Rhizobium and Frankia

Options

A. Are free living in soil, but as symbionts for atmospheric nitrogen fixation
B. Produce nodules on the roots of leguminous plants
C. Are strictly anaerobic
D. More than one option is correct

Answer

### Step-by-Step Solution:

1. **Identify the Organisms**:
- Rhizobium and Frankia are both nitrogen-fixing bacteria. They play a crucial role in the nitrogen cycle by converting atmospheric nitrogen into a form that plants can use.

2. **Living Conditions**:
- Both Rhizobium and Frankia are free-living in the soil. This means they can exist independently in the soil environment before forming symbiotic relationships with plants.

3. **Symbiotic Relationships**:
- Rhizobium forms symbiotic relationships specifically with leguminous plants (like peas, beans, and clover), leading to the formation of root nodules where nitrogen fixation occurs.
- Frankia, on the other hand, forms symbiotic relationships with non-leguminous plants (such as alder trees).

4. **Nodule Formation**:
- Rhizobium produces nodules on the roots of leguminous plants, which are essential for nitrogen fixation.
- Frankia does not produce nodules on leguminous plants but instead forms nodules on non-leguminous plants.

5. **Oxygen Requirements**:
- Rhizobium is classified as a strictly anaerobic bacterium, meaning it thrives in low-oxygen environments, which is suitable for the nodules it forms.
- Frankia, however, is an aerobic bacterium, which means it requires oxygen for its metabolic processes.

6. **Conclusion**:
- Based on the above points, the correct statements regarding Rhizobium and Frankia are:
- Both are free-living in the soil.
- Rhizobium forms nodules on leguminous plants, while Frankia forms nodules on non-leguminous plants.
- Rhizobium is anaerobic, while Frankia is aerobic.