Endolith

An endolith or cryptoendolith is an organism (archaea, bacterium, or fungus) that lives inside rock, coral, animal shells, or in the pores between mineral grains. Many are extremophiles; living in places previously thought inhospitable to life. They are of particular interest to astrobiologists, who theorize that endolithic environments on Mars and other planets constitute potential refugia for extraterrestrial microbial communities.

Environment
Endoliths have been found in rock down to a depth of 3 km (9,600 feet), though it is unknown if that is their limit (due to the cost involved in digging so deeply). The main threat to their survival seems not to result from the pressure at such depth, but from the increased temperature. Judging from hyperthermophile organisms, the temperature limit is at about 120°C (the recently-discovered Strain 121 can reproduce at 121°C), which limits the possible depth to 4-4.5 km below the continental crust, and 7 or 7.5 km below the ocean floor. Endolithic organisms have also been found in surface rocks in regions of low humidity (hypolith) and low temperature (psychrophile), including the Dry Valleys and permafrost of Antarctica.

Survival
Endoliths can survive by feeding on traces of iron, potassium, or sulfur. (See lithotroph.) Whether they metabolize these directly from the surrounding rock, or rather excrete an acid to dissolve them first, remains to be seen. The Ocean Drilling Program found microscopic trails in basalt from the Atlantic, Indian, and Pacific oceans that contain DNA. Photosynthetic endoliths have also been discovered.

As water and nutrients are rather sparse in the environment of the endolith, they have a very slow procreation cycle. Early data suggests that some only engage in cell division once every hundred years. Most of their energy is spent repairing cell damage caused by cosmic rays or racemization, and very little is available for procreation or growth. It is thought that they weather long ice ages in this fashion, emerging when the temperature in the area warms.

Slime
As most endoliths are autotrophs, 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 where these different endolithic species live together is called SLiME (Subsurface Lithotrophic Microbial Ecosystem).