Geomicrobiology refers to the activities of microorganisms (usually bacteria) that live beneath the surface of the Earth. The field of study is also referred to as bio-geochemistry and subsurface microbiology. Habitats of the organisms include the ocean and deep within the rock that makes up Earth's crust. The study of the identities and activities of such organisms is important from a basic science standpoint and for commercial reasons.
Microorganisms are a vital part of the cycling of carbon, nitrogen, and sulfur between the surface of Earth and the surrounding atmosphere. These cycles in turn support the diversity of life that exists on the planet. As well, microorganisms break down other compounds that are present in water, soil, and the bedrock.
Many of the bacteria involved in geomicrobiological activities live in environments that are extremely harsh to other life forms. For example, bacteria such as Thermus aquaticus thrives in boiling hot springs, where the temperature approaches the boiling point of water. Such bacteria have been dubbed "extremophiles" because of their extraordinary resilience and adaptation to environmental pressures of temperature, pressure, acidity, salt concentration, or radiation. Other extremophiles live deep in the ocean under enormous atmospheric pressure. The bacteria that live around hot vents at the ocean floor, for example, use the minerals expelled by the vent in a way that supports the development of all the other life that can exist in the vicinity of the vent. Another type of bacteria lives within rock located miles under the surface of the Earth. Indeed, bacteria have been recovered from almost two miles beneath the Earth's surface, an environment that is hostile to all other forms of life. It is presumed that the ancestors of these bacteria entered the rock through nearby oil deposits or by percolating into the rock through microscopic cracks.