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Some are known to live on surface rocks, and make use of photosynthesis for energy.
Those that live in deeper rocks cannot use photosynthesis to gather energy, but instead extract energy from minerals around them. They live in cracks in the rock where water seeps down. The water contains dissolved carbon dioxide (CO2) which the organisms use for their carbon needs. They have been detected in rocks down to depths of nearly three km, where the temperature is approximately 75 °C.