Which archaeon was the first microorganism found growing naturally at temperatures above 100°C?

Pyrolobus fumarii was the first microorganism found growing naturally at temperatures above 100°C.

This archaeon was isolated from a hydrothermal vent on the Mid-Atlantic Ridge and was described in 1997. It grows optimally at about 106°C and can reproduce at temperatures near 113°C under high pressure. Such conditions are possible because pressure raises the boiling point of water around deep-sea vents.

P. fumarii is a chemolithoautotroph: it obtains energy by using inorganic chemicals rather than sunlight or organic food. It uses hydrogen as an electron donor and nitrate as an electron acceptor, producing nitrite. The organism fixes carbon dioxide to build cellular material.

The term archaeon is important because archaea are a distinct domain of life, separate from bacteria and eukaryotes. Pyrolobus is therefore not simply an unusually heat-resistant bacterium. Methanopyrus kandleri is another extreme thermophile, but it is known for growth at very high temperatures and methane production rather than for being the first reported organism growing above 100°C.