Rhizobium is

Options

A. Coccus
B. Spiral
C. Rod-shaped
D. filamentous

Answer

**Step-by-Step Solution:**

1. **Definition of Rhizobium**:
- Rhizobium is a genus of bacteria that is known for its role in nitrogen fixation. It is classified as a gram-negative bacterium, which means it has a specific cell wall structure that can be identified through Gram staining.

2. **Shape of Rhizobium**:
- The bacteria are characterized by their rod-shaped morphology. This shape is typical for many types of bacteria and can influence their movement and interaction with their environment.

3. **Role in Nitrogen Fixation**:
- Rhizobium plays a crucial role in converting atmospheric nitrogen (N₂) into ammonia (NH₃). This process is vital for plants, particularly leguminous plants, as they require nitrogen in a form that can be assimilated for growth.

4. **Association with Leguminous Plants**:
- Rhizobium forms symbiotic relationships with the roots of leguminous plants (such as peas, beans, and lentils). In this association, the bacteria reside in root nodules, where they convert nitrogen from the atmosphere into a usable form for the plant.

5. **Aerobic Nature**:
- Rhizobium is classified as an aerobic bacterium, meaning it requires oxygen for its metabolic processes. This is important for its survival and function within the root nodules of plants.

6. **Free-living Nitrogen Fixing Bacterium**:
- In addition to its symbiotic role, Rhizobium can also exist as a free-living organism in the soil, where it can still contribute to nitrogen fixation, albeit to a lesser extent compared to when it is in symbiosis with plants.