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Geomicrobiology is a science that combines geology and microbiology, and studies the interaction of microscopic organisms with their inorganic environment, such as in sedimentary rocks. The field is especially important when dealing with microorganisms in aquifers and public drinking water supplies.
Another field of study in geomicrobiology is the study of extremophile organisms, the microorganisms that thrive in environments normally considered hostile. Such environments may include extremely hot (hot springs or mid-ocean ridge black smoker) environments, extremely saline environments, or even space environments such as Martian soil or comets.
Recent observations and research in hyper-saline lagoon environments in Brazil and Australia have shown that anaerobic sulfate-reducing bacteria may be directly involved in the formation of dolomite. This suggests the alteration and replacement of limestone sediments by dolomitization in ancient rocks was possibly aided by ancestors to these anaerobic bacteria.
Some bacteria use metal ions as their energy source. They convert (or chemically reduce) the dissolved metal ions from one electrical state to another. This reduction releases energy for the bacteria's use, and, as a side product, serves to concentrate the metals into what ultimately become ore deposits. Certain iron, uranium and even gold ores are thought to have formed as the result of microbe action.
Microbial life has been found at great depths in sedimentary and other rocks, several kilometers below the surface. The energy sources for growth of these microbes includes hydrogen gas generated in the interior of the planet, and it is estimated the biomass of these sub-surface microbial communities may equal the weght of all ocean and land plants. 
- Gold mines may be formed by bacteria - PDF file
- Biomineralization of magnetic minerals
- BBC News, 28 September, 2001: The microbes that 'rule the world' Citat: "... The Earth's climate may be dependent upon microbes that eat rock beneath the sea floor, according to new research....The number of the worm-like tracks in the rocks diminishes with depth; at 300 metres (985 feet) below the sea floor, they become much rarer..."
- BBCNews: 16 January, 2002, Tough bugs point to life on Mars Citation: "...This research demonstrates that certain microbes can thrive in the absence of sunlight by using hydrogen gas..."
- Exploration of deep intraterrestrial microbial life: current perspectives. Pedersen K. (2000). FEMS Microbiol Lett. Apr 1;185(1):9-16.