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PFAS (PolyFluorinated Alkyl Substances) are popularly known as "forever chemicals", because they are extremely inert and, being entirely artificial, no natural organism eats them--so, they don't break down, and stick around in the environment pretty much forever.

However, there are at least 5 naturally-occurring organofluorine molecules produced by plants--so biological manipulation of carbon-fluorine bonds is clearly possible! There's just very little evolutionary pressure to figure it out, except for plants producing poisons.

But suppose we change the pressures, and artificially select, or directly engineer, microbes to fix carbon by stripping fluorines from organofluorine substances, to clean them from the environment. What's the safest way for such organisms to dispose of the resulting excess fluorine, that doesn't just result in yet another "forever chemical" polluting the environment?

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Clumpy-bug.

When they're being engineered they need to be fixed to behave like a cross between slime-moulds and coral.

Slime-moulds start as individual amoeba-like cells, foraging for scraps without paying attention to other's of their kind - then when there's a trigger (resource-scarcity, temperature, light duration of diurnal cycle, getting a big enough load of fluorine etc..) they suddenly get frisky. They gather in great masses, their cells fusing so that the form one big cell (plasmodium) with many thousands of nuclei partying within.

That's gotten them in one place.

Coral lays down a mineral skeleton, this would be taken as a behaviour by the geneticists and adapted to the needs of laying-down a relatively inert fluorine matrix, say: magnesium fluoride, magnesium being present in seawater at just over 0.1 % by mass of water. This will produce nice resources of magnesium too without all that terrible mining wrecking the countryside. Many sea creatures use Mg.

If the creatures can be encouraged to make them into lumps, say briquettes that can just be visually recognised and collected by underwater drones, then all the better. As Willk suggested, a coral corral. ;)

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    $\begingroup$ /Corral lays down a mineral skeleton/ Great idea! Calcium and magnesium fluorides added to the reef aka coral corral. $\endgroup$
    – Willk
    Commented May 18, 2022 at 16:33
  • $\begingroup$ Is there any particular advantage to magnesium over calcium? The solubilities of MgF2 and CaF2 look pretty similar. $\endgroup$ Commented May 18, 2022 at 19:23
  • $\begingroup$ Alloy wheels you can't drive through a puddle with? @LoganR.Kearsley $\endgroup$ Commented May 18, 2022 at 21:34
  • $\begingroup$ @JiminyCricket. That's an advantage to humans who might want to use the deposited metals, but that doesn't mean it's the optimal choice for the organisms themselves or the environment they are supposed to be cleaning up. $\endgroup$ Commented May 18, 2022 at 21:36
  • $\begingroup$ As far as the environment goes, oysters urchins and various seaweed are periodically checked for toxic levels of the magnesium as populations keep dying-off, there's too much of it (see the sea-creatures link). In terms of energetics of the reactions needed for this solution to work, you'd need to ask someone better versed in chemistry than I to be certain, but as far as I'm aware it's similar to calcium. @LoganR.Kearsley $\endgroup$ Commented May 18, 2022 at 21:44
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Turn it into fluorite, calcium fluoride, which is fairly common natural mineral and mostly harmless. it requires 1 calcium atom to bind 2 fluorine atoms so it even is efficient.

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Release it as fluoride

F- (fluoride) is pretty stable as an ion. Just let it loose that way, and what you have is a bacterium that fluoridates drinking water. That isn't always a good thing - there are regions where fluorosis, excessive fluoride in drinking water, is common (see slide 11). But in much of the world, you're actually doing good by increasing the fluoride level in the water, if anything, and it will eventually wash away to the sea regardless.

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