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I've looked up the different "Colorations" of blood online, different metal-based proteins have differing levels of oxygen transfer efficiency, but those were answers I didn't find compelling enough to explain wacky blood colors for my own species.

I was wondering if perhaps, due to a change in diet, or because of location (ie. a different star system), or medicine technology (plasma or other immune/blood cells or primitive nanobots) would be able to change coloration to be purple/magenta.

I was mostly inspired by an article in a wiki for the Sangheili species in the Halo franchise, and they describe "the coloration is most likely caused by bimetallic hemoglobin, possibly cobalt-iron, whereas the crimson-blooded Humans possess monometallic iron-based hemoglobin." - "Sangheili" Article from "Halo Alpha" Halo Wiki.

I could just handwave the issue like in the referenced article, but I am curious as to whether or not that could scientifically happen in nature. Could a species likely or theoretically develop bimetallic blood efficient enough to rival the performance of monometallic blood like in humans and animals on Earth?

The purpose mostly being aesthetic, but a scientific explanation would be some fascinating flavor text for my story.

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Many mollusks often have both hemoglobin and hemocyanin in their circulatory systems. I found lots of unsourced mentions online for this, but only one from a university (ugly as the page may be).

About snails:

Blood is colorless, and has both hemoglobin and hemocyanin in most species

About clams:

There are a few examples of where haemoglobin [sic] is dissolved in the plasma and hemocyanin may also occur.

This does not make their blood purple. Rather, it's quite translucent and with a bluish hue, perhaps due to the low concentration of both hemes and the cyanin one being more prevalent.

Allow me to insert (the word) penis here by means of the Priapulida annelids (from Gr. πριάπος, priāpos 'Priapus' + Lat. -ul-, diminutive, so it kinda reads like a small but unrelenting erection, despite the worm itself not ever getting close to being stiff), often called penis worms. These creatures, along with other less phallic worms, use hemerythrin as the oxygen carrier in their blood. This makes their blood colorless when deoxygenated, but purple when oxygenated.

Hemerythrin has a structure that differs from hemoglobin, and the actual oxygen capturing mechanism is quite complex, but iron is the only metal involved.

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The "blood colors" stuff you'll find on the internet refer specifically to real organisms on Earth, and to the minor variations on these biochemistry themes that are obvious. For a completely novel biology (tree of life) from another planet, there are certainly enough plausible potentials for you to use any color you might like, including magenta.

If you specifically want it to implicate more than one metallic element, that doesn't seem too far-fetched at all. Biology uses all sorts of weird elements here and there, besides the usual H/C/O/N/P. The only real limitations would be for those elements that are so rare that there couldn't plausibly be enough of it on the planet to sustain evolution for a few hundred-million years (iridium? technetium doesn't occur naturally, etc).

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On our planet we have found bloods of various colors, all depending on the type of metallo-protein used in the blood, but all of them use a single type of metal, be it iron, copper or whatever else.

I would dare to say that, if there was any advantage, or at least no disadvantage, in having a bimetallic blood, we would have found it in some organism.

Maybe the reason is that it is already difficult to find a single metal biologically available is already challenging, having to find two of them in the right amount would simply be more difficult, increasing the costs, with no clear benefits.

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    $\begingroup$ That's a very geocentric view, though, both insomuch as bioavailability of metals on Earth as well as environmental factors here versus elsewhere in the universe. You're definitely not wrong, but there's an opposite plausibility to posit - that bioavailable metals are actually higher on Earth than elsewhere and organisms could develop on another planet that use 2+ metals due to scarcity (being and either-or-both approach required for maintenance of life). $\endgroup$ Apr 1, 2021 at 17:56

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