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We don't to know much about organisms living deep below the Earth's crust. Recently a team led by S. Giovanni discovered some microbes 300 m below the ocean floor. The microbes were found to be a completley new and exotic species and apparently they feed off hydrocarbons like methane and benzene. Scientists speculate that life may exist in our Solar System far below the surface of some planets or moons. This raises some questions:

  1. What is the theoretical minimum distance from Earth's core where life can still exist. Please explain how you came up with this number. For example, there are temperature-imposed limits on many biochemical processes.

  2. Is there the potential to discover some truly alien life forms in the Earth's mantle (by this I mean, life which is not carbon based, or life which gets its energy in ways we have not seen before, or non DNA-based life, or something along these lines)?

  3. What is the greatest distance below the Earth's crust that life has been discovered? I believe it is the 300 m I cited above, but I am not 100% sure.

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  • $\begingroup$ Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy. $\endgroup$ – MattDMo Feb 17 '15 at 2:18
  • $\begingroup$ Are you serious? I am in high school, and I don't take a biology class. This is not homework and I doubt any professor would assign a homework question like this? I don't know much about biology, especially about organisms living deep below the Earth's crust, which is why I'm asking the question...How in the world do you expect someone who knows nothing about biology to "show an attempt at an answer". In my opinion this is a totaly valid question and its not homework. $\endgroup$ – Joshua Benabou Feb 17 '15 at 3:27
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    $\begingroup$ @JoshuaBenabou Homework does not mean homework in school. Homework refers to your personal effort in the sense "doing your homework before asking others". Showing an attempt means looking up or at least doing a google search. And please dont give this "I don't know biology" argument. When I ask questions in stackoverflow I make sure that at least I read the manual page once. If you don't know any biology then start by learning some. $\endgroup$ – WYSIWYG Feb 17 '15 at 4:52
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    $\begingroup$ Alright I've edited the question to include some background information. Is it good now? $\endgroup$ – Joshua Benabou Feb 18 '15 at 2:41
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    $\begingroup$ I think the second subquestion should be removed, since it is rather nonsensical. It can be restated as: "What is the potential of discovering something completely unknown?". Of course we can discover something entirely new and alien, but to evaluate the possibility of this is far too speculative. $\endgroup$ – fileunderwater Feb 18 '15 at 12:32
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There's a lot we don't know about life in deep caves, but we can bound the deepest living organism to at least 3.5 kilometers down, and probably not more than 30 kilometers down.

The worms recovered from deep mining boreholes are not particularly specifically adapted to live that far down: they have similar oxygen/temperature requirements as surface nematodes.

The Tau Tona mine is about 3.5 kilometers deep and about 60˚ C at the bottom. Hydrothermal vent life does just fine up to about 80˚C, and the crust gets warmer at "about" 25˚C per kilometer. It's entirely reasonable to expect life to about 5 kilometers down, but further than that is speculation.

Increasing pressure helps to stabilize biological molecules that would otherwise disintegrate at those temperatures, so it's not impossible there could be life even deeper. It may even be likely, given that the Tau Tona life breathes oxygen.

I am certain no life we might recognize as life exists in the upper mantle.

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At least 283 bacterial species (as of June, 2017) have been found in deep mines, deep seas, or deep-sea sediments; for example:

Abyssivirga alkaniphila, 2.3 km; Alcanivorax dieselolei, 5.0 km; Alcanivorax marinus, 2.5 km; Alcanivorax nanhaiticus, 2.1 km; Alkalimonas collagenimarina, 4.0 km; Alkaliphilus transvaalensis, 3.2 km; Altererythrobacter atlanticus, 2.6 km; Altererythrobacter marinus, 1.5 km; Amycolatopsis albispora, 2.9 km; Anoxybacter fermentans, 2.9 km; Arthrobacter ardleyensis, 5.0 km; Aurantivirga profunda, 1.0 km; Arthrobacter subterraneus, 0.5 km.


For links to the articles describing these bacterial species, see:

https://www.researchgate.net/publication/310900732_Bacteria_I_Names http://bacteria.martinklvana.com/

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