The endolithic biome is a recently discovered biome located beneath the Earth’s surface consisting entirely of extremophile microbes that live in tiny pores and crevices of rocks. They consume reduced iron, potassium, ammonia or sulfur compounds for energy and reproduce slowly due to scarce nutrients. The main limiting factor in the penetration of subsurface microbes is temperature. The biome may account for a substantial proportion of global biomass and is being studied for its implications for the possibility of extraterrestrial life.
The endolithic biome (meaning “inside the rock”) is a biome located beneath the surface of the Earth. Other examples of biomes include savannah, desert, and jungle. The endolithic biome consists entirely of microbes that live in tiny pores and crevices of rocks and extends down to at least 3 km (9,600 ft) below the surface. The endolithic biome can be considered the hardest biome on Earth, due to the almost complete absence of sunlight, oxygen and most nutrients. Endolithic microbes have been affectionately called “bugs from hell.”
The endolithic biome is the most recently discovered biome. Serious exploration only began in the early 1990s. The most popular place to study these microbes are gold mines, some of which extend up to two miles below the surface. The microbes found are bacteria or archaea. All microbes in the endolithic biome are extremophiles, meaning they thrive in extreme conditions of heat or lack of nutrients.
Endolithic microbes are rock eaters or lithotrophs. They consume traces of reduced iron,
potassium
, ammonia or sulfur compounds. These reduced compounds are oxidized using oxygen, nitrite or sulfur compounds. This produces energy and free electrons, which are channeled into respiration cycles to produce ATP. Endolithic microbes get
carbon
for their cellular structures both by taking carbon from small amounts of
carbon dioxide
between rocks, like plants, or by breaking down and consuming organic material found in traces, like animals. These are called lithoheterotrophs and lithoautotrophs, respectively. When a microbe employs a mix of the two strategies, it is called a mixotroph.
Microbes in the endolithic biome reproduce very slowly, due to scarce nutrients. Much energy is also expended on repairing cosmic ray damage. Preliminary studies suggest that some species may engage in cell division as infrequently as once every 100 years. Conversely, some surface microbes replicate every 30 minutes.
Investigators of endolithic biomes report that the main limiting factor in the penetration of subsurface microbes is temperature rather than rock density or chemical factors. Judging by hyperthermophilic (“super-heat-loving”) organisms such as the recently discovered strain 121, capable of reproducing at 121°C (250°F), the temperature of an autoclave, endolithic organisms can exist up to 4.5 km (2.8 mi) below the continental crust and 7.5 km (4.6 mi) below the ocean floor. Due to the costs involved in digging so deep, no serious investigations of microbial life at such depths have yet been conducted. But scientists suspect that the endolithic biome may account for a substantial proportion of global biomass.
Because endolithic biome conditions resemble those that might be found on planets such as Mars or the moons of Jupiter, biome studies have been funded by agencies such as NASA, with an interest in their implications for the possibility of extraterrestrial life.
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