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I’m currently working on a precursor species who have mercury based blood. They are bipedal reptiloids (similar to the Silurians in Dr. Who) whose skin is covered in iridium/osmium scales. A single individual's lifetime can often last a few hundred thousand years. They evolved roughly 10.5 billion years ago, they are highly adaptable, capable of surviving the most extreme conditions. But I need help in constructing their biology. Would mercury facilitate oxygen-based respiration or would they have to breathe something else; ammonia or methane for example? What elements or compounds would work in forming their musculoskeletal system? What could be used to form mercury-based amino acids… basically what would they be made of. Humans are primarily carbon, oxygen, and hydrogen, what would they be?

I know it’s a very broad question but I do not know anything about xenobiology.

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    $\begingroup$ It depends on what they use mercury for, really. Is it an analog to iron in Earth hemoglobin? Is it merely a required mineral for some biological pathway other than respiration? Both of these are semi-plausible. Respiration of gases is almost certainly oxygen, there doesn't seem to be any other respiration chemistry that makes sense. $\endgroup$
    – John O
    Oct 12, 2022 at 16:49
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    $\begingroup$ Their plumbing would require some minor adjustments. youtube.com/watch?v=GvVaaZ21C44 $\endgroup$
    – BillOnne
    Oct 12, 2022 at 16:55
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    $\begingroup$ Would there have even been enough time for enough heavy metals to be formed in the universe for them to have evolved 10 billion years ago? At least carbon forms a lot quicker and more abundantly early in stellar formation. The cycle of their species evolution would have needed to be extremely short (1-2 billion from microscopic to full intelligence). $\endgroup$
    – DWKraus
    Oct 13, 2022 at 12:36
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    $\begingroup$ Mercury as a fluid for circulation is problematic in any number of ways. We can assume that it's some metallic organism, but many metals and minerals dissolve in it, presumably more than a few that would be necessary for a metallic organism. Furthermore, it's not even liquid much colder than is on Earth, which is probably the wrong temperature range for such an organism to have evolved. Some of the aliens in James Hogan's Code of the Lifemaker used solid mercury as components. $\endgroup$
    – John O
    Oct 14, 2022 at 13:41
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    $\begingroup$ It's a decent rule of thumb that replacing a molecule with an element in any system is going to have weird effects and may be outright impossible. Water has a lot of really useful properties that mercury emphatically does not. Being liquid is not enough. $\endgroup$
    – jdunlop
    Oct 14, 2022 at 17:54

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Short answer: The reactivity of mercury might cause problems.

It was pointed out before that mercury is hydrophobic. However, I don't see this as a big problem. Water (or blood) can dissolve hydrophobic substances using solvents in low concentrations, so I guess most hydrophilic substances could be dissolved in mercury with some solvents. In the human body, there are a lot of hydrophilic molecules and few hydrophobic ones. I think it is possible to invert the ratio there and still have an organism that somehow works.

@Sean OConnor correctly pointed out that it is a bit of a strain to refer to hydrocarbons and mercury both as non-polar. While they both are not polar it might imply that they can mix freely like ethanol/water which is not the case. Solubility with hydrocarbons is limited, but possible (https://pubs.acs.org/doi/10.1021/acs.jced.6b00173, https://www.dcceew.gov.au/environment/protection/npi/substances/fact-sheets/mercury-compounds)

In contrast to water, mercury is quite reactive. It reacts with organic molecules and most metals, so it might be difficult to find sufficient substances that don't react with mercury. But assuming there are substances that are inert enough, it should work.

We can only speculate, what a musculoskeletal system or amino acids would look like. Except that they must be resistant against pure mercury. I guess they would still require some amount of non-metallic atoms like carbon since the ability of mercury to form complex molecules is limited. Mercury seems to create two bonds to non-metallic atoms when it forms compounds with them, so maybe it could replace oxygen with a lot of changes in the biochemistry.

But I must say I wonder where your species is gonna find all that mercury.

