Organisms that fix nitrogen in aquatic habitats are

Options

A. green algae
B. cyanobacteria
C. brown algae
D. protozoa

Answer

To solve the question "Organisms that fix nitrogen in aquatic habitats are," we will follow these steps:

### Step 1: Understand Nitrogen Fixation
Nitrogen fixation is the process by which certain organisms convert atmospheric nitrogen (N2) into ammonia (NH3) or related compounds, making nitrogen available for use by living organisms.

**Hint:** Remember that nitrogen fixation is crucial for the nitrogen cycle and is performed by specific types of organisms.

### Step 2: Identify Aquatic Habitats
Aquatic habitats include freshwater and marine environments where various organisms live. We need to focus on organisms that thrive in these environments and can fix nitrogen.

**Hint:** Think about the types of organisms that are commonly found in water bodies.

### Step 3: Consider the Options
The options provided are:
- Green algae
- Cyanobacteria
- Brown algae
- Protozoa

We need to evaluate which of these are known for their ability to fix nitrogen.

**Hint:** Recall which of these groups are known for their nitrogen-fixing capabilities.

### Step 4: Evaluate Each Option
- **Green Algae:** While some green algae can perform photosynthesis, they are not primarily known for nitrogen fixation.
- **Cyanobacteria:** This group includes organisms like Nostoc and Anabaena, which are well-known for their nitrogen-fixing abilities in aquatic environments.
- **Brown Algae:** These are primarily marine algae and do not fix nitrogen.
- **Protozoa:** These are single-celled organisms that generally do not fix nitrogen.

**Hint:** Focus on the specific characteristics of cyanobacteria that enable them to fix nitrogen.

### Step 5: Conclusion
Based on the evaluation, the organisms that fix nitrogen in aquatic habitats are **Cyanobacteria**.

**Final Answer:** Cyanobacteria