Organisms that are capable to fix nitrogen in aquatic habitats are ……………………………….

Explanation

**Step-by-Step Solution:**

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

2. **Identifying Organisms**: In aquatic habitats, certain organisms are known to have the capability to fix nitrogen. Among these, cyanobacteria are the most prominent.

3. **Cyanobacteria**: These are photosynthetic bacteria that can perform nitrogen fixation. They are often found in various aquatic environments.

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