A nitrogen-fixing microbe associated with Azolla is:

Options

A. Anaebaena
B. Spirulina
C. Totypothrix
D. Nostoc

Explanation

To answer the question "A nitrogen-fixing microbe associated with Azolla is:", we can follow these steps:

### Step 1: Understand Azolla
Azolla is a genus of aquatic ferns that has a symbiotic relationship with certain nitrogen-fixing microbes. This relationship is beneficial for both the plant and the microbe.

**Hint:** Azolla is a type of fern that grows in water and has a special relationship with certain microbes.

### Step 2: Identify Nitrogen-Fixing Microbes
Nitrogen-fixing microbes are organisms that can convert atmospheric nitrogen (N2) into ammonia (NH3) or related compounds, which plants can use. These microbes can be free-living or symbiotic.

**Hint:** Look for microbes that can convert nitrogen gas into a form usable by plants.

### Step 3: Recognize the Symbiotic Relationship
In the case of Azolla, it forms a symbiotic relationship with a specific type of cyanobacteria. This means that the cyanobacteria live in association with the Azolla plant and provide it with nitrogen.

**Hint:** Symbiotic relationships involve mutual benefits; the plant provides shelter and nutrients, while the microbe supplies nitrogen.

### Step 4: Identify the Specific Microbe
The cyanobacteria associated with Azolla that is known for its nitrogen-fixing ability is called Anabaena. This microbe is crucial for the nitrogen-fixing process in the Azolla plant.

**Hint:** Look for the cyanobacteria that specifically associates with Azolla for nitrogen fixation.

### Step 5: Eliminate Incorrect Options
Other options like Spirulina and Trichodesmium are also cyanobacteria but do not form a symbiotic relationship with Azolla. Nostoc is another nitrogen-fixing cyanobacterium but is not specifically associated with Azolla.

**Hint:** Not all cyanobacteria have the same relationships; focus on the one that is known to work with Azolla.

### Conclusion
The nitrogen-fixing microbe associated with Azolla is **Anabaena**.

**Final Answer:** Anabaena