Endolith

Rock boring microbe or extremophile organism (archaea, bacterium, or fungus) that lives inside rock or in the interstitial spaces between mineral grains in deep granitic and basaltic formations. Most endoliths are prokaryotic or mesoscale prekaryotes and are among the most common naturally-occurring life-forms known. Recently, it has been suggested that those species are relics of the Earth’s earliest life forms, from a time when the atmosphere contained little or no oxygen. It has also been suggested that the amount of “biomass” inside the Earth may be as great as or even greater than that in what has been traditionally portrayed as the biosphere.

        Three main types of endoliths have been identified. Cryptoendoliths live in rock at the Earth's surface and can tolerate extreme surface conditions; examples include lichen. Subsurface endoliths are found in aquifers or solution caverns and use nutrients in groundwater. Deep biosphere endoliths are found in rock miles from the surface in extremely hot (over 100° C) or cold (up to -15° C) conditions and also in deep mines under anoxic conditions, extreme temperatures, intense pressures, and without access to sunlight.

        Endoliths can survive by feeding on traces of iron, potassium, or sulfur. Whether they metabolize those elements directly from the host rock or excrete acidic solutions to dissolve them first remains unknown. Many endoliths are autotrophs that make their own organic compounds by using gas or dissolved nutrients from volatiles or water moving through joints, pores, and fractures in the rock. As water and nutrients are far from plentiful at depths, endoliths have a very slow procreation cycle, with cell division occurring only once every hundred years.
        Endoliths have been found as deep as two miles; though at this time no one knows if that constitutes a lower limit. Just think about the hyperthermophile organisms living around the black smokers in the ocean, whose temperature limit is at about 110° C. That would put the downward range of the endoliths at around three miles below the continental crust and not quite five miles below the ocean floor. Interestingly, endolithic organisms have also been found in regions of great cold and low humidity, including Antarctica.
        Author’s Note: When I was a young paleontology student some four decades ago, I was taught that the biosphere descended some several tens of yards or so into the Earth’s solid surface, where it stopped. According to theory then accepted by nearly everyone, heat and pressure beyond those depths created an inhospitable environment that was incapable of sustaining life. That idea existed well before rock cores that are brought routinely when drilling for oil and other goodies were analyzed and sterile handling of those samples demonstrated that lithotrophic (rock-eating) bacteria and even higher multi-cellular protozoa that fed on the bacteria live at great depths in the Earth’s interior — perhaps as deep as 12,000 feet. The lesson is simple: science truly is capable of astounding you, but only if your eyes and minds are open.