Fill in the blanks:
Blue green algae fix _________ directly from air to enhance fertility of soil.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Role of Blue-Green Algae**: Blue-green algae, also known as cyanobacteria, are microorganisms that play a significant role in the ecosystem, particularly in soil fertility.

2. **Identifying What They Fix**: The question specifically asks what blue-green algae fix directly from the air. The key term here is "fix," which refers to the process of nitrogen fixation.

3. **Nitrogen Fixation Process**: Nitrogen fixation is the process through which certain organisms convert atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds, making it available for plants.

...