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 consitute potential refugia for extraterrestrial microbial communities.

Environment

Endoliths have been found in rock down to a depth of 9,600 feet (3 km), though it is unknown if that is their limit (due to the cost involved in digging so deeply).Two miles underground — Gold mines present "ideal environment" for geologists studying subsurface microbesLooking for life in all the wrong places — research on cryptoendoliths Discover, May, 1997 by Will Hively 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 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 surface rocks in regions of low humidity (hypolith) and low temperature (psychrophile), including the Dry Valleys and permafrost of Antarctica.Microbial Diversity of Cryptoendolithic Communities from the McMurdo Dry Valleys, 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.Glass Munchers Under the Sea — NASA Astrobiology Institute — Leslie MullenMicrobial Populations in Ocean Floor Basalt: Results from ODP Leg 187 Photosynthetic endoliths have also been discovered.

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. 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).Frequently Requested Information about the SLiME Hypothesis

See also

• Lithotroph
• The controversial Deep biogenic petroleum theory proposes that petroleum is produced by endolithic bacteria, and not decaying organic matter

References

External links

• Endoliths General Collection — This collection of online resources such as news articles, web sites, and reference pages provides a comprehensive array of information about endoliths.
• Endolith Advanced Collection — Compiled for professionals and advanced learners, this endolith collection includes online resources such as journal articles, academic reviews, and surveys.

Extremophiles