Microorganisms are living organisms that are not visible to the naked eye. Usually they can be seen only with the help of a microscope. The main difference to more complex organisms such as plants and animals, is their relatively simple biological composition (e.g. no or no real tissue). Based on their cell structures, microorganisms can be divided into bacteria, protozoa (single-celled organisms), fungi and algae. In water or water-bearing systems, these are usually undesirable, since they permanently impair the water quality or, if it is drinking water, for example, microorganisms can also trigger diseases.