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 can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: We need to identify organisms that are capable of fixing nitrogen and are found in aquatic environments.

2. **Evaluate the Options**:
- **Option 1: Green Algae**: Green algae are primarily photosynthetic organisms and do not have the capability to fix nitrogen. Therefore, this option is incorrect.
- **Option 2: Cyanobacteria**: Cyanobacteria, also known as blue-green algae, are known for their ability to fix nitrogen. An example of a nitrogen-fixing cyanobacterium is Anabaena, which can be found in various aquatic habitats. This option is correct.
- **Option 3: Brown Algae**: Brown algae are also primarily photosynthetic and do not fix nitrogen. Thus, this option is incorrect.
- **Option 4: Protozoa**: Protozoa are single-celled organisms that do not have the capability to fix nitrogen. Therefore, this option is incorrect.

3. **Conclusion**: After evaluating all the options, the only organism that fixes nitrogen in aquatic habitats is cyanobacteria.

### Final Answer:
The correct answer is **Cyanobacteria**.

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