A microorganism or microbe is a living thing (organism) that is too small to be visible to the human eye (microscopic). The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea or protists.
Viruses and prions are not called microorganisms because they are classified as non-living. Micro-organisms are often described as single-celled, or unicellular organisms, but there are unicellular protists that are visible to the human eye, and some multicellular species are microscopic.
Microorganisms live almost everywhere on earth where there is liquid water, including hot springs on the ocean floor and deep inside rocks within the earth's crust.
Microorganisms are critical to nutrient recycling in ecosystems, because they act as decomposers. Because some microorganisms can also take nitrogen out of the air, they are an important part of the nitrogen cycle. Pathogenic, or harmful, microbes can invade other organisms and cause disease.
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