Protozoa are a diverse assemblage of over 65,000 minute, mobile, single-called organisms. Moisture is their prime ecological requirement and the majority are found in the sea or freshwater, although others (particularly flagellates, small amoebae and ciliates) are ubiquitous in soil. Protozoa play an extremely important role in many ecological processes, making up a large portion of the biomass of many aquatic systems. Due to their complex taxonomy and the fact that only half of the existing species has been described it is difficult to know the exact threats that endanger their survival. But it is clear nevertheless, that survival is influenced by food supply, light (for certain species), temperature, oxygen, carbon dioxide, acidity of the medium and water (for active life).