An endolith is an organism (archaeum, bacterium, fungus, lichen, alga or amoeba) that lives inside rock, coral, animal shells, or in the pores between mineral grains of a rock. 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.
The term "endolith", which defines an organism that colonizes the interior of any kind of rock, has been further classified into three subclasses:
Chasmoendolith: colonizes fissures and cracks in the rock (chasm = cleft)
Cryptoendolith: colonizes structural cavities within porous rocks, including spaces produced and vacated by euendoliths (crypto = hidden)
Euendolith: penetrates actively into the interior of rocks forming tunnels that conform with the shape of its body, rock boring organism (eu = good, true)
Endoliths have been found in rock down to a depth of 3 kilometres (1.9 mi), 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, the Alps and the Rocky Mountains.
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