Which saprophytic prokaryote becomes an endosymbiotic diazotroph ?

Options

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

Answer

To answer the question "Which saprophytic prokaryote becomes an endosymbiotic diazotroph?", we will analyze the options provided and identify the correct answer step by step.

### Step 1: Understand the terms
- **Saprophytic prokaryote**: These are bacteria that obtain nutrients by decomposing organic matter.
- **Endosymbiotic diazotroph**: This refers to organisms that can fix atmospheric nitrogen and live in a symbiotic relationship within the cells of a host organism.

### Step 2: Analyze the options
1. **Rhizobium**: A genus of gram-negative bacteria that forms symbiotic relationships with legumes, fixing nitrogen in root nodules.
2. **Clostridium**: A genus of bacteria that are primarily anaerobic and do not typically form symbiotic relationships with plants.
3. **Nostoc**: A genus of cyanobacteria that can fix nitrogen but does not typically form endosymbiotic relationships with plants.
4. **Azotobacter**: A free-living nitrogen-fixing bacterium that does not form symbiotic relationships.

### Step 3: Identify the correct option
- Among the options, **Rhizobium** is the only saprophytic prokaryote that becomes an endosymbiotic diazotroph. It colonizes the root nodules of leguminous plants and converts atmospheric nitrogen into ammonia using the enzyme nitrogenase.

### Conclusion
The correct answer to the question is **Option A: Rhizobium**.

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