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    $\begingroup$ Water is incredibly reactive, and quite a bit of metabolic effort goes into preventing or repairing spontaneous hydrolysis. We're just preferentially built out of things that happen to react slowly or usefully with water, rather than things that react violently with it, because we use water as our primary biosolvent. $\endgroup$ Oct 14, 2022 at 15:52
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Mercury is nothing like water and probably cannot be used as a water substitute.

One of the most important chemical qualities of water is its polarity, which allows there to be chemicals (lipids) which have a hydrophobic and a hydrophilic side. This is what allows cell membranes to exist, and without cell membranes it would be difficult for life to form in a manner that separates between its inside and its outside. Ammonia, notably, is a polar molecule, which is why you often see it as a replacement for water in speculative xenobiology.

Mercury is a non-polar elemental metal and cannot be used in this manner. So I would not expect to see mercury-based life.

Mercury does have the interesting property of being able to "dissolve" most metals, forming amalgams which can flow like liquids at temperatures much lower than these metals would normally melt. If you want an organism that uses mercury in its biochemistry somewhere, perhaps it has a mercury-based secondary circulatory system which allows it to digest metals (perhaps by spitting up mercury on them and then sucking up the amalgam) and assimilate them into its physiology, like its bones or shell or something.

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  • $\begingroup$ But one is assuming that they are based on polar compounds, surely non-polar compounds such as oils and alkalis rather than amino acids, if one abandons the norms of terrestrial biochemistry, surly the possibility of non-polar life exists. $\endgroup$ Oct 13, 2022 at 12:27
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    $\begingroup$ @PrinceThomasthe42nd Trying to build a cell without a polar medium is like trying to build a car without tools. You don't necessarily need a specific set to tools to make it happen, but you cant just mash rocks and coal together with your hands and get a car. $\endgroup$
    – Nosajimiki
    Oct 13, 2022 at 13:28
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    $\begingroup$ @Nosajimiki What about a cell membrane with the polar sides on the inside and non-polar sides on the outsides? Obviously, it does not exist, but it does not seem completely implausible either. $\endgroup$
    – Matthias
    Oct 13, 2022 at 13:46
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    $\begingroup$ @Matthias OP is looking to use mercury which is by definition non-polar. So even if some exotic form of life was based on this idea, it would not match the OP's description. $\endgroup$
    – Nosajimiki
    Oct 13, 2022 at 15:37
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    $\begingroup$ It's not just polarity. The polar/non-polar solubility rule is a simplification of a load of more complicated solubility trends. Mercury isn't miscible with oil, despite them both being non polar. Mercury is really only miscible with other metals. It does some complicated liquid metallic bonding. This means it's more or less useless as a biolgical solvent, unless you get very creative. $\endgroup$
    – user86462
    Oct 14, 2022 at 1:27
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How biologically different would a species that used mercury rather than water be?

In short: enormously. Chemical details aside, they would be about 13 times denser than water-based organisms, which would have enormous effects on biomechanics.

Addressing details of biochemistry is difficult as there is very little information available on the properties of mercury as a solvent. Would mercury facilitate oxygen based respiration? Well... maybe? It certainly doesn't dissolve very much oxygen. A hydrogen-breathing or nitrate-based metabolism might be more practical, on the basis that hydrogen can basically diffuse through anything and mercury nitrate will dissolve in pure mercury. What elements or compounds would work in forming their musculoskeletal system? Who the heck knows! Mercury is weakly soluble in some organic solvents, so it makes sense that hydrocarbons may be weakly soluble in mercury as well--perhaps there are organometallic groups that are more thoroughly soluble (since mercury is very good at dissolving other metals, with the notable exception of iron), in which case muscles, and tissues generally, would still be basically "organic", but with lots of incorporated metals. What would bones be made out of? Well, something that mercury doesn't easily dissolve! Iron-based bones would be cool, but calcium carbonate and calcium phosphate should also still work just fine, or some kind of organometallic polymer analogous to lignin or chitin.

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