Alternative elements for oxygen transport in an alien blood
I am designing an alien civilization for a story, and I am currently in the biological part. They will be oxygen breathers, so they will need to have that oxygen delivered inside their bodies, and I am looking for alternatives to that function: I don't want to use iron oxide to transport the oxygen, or copper (hemoglobin or hemocyanin). Do you have an idea of which some other elements could be efficiently used for that function? I am not asking about the blood dynamics (I will invent that later). I just want to find a plausible logical alternative to iron or copper.
EDIT 01:
They come from an Earth-like planet with 1.15 times the gravity of Earth, with an oxygen concentration of 35%. They evolved from sea creatures and have squid-like bodies with four tentacles evolved into legs, and two of the tentacles used for manipulation. They are about twice the size of a human when they stand, and they need oxygen masks when they come to Earth (because of the 21% of oxygen in our atmosphere). The protagonist of the story is precisely an alien who chooses not to use mask and adapt himself to the low oxygen ratio (like an Everest climber). Aliens with masks can jump higher and run faster because of the reduced gravity, but this one preferes the freedom of living without mask, even if that means getting tired very easily.
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Coboglobin is one of the main iron- and copper- free proteins that can be used for oxygen transport. It was first synthesized by humans in 1970, where cobalt was intentionally substituted for iron in a hemoglobin-like protein. It also looks like it's been talked about before on Worldbuilding. To have a creature evolve to use coboglobin, you'd want an environment that . . .
- is relatively warm.
- has a higher concentration of oxygen.
Coboglobin is less efficient that hemoglobin, so you probably wouldn't see it arise naturally on a planet unless iron was in short supply and cobalt was much more abundant than it is on Earth.
Regrettably, I had to rule out a number of more promising hemoglobin-like oxygen carriers proteins because they contain iron or copper (note also that some are simply not as efficient as hemoglobin):
- Myoglobin, which contains iron and is found in muscles.
- Chlorocruorin, which contains iron is found in a number of worms.
- Hemocyanin, which is very common in a number of molluscs but contains copper.
- Leghemoglobin, which has iron and is found in planets as an oxygen-carrier.
Now, it's notable that under certain conditions, proteins may only have to account for minimal oxygen transport. Even in humans, the normal plasma in blood can transport some oxygen on its own, but members of the Notothenioidei order of fish require virtually no hemoglobin. Part of the reason for this is that the water they swim in (cold, polar water) is extremely oxygen-rich, and so efficiency of oxygen transport is less of an issue. Yes, coboglobin works well in warmer environments, not cold ones, but if your world has an atmosphere rich in oxygen - say, perhaps 50% by mass - then maybe efficiency would be less important.
Fortunately, your additional specifications seem to conform to these requirements. A 35% oxygen atmosphere is pretty close to twice as oxygen-rich as our own - maybe not quite enough for an organism to not need a hemoglobin-like molecule, but still good enough that efficiency is less of an issue. Plus, given that you've said that the aliens evolved from aquatic creatures, perhaps their ancestors were like the Notothenioids, and had very little hemoglobin (or, here, coboglobin).
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