Microorganisms and overpopulation are ........pollutants .

Explanation

**Step-by-Step Solution:**

1. **Identify the Types of Pollutants**: Understand that pollutants can be classified into different categories based on their origin and nature. The main categories include physical, chemical, and biological pollutants.

2. **Define Biological Pollutants**: Biological pollutants are substances that come from living organisms. They can include microorganisms such as bacteria, viruses, fungi, and even larger organisms that can affect the environment and human health.

3. **Analyze Microorganisms**: Microorganisms like bacteria and fungi can cause diseases and contribute to environmental issues. For example, they can lead to algal blooms in water bodies, which can deplete oxygen and harm aquatic life.

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