Endolith
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An endolith is an extremophile organism (archaea, bacterium, or fungus) that lives inside rock or in the pores between mineral grains.
Environment
Endoliths have been found in rock down to a depth of ~2 miles (3 km), though it is unknown if that is their limit. Possible limitations do not result from the pressure in such depth, but from the increased temperature. Judging from hyperthermophile organisms, the temperature limit is at about 110°C, which limits the possible depth to 4 km below the continental crust, and 7 km below the ocean floor. Endolithic organisms have also been found in regions of low humidity, including the Dry Valleys of Antarctica - see hypolith for more on this variety.
Survival
Endoliths can survive by feeding on traces of iron, potassium, or sulfur. Whether they metabolize these directly from the surrounding rock, or rather excrete an acid to dissolve them first, remains to be seen.
As water and nutrients are rather sparse, endoliths have a very slow procreation cycle. Early data suggests that some only engage in cell division once every hundred years.
SLiMEs
As most endoliths are autotroph, they can generate organic compounds essential for their survival on their own from inorganic matter. Inevitably, some endoliths have specialized in feeding on their autotroph relatives. The micro-biotope of the different endolithic species is called SLiME (Subsurface Lithotrophic Microscopic Environment).