizobium, and Anabaena.

3. **Analyzing Each Option:**
- **Azotobacter:** This is a free-living, aerobic, and saprotrophic nitrogen-fixing bacterium.
- **Rhodospirillum:** This is also a nitrogen-fixing bacterium but is often involved in symbiotic relationships.
- **Rhizobium:** This is a symbiotic nitrogen-fixing bacterium that forms nodules on the roots of legumes and is anaerobic.
- **Anabaena:** This is a filamentous cyanobacterium that is also symbiotic and not free-living.

4. **Conclusion:**
- Based on the definitions and characteristics, the only microbe that fits all the criteria (free-living, aerobic, saprotrophic, and nitrogen-fixing) is **Azotobacter**.

### Final Answer:
The free-living, aerobic, and saprotrophic N2-fixing microbe is **Azotobacter**.