Endolith

Endolith lifeform found inside an Antarctic rock

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.^{[citation needed]}

Subdefinitions

The term "endolith", which defines an organism that colonizes the interior of any kind of rock, has been further classified into three subclasses:^[1]

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)

Environment

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).^[2][3] 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,^[4] the Alps^[5] and the Rocky Mountains.^[6][7]

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.^[8][9] Photosynthetic endoliths have also been discovered.

As water and nutrients are rather sparse in the environment of the endolith, they have a very slow reproduction 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 reproduction or growth. It is thought that they weather long ice ages in this fashion, emerging when the temperature in the area warms.^[3]

Slime

As most endoliths are autotrophs, they can generate organic compounds essential for their survival on their own from inorganic matter. Some endoliths have specialized in feeding on their autotroph relatives. The micro-biotope where these different endolithic species live together has been called a subsurface lithotrophic microbial ecosystem (SLiME).^[10]

See also

	Environment portal
	Ecology portal
	Earth sciences portal
	Sustainable development portal

• Lithotroph
• Lithophile

References

1. ^ Stjepko Golubic, E.Imre Friedmann, and Jürgen Schneider (1981) The lithobiotic ecological niche, with special reference to microorganisms. Journal of Sedimentary Research, Vol.51, No. 2, June: pages 475-478
2. ^ Two miles underground — Gold mines present "ideal environment" for geologists studying subsurface microbes
3. ^ ^{a b} Looking for life in all the wrong places — research on cryptoendoliths Discover, May, 1997 by Will Hively
4. ^ Microbial Diversity of Cryptoendolithic Communities from the McMurdo Dry Valleys, Antarctica
5. ^ Horath Th, Bachofen R (2009) Molecular Characterization of an Endolithic Microbial Community in Dolomite Rock in the Central Alps (Switzerland) Microb Ecol 58:290–306
6. ^ Walker JJ, Spear JR, Pace NR (2005) Geobiology of a microbial endolithic community in the Yellowstone geothermal environment. Nature 434:1011–1014
7. ^ Walker JJ, Pace NR (2007) Phylogenetic composition of Rocky Mountain endolithic microbial ecosystems. Appl Environ Microbiol 73:3497–3504
8. ^ Glass Munchers Under the Sea — NASA Astrobiology Institute — Leslie Mullen
9. ^ Microbial Populations in Ocean Floor Basalt: Results from ODP Leg 187
10. ^ Frequently Requested Information about the SLiME Hypothesis

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.