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I'm working on a hard sf novel about humans being forced to survive on a metal-poor planet. They have to take dietary metal supplements at first but eventually a bio-engineer rigs the biochemical pathways from a native species to work in their offspring using bacterial and viral vectors. I'd like a non-metallic element to assist in oxygen gas exchange. An alternative is that they use iron but that it is recycled, somehow, within the body. So, for example, heavy blood loss would be a serious accident since the iron in the blood would be hard to replace. Any ideas would be appreciated.

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    $\begingroup$ Hey tgrignon, I've added the tag chemistry, as it might be useful to have that specialty on-board with this question. If this goes against your wishes, feel free to remove it. $\endgroup$ – We are Monica. Aug 14 at 16:02
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    $\begingroup$ You're going to need more than just a replacement for iron, you're going to need a whole new molecule that isn't hemoglobin. Hemoglobin is designed to work with iron, and its molecular structure wouldn't allow anything but a metal to be used (it's a chelated compound). Also: The liver works very hard to recycle iron already. $\endgroup$ – stix Aug 14 at 16:03
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    $\begingroup$ @Willk This is not a duplicate. All the answers to that previous question involve metal ions, while this one is specifically asking for non-metallic options. $\endgroup$ – Logan R. Kearsley Aug 14 at 16:50
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Fluorine.

Specifically, fluorine bound into perfluorocarbon molecules. These substances can dissolve ridiculously high concentrations of oxygen--enough so that they can be used for liquid breathing. Some perfluorocarbons can even carry more oxygen and CO2 than blood can (that's by bulk measure, though, and you definitely don't want to simply replace blood entirely with perfluorocarbons to get that performance--you'd break a lot of other stuff in the process!).

Instead of normal red blood cells, your modified people would have perfluorocarbon vesicles--basically, fancy oil droplets--suspended in their bloodstream.

Of course, there are numerous disadvantages to such a system. For one, fluorine isn't exactly a common element either, so they may be better off sticking with the iron supplement anyway. For another, haemoglobin alters its binding strength according to pH, so it selectively releases oxygen in areas that need it most (i.e., tissues which are producing a lot of metabolic byproducts like CO2 or lactic acid). Oxygen-solvating vesicles would not have that kind of specific response, thus further reducing the efficiency of oxygen delivery.

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  • $\begingroup$ "fancy oil droplets" suspended in the bloodstream sounds like a sure recipe for an embolism to me... No thanks... $\endgroup$ – stix Aug 14 at 17:50
  • $\begingroup$ This answer surprised me. It is a gas though, right? Until it's bound into the PFC? $\endgroup$ – tgrignon Aug 14 at 17:54
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    $\begingroup$ @stix and fancy globs of protein encased in fat don't? $\endgroup$ – Logan R. Kearsley Aug 14 at 18:21
  • $\begingroup$ @tgrignon In elemental form, yes, but you wouldn't acquire it that way, just like you don't get dietary calcium or sodium in highly reactive elemental form. It would most likely enter the biosphere in the form of various fluoride salts, which are solids. $\endgroup$ – Logan R. Kearsley Aug 14 at 18:23
  • $\begingroup$ @LoganR.Kearsley Yes, actually. PFCs have extremely high viscosity and poor CO2 removal. This is a problem in their use as a medical treatment and in liquid breathing. $\endgroup$ – stix Aug 14 at 18:24
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Water

Everyone loves water for biology. A water molecule can carry an extra oxygen and in doing so becomes hydrogen peroxide. Our bodies generate hydrogen peroxide which circulates around and is induced to give up its oxygen via various enzymatic reactions.

https://www.ncbi.nlm.nih.gov/pubmed/23849856

Hydrogen peroxide is generated in numerous biological processes and is implicated as the main transmitter of redox signals. Although a strong oxidant, high activation energy barriers make it unreactive with most biological molecules.

Water is not a metal. It can carry oxygen as the peroxide and as opposed to sulfates or borates, carries it loosely. As the iron containing heme ring is shepherded about by the huge protein hemoglobin, so there would be a hemoglobin analogous protein which would protect one or more molecules of hydrogen peroxide, and mediate the release and reuptake of the oxygen.

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  • $\begingroup$ For reference: the average molarity of haemoglobin in normally-saturated blood is around 2mM. Haemoglobin can carry up to 4 oxygen molecules, but most haemoglobin in arterial blood is not fully saturated, so typical oxygen molarities are around 5mM. Each peroxide molecule carries half an oxygen molecule, but at slightly higher oxidizing potential, so you're looking at 8-9mM concentration of hydrogen peroxide in arterial blood. That's actually not too bad. I can easily believe stabilizing protein complexes could keep that safe. $\endgroup$ – Logan R. Kearsley Aug 14 at 19:15

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