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Here's a nice article about Earth's subsurface ecology - surprisingly large and robust for an environment with very little available energy.

Turns out that a bunch of it is apparently living on the products of radioactive decay.  Something decays, knocks a water molecule apart splitting off a hydrogen atom and then an HO which quickly binds to something forming various radicals and peroxides. 

And some cells use the Hydrogen as fuel, and some cells use the radicals to break down normally-inaccessible sources, and some use the peroxides, eventually breaking them down at slightly less energy cost than the yield from the Hydrogen & Oxygen they liberate from it.

So this vast subterranean ecology lumbers very slowly onward into eternity, using an energy source it's almost impossible for any surface event to destroy or limit.  No matter what happened to surface life, whenever it becmes possible for there to *be* surface life there will be living cells down there to cough up a new generation of one-celled surface organisms and recolonize the planet starting from zero.

Of course, there ain't no redux in another couple billion years when the sun starts to eat the place, but  this is how life laughs at our silly worries that we might wipe it out.  If pressed, we could alter the climate and ecology to be no longer compatible with *US* living here, but life, in general, has nothing to fear as long as this planet exists.
The more I think about this the deeper its implications get. We're talking about lifeforms that can literally live for millions of years inside a rock. That's long enough to make some kinds of panspermia theories quite viable. The scenario where life started on Mars and then somehow spread to Earth now has a viable vehicle. Having arrived on a world where the environment is hellishly different from that where it came from, our little radiosynthetic organisms just. don't. care. They can live for millions more years inside their rock on the new world. Adapting to conditions closer to the surface of the rock is something that can take a leisurely pace, with enough time for evolution to actually work. Try something like that with one of us fast-metabolism critters that depends much more closely on surface conditions, and we'd just die with no time to evolve and adapt.

If you're a terraformer and taking a "long view" of things, you have a readymade organism to seed almost any world with. Establish a colony of deep radiosynthetic organisms, and from that moment onward every time the planet's surface is compatible with any kind of life based on that basic biochemistry, one-celled organisms will, within a million years, start appearing on the surface. And billions of years later other organisms, with more conventional metabolisms and brains, that require an entire ecology just to exist, will wonder how abiogenesis actually happened on their world.

And your 'seeds' can take a long time in transit. It is a self-contained ecology that can exist, inside a rock drifting through interstellar space, nourished by cosmic rays and secondary radiation from being hitt by high-speed particles. Just pack 'em up with some long-lived isotope so they don't drift toward absolute zero, and off they go. Maybe in a billion years one of your seeds figures out how to be an orwood tree.
Various thoughts here:

a) This is another example of the universe operating on timescales that humans can likely literally not comprehend.

b) This reminds me of mentions of seen of the deep ocean volcanic vents (black smokers etc.) and how if the Earth dropped into another ice age or the like, they might not even notice.

c) OA being OA, this sounds like something that should be written up in the context of the setting, either as a project(s) being done by some group of Terragens (first thought -the Negentropy Alliance is very interested in preserving and spreading order into Deep Time so might do this. Second thought - perhaps a cooperative project between the Negents and either the Biopolity or the Red Star M'pire, both of which are very big into biotech). Could be the creation/seeding of a planet with such organisms to see what happens over the long term and/or the spreading of 'seeded' rocks through space (either in a lot of star systems or even through interstellar space) with the idea that if something happens to Terragens/life in the Milky Way, the seeds could form the basis for a reboot some millions or billions of years in the future.

Just some thoughts,

A lot of uranium deposits are biogenic, though their activity is more so related to concentrating them and not living off them. And the geology does most of the work but anyways, this raises an interesting theory where the microbes do live off them but also have evolved to concentrate them maybe to support that lifestyle.

Quote:Nikolai, Prince of Denmark — 05/14/2021
But where does it being a main food source for all deep crustal life come from?

destrucules — 05/14/2021
Oh, that? That comes out of studies of Kidd Creek, where microbiologists were looking at fluid inclusions that have been hydrologically and chemically isolated for ~2 Gyr. It was found that quiescent radioactive decay from the walls of the inclusions was sufficient to create a small chemical disequilibrium in the water, and active chemolithoautotrophs (we need an acronym for that or something) were found feeding on that disequilibrium. As far as I'm aware, only a minority of organisms directly feed on radiation, but since uranium is one of the three main radioactive elements in Earth's crust, and since the deep biosphere, or at least the deep continental biosphere, seems to be largely fed by radioactivity, uranium must be one of the major energy sources for the deep biosphere. It's not the main source or anything, but it's one of the main ones
Some planets would be more radioactive than Earth, so might support more chemolithoautotrophs than Earth does. If deep biospheres of this kind are commonplace in the universe, these organisms may be the most common (and/or the most widespread) lifeform by mass (and/ or by number).

Also more possible sites for the evolution of sophont biofilms, or natural biogeocomputing substrates (BCGs). BCGs not only appear in Orion's Arm, but are also present in Stephen Baxter's stories - they are a major player in the Proxima duology, and have occasionally appeared in the Xeelee Sequence, too.