Microscopic organisms

A microorganism or microbe is a microscopic organism, which may be single-celled or multicellular. The study of microorganisms is called microbiology, a subject that began with the discovery of microorganisms in the 1670s by Antonie van Leeuwenhoek.Robert Hooke coined the term "cell" after viewing plant cells under his microscope. Later, in the 19th century, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In 1876 Robert Koch discovered that microorganisms cause diseases.

Microorganisms are very diverse and include all bacteria, archaea and most protozoa. This group also contains some fungi, algae, and some micro-animals such as rotifers. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists classify viruses and viroids as microorganisms, but others consider these as nonliving. In July 2016, scientists identified a set of 355 genes from the last universal common ancestor of all life, including microorganisms, living on Earth.

Microorganisms live in every part of the biosphere, including soil, hot springs, inside rocks at least 19 km (12 mi) deep underground, the deepest parts of the ocean, and at least 64 km (40 mi) high in the atmosphere. Microorganisms, under certain test conditions, have been observed to thrive in the near-weightlessness of space and to survive in the vacuum of outer space. Microorganisms likely far outweigh all other living things combined. The mass of prokaryote microorganisms including the bacteria and archaea may be as much as 0.8 trillion tons of carbon, out of the total biomass of between 1 and 4 trillion tons. Microorganisms appear to thrive in the Mariana Trench, the deepest spot in the Earth's oceans. Other researchers reported related studies that microorganisms thrive inside rocks up to 580 m (1,900 ft; 0.36 mi) below the sea floor under 2,590 m (8,500 ft; 1.61 mi) of ocean off the coast of the northwestern United States, as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan. In August 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft; 0.50 mi) below the ice of Antarctica. According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."

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