How is algae useful to us?

Explanation

**Step-by-Step Solution:**

1. **Oxygen Production**: Algae play a crucial role in increasing the level of oxygen in water bodies through the process of photosynthesis. This oxygen is essential for the survival of aquatic animals.

2. **Food Source**: Many types of algae are consumed by humans and other organisms. They serve as a nutritious food source, providing essential nutrients.

3. **Fertilizer**: Algae can be used as a natural fertilizer in agriculture. They enrich the soil and promote plant growth.

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