Some follow-up thoughts on this question...

Hydrogen isn't very soluble in water. Oxygen is more so, but still sufficiently insoluble that most oxygen-breathing Earth creatures use special oxygen carrying proteins (most famously, haemoglobin) to move oxygen through their blood rather than just relying on its solubility in said blood. Ergo, it would seem that hydrogen-breathing animals would also be in great need of some gas-transport assistance.

As explored in that other question, ammonia is likely to also be quite common in a hydrogen-breather's environment, and can also be used as an electron donor for respiration, and doing so seems like a good strategy for aquatic organisms, which would therefore not need to care about efficient hydrogen transport; however, a land creature would be well advised to minimize its metabolic destruction of ammonia so as not to upset the pH and osmotic balance of its own bodily fluids.

So, what would make a good equivalent for the red-blood-cell mechanism that we use to transport oxygen for a hydrogen-breathing creature? Do they use a reversibly-binding metal-protein complex like us? Do they rely on dissolving hydrogen in non-polar vesicles? Or something else entirely?

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    $\begingroup$ wouldn't any acid do as proton carrier? $\endgroup$
    – L.Dutch
    Commented Jul 24, 2019 at 19:30
  • $\begingroup$ We use oxygen as an oxidizer (i.e. to literally burn sugar through respiration), to answer your question, it would be better to know how your biochemistry works and what your creatures use as an oxidizer, as you can't directly substitute hydrogen for oxygen, as it's a relatively poor oxidizer. $\endgroup$
    – stix
    Commented Jul 24, 2019 at 19:35
  • $\begingroup$ @L.Dutch Yes, but we don't care about transporting ionized protons. We care about hydrogen gas. $\endgroup$ Commented Jul 24, 2019 at 19:35
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    $\begingroup$ Many metals, like palladium, readily form hydrides. This should be the idea, but full answer would require a lot more research. $\endgroup$
    – Alexander
    Commented Jul 24, 2019 at 19:35
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    $\begingroup$ @stix The counterpoint to that argument, is that building biomass via photosynthesis in a reductive environment requires between 5 and 10 times less energy, which means that photosynthesis becomes viable at much lower light levels. ncbi.nlm.nih.gov/pmc/articles/PMC4284464 $\endgroup$ Commented Jul 24, 2019 at 20:18

3 Answers 3


Another thread (Other blood colors) had a user Jim2B post a very in depth post on possible blood colours for binding oxygen, but also referenced this gem: Chloro-carbonyl-bis(tri phenylphosphine)-iridium

An excerpt from the book Xenology (An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, by Robert A. Freitas Jr. http://www.xenology.info/Xeno/10.4.htm) had this to say:

Another interesting though less likely possibility is iridium-based blood. One simple compound with an absolutely horrible name (chloro-carbonyl-bis(tri phenylphosphine)-iridium) has recently been shown to undergo reversible oxygenation. This substance is insoluble in water and other polar media such as liquid ammonia and alcohols, but this presents no barrier to its use in blood. The vanadium chromagen found in ascidians is also insoluble in water.

In solution, the compound takes up one atom of oxygen per molecule to change from brilliant yellow to sullen orange. The reaction is not quite as fast as with the cobalt complexes, so a more convoluted lung would be necessary.

In the oxygenated condition, the iridium-based blood of extraterrestrials would have to be protected from light because it is very photosensitive. The pigment slowly decomposes over a period of days or weeks when exposed to strong light, gradually changing color from orange to green and finally to a deep bluish-black. Such aliens would therefore either have very dark skin, or would inhabit a dimly lit world. (In the absence of light, the molecule is stable for years.)

The iridium complex has one additional property which is extremely fascinating to xenobiologists. In addition to oxygen, the molecule is also capable of reversibly binding hydrogen as well!

So, if you were looking for a a transport protein that could potentially work for your species that breathes hydrogen, and wanted a metal-protein complex like we have, this could do the job. You'd have the restrictions of the photosensitivity I'm afraid, but you wouldn't need to change much from protein blood carriers beyond having organisms living in the dark or protecting their skin from light. Plus you get the cool effect of the blood being coloured something more exotic than red - yellow when oxygenated and dull orange when not.

Hope this tiny bit of into might be able to play a part in figuring out your problem!


Carrying hydrogen gas directly would be difficult, and perhaps not useful. In real biology, the oxygen isn't carried as a gas, but as oxygen radicals attached to the iron atoms in the heme molecule.

However, it is possible. Your best approach would be to use a chelated compound with palladium at its core.

You can think of a chelated compound as a sort of organic "claw" molecule that "grabs" and holds a metal ion. The claw part is called a ligand.

These kinds of compounds are very common in biology, actually. The aforementioned heme (part of hemoglobin) uses iron as its metal, which of course is why blood is red (it is literally rust).

