How many types of bacteria are identified as being involved in nitrogen fixation?

Options

A. 12
B. 9
C. 6
D. 3

Answer

**Step-by-Step Solution:**

1. **Understanding Nitrogen Fixation**: Nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can be used by plants.

2. **Types of Nitrogen-Fixing Bacteria**: There are three main types of bacteria involved in nitrogen fixation:
- **Free-Living Nitrogen Fixers**: These bacteria live independently in the soil and do not form associations with plants. Examples include:
- *Azotobacter*
- *Bacillus*
- *Clostridium*

3. **Symbiotic Nitrogen Fixers**: These bacteria form a symbiotic relationship with plants, particularly legumes. They establish root nodules where nitrogen fixation occurs. Key examples include:
- *Rhizobium*: Associated with leguminous plants.
- *Frankia*: Associated with non-leguminous plants like Casuarina.

4. **Loose Symbiotic Nitrogen Fixers**: These bacteria associate with plants but do not form nodules. An example is:
- *Azospirillum*: Found in the roots of maize.

5. **Conclusion**: Based on the types of nitrogen-fixing bacteria identified, we conclude that there are three distinct types involved in nitrogen fixation.

6. **Final Answer**: The correct answer to the question is option 4, which states that there are 3 types of bacteria involved in nitrogen fixation.

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