Bacteria trapped in crystal inclusions found 'alive' after 50,000 years - what were they eating all that time?

Question

The phys.org article Biologists find weird cave life that may be 50,000 years old describes the announcement by NASA Astrobiology Institute director Penelope Boston at the 2017 AAAS meeting$^{(1)}$ of micro-organisms found in small inclusions within crystals of hydrated Calcium Sulphate (Gypsum) that had grown while underwater in a cave in Naica, Mexico. It is estimated that some of these organisms have been isolated within inclusions for as long as 50,000 years, and yet can grow and reproduce when carefully extracted and provided with fresh chemosynthetic nutrients.

(BBC Radio interview with Penelope Boston)

This particular discovery has just been formally announced so there is no peer-reviewed material to read yet, but it's possible someone here in Biology SE attended the meeting or has read further about the announcement.

50,000 years is a long time (for a bacteria, not a crystal), and if I understand correctly the energy source for these organisms is chemosynthesis. Put simply, wouldn't they have eventually used up all their food and died? I'm thinking the crystal is a good electrical insulator and the cave was dark, so there couldn't be external energy sources to replenish the oxidation state of the iron or sulphur or whatever they were eating.

The elevated temperature and ubiquitous radiation would have presented a relentless, potential source for DNA damage mechanisms, and repair would require a constant supply of energy. So I'm guessing there had to be some minimal source of energy to keep them viable, if not actually 'alive' for 50,000 years.

Is this thinking roughly correct? If so, what might that source of energy have been?

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Below: Giant gypsum crystals in a cave in Naica, Mexico, from here. Note the person for scale at lower right.

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(1) Currently 2017 AAAS meeting links are all re-directing to the 2018 meeting, so I can't find a link to the particular talk or session.

Answer 1

The question is interesting, but I must say it is too early to say anything. But let me tell you what I can.

Dormancy, first of all, is a state in which all metabolic activities of an organism temporarily stop or slow down, and when saying all, I literally mean it. Dormancy can be considered, in layman's terms, as molecular level of hibernation. Any organism, in dormant state, does perform all kinds of activities, but in super slow motion. Again, we don't know anything about those microbes, so I can just tell the currently known mechanisms by which those microbes could have survived. Gypsum i.e. $CaSO_4.2H_2O$ contains sulfate ($SO_4^{2-}$), so those microbes (let me call them X from now) must be sulfur metabolizing ones to survive. Now, known mechanisms of sulfur metabolism include:

• sulfur oxidation
• sulfate reduction
• sulfite reduction

which are summarized as:

source

Now, with sulfate already available from gypsum (considering dissociation) as:

$CaSO_4.2H_2O \rightarrow Ca^{2+} + SO_4^{2-} + 2\hspace{1mm}H_2O$

The reactions it can perform are sulfate reduction and sulfite reduction. Some of the mechanisms are:

source

In the presence of $CO_2$, glucose can also be formed as:

$12\hspace{1mm}H_2S + 6\hspace{1mm}CO_2\hspace{1mm}\rightarrow\hspace{1mm}C_6H_{12}O_6 + 6\hspace{1mm}H_2O + 12\hspace{1mm}S$

The required $CO_2$ might come from respiration. During dormancy, this process is highly slowed down, so that X may have survived such a long time by just this process.

Again, this is just the known mechanism, X may have evolved other, possibly even more efficient, mechanism. The above example was just to suggest how X could have survived in gypsum crystals for as long as 50K years. We don't know anything about X yet. From the article you cite:

If confirmed, the find is yet another example of how microbes can survive in extremely punishing conditions on Earth.

meaning we don't even know yet whether they are really viable or not.

The life forms—40 different strains of microbes and even some viruses—are so weird that their nearest relatives are still 10 percent different genetically. That makes their closest relative still pretty far away, about as far away as humans are from mushrooms, Boston said.

Now, viruses ain't alive. And 90% similarity means a lot of difference, which means they almost certainly have different mechanisms for survival. At last, I would again remind that it is too early to say anything unless some scientific studies are performed and published.

P.S. talking about energy source, even the heat and radiation from environment could be used as a source of energy. Obviously, harnessing energy directly from external heat is pretty difficult, but when one finds fungi using radiation for growth, then you never know!

Answer 2

Why do you presuppose bacteria needed to "eat" something all the time to survive. Let me give you an example-if I take a batch of a billion bacteria, throw it on a meteorite and let it bathe on the "sunshine" in space for a million years, then took it back to Earth, put it in a culture where the bacteria can grow again and out of those billion bacteria a single one was able to restart its metabolism and grow a new colony of billions of bacteria, would you consider it alive all those years it was in space? Especially, when it comes to bacteria, because they are so small they can easily go to huge numbers and if just a tiny fracture of those manage to survive you have Life Immortal.

The matter of Life is it can look quite, for the lack of a better word, "dead" for a very long time and despite not having the "need" to eat anything it can(when given the right conditions)flourish back to its full potential. Life can be pretty resilient and biomolecules remain unscanted for very long periods of time, so the question-"What it feeds on?" can be surmised as-"Can it survive unscented for so long?". And be sometime the answer is-Yes!

You may be interested to read this here and see the original paper here