A microorganism (from the Greek: μικρός, mikros, "small" and ὀργανισμός, organismós, "organism") is a microscopic organism, which may be a single cell or multicellular organism. The study of microorganisms is called microbiology, a subject that began with Antonie van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design.
Microorganisms are very diverse and include all the bacteria and archaea and almost all the protozoa. They also include some members of the fungi, algae, and animals such as rotifers. Many macro animals and plants have juvenile stages which are also microorganisms. Some microbiologists also classify viruses as microorganisms, but others consider these as nonliving. Most microorganisms are microscopic, but there are some bacteria such as Thiomargarita namibiensis and some protozoa such as Stentor, which are macroscopic and visible to the naked eye.
Microorganisms live in every part of the biosphere including soil, hot springs, on the ocean floor, high in the atmosphere and deep inside rocks within the Earth's crust (see also endolith). Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers. As some microorganisms can fix nitrogen, they are a vital part of the nitrogen cycle, and recent studies indicate that airborne microorganisms may play a role in precipitation and weather.
On 17 March 2013, researchers reported data that suggested microbial life forms thrive in the Mariana Trench. the deepest spot in the Earth's oceans. Other researchers reported related studies that micro-organisms thrive inside rocks up to 1900 feet (580 metres) below the sea floor under 8500 feet (2590 metres) of ocean off the coast of the northwestern United States. According to one of the researchers,"You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."
Micro-organisms are also exploited in biotechnology, both in traditional food and beverage preparation, and in modern technologies based on genetic engineering.
A small proportion of micro-organisms are pathogenic and cause disease and even death in plants and animals.
In popular culture, the term microbe is often used to refer to micro-organisms, although often in a negative context.