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In my latest draft, a worm-like species buries itself deep below the crust of their world and consumes/gathers tiny amounts of heavy metals.

Can you think of any plausible reason why a wormlike creature would do so? Why would they spend time and energy digging deep and gathering small pieces of metal?

Some Details:

  • Location: Earth-analog
  • The species itself is not really fleshed out right now.
  • They do return to the surface. Its either part of their lifecycle or daily/weekly/monthly routine.
  • the intake of metal takes places many hundreds of metres below the surface, at least 1000 metres below. I should make the heat-resistant, or should I?
  • the metals take a significant amount of their worms bodyweight.

Bonus Question: Is that deep below the surface a suitable place for heavy metals or do I need to mess with the geology of my earth analog for this?

Edit: As Suggested by John Dallman the plot reason for this:

The worms are harvested by humans, who need the heavy metals. It kind of harvesting spider silk in terms of quantity, maybe a order of magnitude higher in output. This work should be dirty, dangerous and really unpleasent for the poor peasants.

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    $\begingroup$ What is the plot reason for them doing this? A clearer idea of that would help people produce in-world explanations for it. $\endgroup$ – John Dallman Jul 23 '16 at 22:45
  • $\begingroup$ The book Half Way Home by Hugh Howey has a creature that is basically like this - might be worth checking out! $\endgroup$ – Jesse Williams Jul 26 '16 at 22:15
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With discovery of bacteria such as these mentioned in National Geographic and those used commercially by companies such as BacTech we know certain bacteria can collect heavy metals, materials with magnetic/electrical properties and just about anything else you like.

These worms could have evolved to feed on these bacteria, deep in the mines, similar to the "Devil Worm" here on Earth. Eventually, one can imagine the bacteria becoming part of the digestive process, just like we host many digestive bacteria which are not part of our genetics but certainly an aspect of our biology.

As the worm species branched out, metals would be carried from deeper and deeper toward the surface, just as worms churn nutrients from bedrock up through the soil. So there are plenty of metal veins which would have become mixed into the soil and likely you will have places with plain metal veins still existing too.

There's no need to go to particular depths, but geological formations such as mountains are more likely to have churned up fresh metals. Wherever chances are higher that metals came up from further down in the core via convection and volcanic activity.

Note: You can involve some other processes with this. Worm excrement may be a special alloy or thread for instance. Different species may have branched off to specialize in particular metals or grow armored exoskeletons that can be harvested. Things like that.

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You could frame it as an example of bioaccumulation. The worms consume soil but have no physiological way to digest/secrete/excrete the heavy metals. As they get older the concentration of the metals is several times larger than the ambient concentration in their environment. (This is a real problem in Earth ecosystems as certain animals have the ability to accumulate nearly toxic levels of heavy metals)

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  • $\begingroup$ Thanks! Is there any way to accelerate/magnify the process, making it a viable source of grams to kilos of metal? Even with a very great number of worms, I think you would only get trace amounts with each specimen. $\endgroup$ – openend Jul 24 '16 at 0:24
  • $\begingroup$ Maybe allow the worms to grow enormously large as they age? They could become snake sized in a month and eventually the ancient worms are so large their tunnels cause sinkholes and create huge caverns etc. $\endgroup$ – Dan Burden Jul 25 '16 at 0:09
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Since rare earth metals are by definition rare, I'm not confident your worms could burrow effectively to find them. You may want to consider instead using your planet's oceans to transport the heavy metals (as ionic salts), and have your worms collect them from the passing tides. In the water, heavy metal ions would be more evenly distributed, simplifying the search for them, and your worms could be sessile filter-feeders. If you want to increase the concentration of heavy metals in one place, introduce a predator that eats these worms and either collects or excretes the heavy metals it encounters.

Within that context, Earth already has a broad range of creatures that collect a regular metal (Calcium) from their ocean environment and use it to construct their skeletons. We call them mollusks and corals. It is not unreasonable to expect that on a different world, your worms might find use for heavy metals and collect them from sea water instead. As one option, I suggest that (except for our theoretical predator), the presence of heavy metals in your worms' bodies might make them less desirable as a meal.

NB: you probably don't want to have one worm collect multiple kinds of heavy metals, as each one has distinct chemical properties. Pick one, and have your worms use that.

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The worm would be that deep looking for metals to find bacteria to eat. When they eat the bacteria, the heavy metals accumulate in the body of the worms. The bacteria to eat would be there taking advantage of some as-yet-undiscovered metabolic pathway.

There are many metabolic pathways discovered (https://en.wikipedia.org/wiki/Metabolic_pathway). Its not hard to imagine one that involves or is catalyzed by a rare metal (https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth's_crust).

Alternately, in the more way out science fiction, the bacteria could thrive way down in the aethenosphere where high temperatures (1300 C) allow this alternative metabolic pathway to operate. Any worm that could bore that deep would have to have physiological adaptations that would be very valuable.

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The worms are resistant to heavy metal poisoning, but all other animals, including their predetors, are suseptable, the worms seek them out as a chemical defense mechanism. Milions of years of evolution have programed them to search for the highest consentration of those metals and they bio-accumulate. Their breeding, however requires the surface so they breed when they are toxic enough to roam the surface with inpunity.

Bonus: sinse the resistance is likely specific to a single metal, they search for it selectively and, when melted like ore, release it in pure form.

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An alternative version of Kiran's answer: the worms eat those metals because the high concentration in their bloodstream protects them from parasites.

Another alternative: as they go deeper into the ground, the concentration of oxygen drops. They need a lot of iron to make a lot of hemoglobin and myoglobin so that they can absorb more oxygen when close to the surface, which allows them to burrow deeper than any other burrowing creatures.

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Many metallic compounds can be exploited by a number of bacteria for energy, in the deep earth out of touch with sunlight and biological compounds made with solar energy that's going to be a primary source of cellular energy for any number of creatures so that's why the metals would be taken up. Yes at depth metals are more common in the rocks than on the surface. The worms needn't be particular heat-proof at 1000 metres, the average thermal gradient underground is 25 degrees C per kilometer down to about 400 km of depth so use that as a guide for thermal resistence. The other thing that goes up as you go down is the "background count" so a lot of what the worms pick up will in fact be at least slightly radioactive, the levels of radiation will depend on exactly what rocks these worms are chewing through, intrusive igneous rocks contain more uranium thorium etc... than deeply buried sedimentary rocks. Metals are inter-soluble, they dissolve into each other, so a life-form that harvests metallic compounds will always pick up metals that it doesn't want along with those that it needs. What does that mean for your worms? They're going to accumulate toxic metals in their tissues along with the target metals. These guys are also going to be tough, in terms of physical texture, because they have to deal with very high pressures in their native environment.

In summary you have a creature that is hard to kill/harvest and extremely toxic when you do that yields a lot of radioactive and/or poisonous material along with the target metals that you want to get out of them. That job is going to suck.

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