![E coli at 10000x, original.jpg](https://upload.wikimedia.org/wikipedia/commons/thumb/b/bc/E_coli_at_10000x%2C_original.jpg/300px-E_coli_at_10000x%2C_original.jpg)
Microorganisms, including bacteria, fungi, algae, and protozoa, play an essential role in natural systems, including soil and water. See:
- Microorganisms in secondary wastewater treatment
- Effective Microorganisms - used in soil and for other purposes.[clarification needed]