A free - living , anaerobic, nitrogen - fixing bacterium is

Options

A. Rhizobium
B. Streptococcus
C. Azotobacter
D. Clostridium

Answer

To solve the question "A free-living, anaerobic, nitrogen-fixing bacterium is," we will analyze each of the provided options systematically.

### Step 1: Understand the Characteristics of the Bacterium
- We are looking for a bacterium that is:
1. Free-living (not dependent on a host)
2. Anaerobic (can live without oxygen)
3. Nitrogen-fixing (can convert atmospheric nitrogen into a usable form)

### Step 2: Evaluate the Options
1. **Rhizobium**
- Rhizobium is known for forming symbiotic relationships with leguminous plants (like peas and beans) and is not free-living. Therefore, this option is incorrect.

2. **Streptococcus**
- Streptococcus is a genus of bacteria that primarily includes pathogenic species and is not known for nitrogen fixation. Additionally, they are not classified as anaerobic nitrogen-fixing bacteria. Thus, this option is also incorrect.

3. **Azotobacter**
- Azotobacter is a genus of free-living bacteria that can fix nitrogen, but it is aerobic, meaning it requires oxygen for its metabolic processes. Therefore, this option does not meet the anaerobic requirement, making it incorrect.

4. **Clostridium**
- Clostridium is a genus of bacteria that are known to be anaerobic and can fix nitrogen. They are free-living and do not require a host for their survival. This option meets all the criteria specified in the question.

### Conclusion
After evaluating all the options, the correct answer is **Clostridium**.

### Final Answer
The free-living, anaerobic, nitrogen-fixing bacterium is **Clostridium**.

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