Similar compounds include hemocyanin, which uses copper as its metal instead of iron. This gives blood made of hemocyanin a blue color, as opposed to red (hence the name).

Another example is vitamin B-12, which uses cobalt as the chelated metal.

In chelated compounds, the metal is generally the active site for the molecule as a whole, and the ligands are used to control the reactions associated with it. You can think of the metal as a kind of "power core" in this case.

So we've established, based on real biology, that you most likely want some sort of chelated compound. So why palladium? Palladium is a precious metal of the noble metals group. It has the interesting property of acting like a "sponge" for hydrogen atoms, being able to pack them much closer together than in their gaseous form.

However, the use of palladium and hydrogen here over iron and oxygen introduces its own problems:

In the two heme examples, the molecule is designed to carry highly reactive oxygen radicals that can be passed on to other proteins for chemical reactions. They aren't carrying oxygen gas directly.

Secondly, palladium's "sponge effect" comes from the lattice structure of its metallic crystals. This means you need not just a single metal atom or ion, as in the previously mentioned cases, but hundreds, probably thousands, of palladium atoms in crystalline form. This won't fit into a simple ligated molecule.

Another problem with palladium is that it requires heating to release the hydrogen it's captured. Your creature will have to have differing temperatures across its body; essentially a "cold section" where it breathes in the hydrogen, and a "hot section" where it releases it.

The last problem is palladium's rarity. As a dense and heavy metal, it would sink to the core of any planet its on, and not easily be available to life to use. On Earth, it's about 3 times as common as gold, but iron is literally millions of times more prevalent. Palladium is present in Earth's crust at 15 parts per billion, but iron is present at 63 parts per THOUSAND. There is on average literally more iron in the crust of the Earth than there is salt in the ocean (about twice as much). This will make it difficult for your creatures to find and consume in any kind of meaningful biocycle.

But your question is "how would it work," not "would it work," so let's try and create a scenario where we can use it:

Your creature's blood would need specialized cells, these cells contain small granules of palladium, let's call them "pallophores." Rather than lungs, your creature could breathe through its skin, which is covered thousands of tiny ruffles packed with pallophores in order to maximize surface area. This effectively increases the surface area for respiration, allowing your creature to pump its blood slower. The fact that the pallophores are pumped close to the skin surface allows them to be better cooled for hydrogen uptake.

Because your creature's pallophores are quite literally like little grains of sand in its blood cells, they couldn't be very flexible, and so your creature would need to have relatively large blood vessels, especially capillaries. The tradeoff is that you could get more density of hydrogen atoms packed into the pallophores than you would be able to for oxygen packed into heme.

Now, we need a way to get the hydrogen back out of the pallophores. Probably the easiest option here is for the creature to have "nodes" in its blood stream where pallophore rich blood cells gather, perhaps little sacs of some sort. These sacs are heated, allowing the hydrogen gas in the pallophores to diffuse out into the surrounding tissue. Your creature will need a lot of these pallosacs in order to provide for its hydrogen needs.

The hydrogen gas is reduced with some other chemical, presumably NOT oxygen, into an easily removed (preferably liquid or ionic) waste product. This waste is carried through a second blood loop where it's excreted by a kidney-like organ.

Because of all the biological limitations we've had to place on the creature, its metabolism will necessarily be slow, so in order to survive, the creature must be large and slow. We can use this ecological niche to solve our rarity problem: Let's assume that there are some kind of plant-like organisms that pull palladium out of the soil and concentrate it in their bodies, and your creature eats the plants in order to get the palladium.

  • $\begingroup$ Maybe it could be more energetic if it had more surface area relative to it's total mass. With wrinkly and saggy skin? Or have several tracheal tubes running deep into their bodies, sort of like insects have. Also, hemocyanin usually floats freely in open circulatory systems instead of running through specialized blood vessels like hemoglobin, because it is so big - I imagine the same for those pallophores, even though they are specialized blood cells and not free floating molecules, because they are still relatively big. $\endgroup$
    – aadv
    Commented Jul 24, 2019 at 23:09
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    $\begingroup$ Hemocyanin is also highly inferior to hemoglobin, which is why only a few arthropods use it, and then only in cold environments where it can be more efficient. Bringing the hydrogen atmosphere inside the creature's body will make it harder to cool the pallophores, and palladium needs a temperature difference to be able to absorb hydrogen, so I would imagine evolution would prefer creatures with external lungs (such as perhaps some kind of gills ala an axolotl, or as we've describe, wrinkly skin to increase surface area). $\endgroup$
    – stix
    Commented Jul 24, 2019 at 23:44
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    $\begingroup$ Oh, the ruffles slipped my mind, sorry, didn't mean to be redundant. In defense of hemocyanin: it is more cosmopolitan than it is usually given credit for. It is used in several warm places without crushing pressures, like the tropical beaches of terrestrial crabs and the arid and rocky deserts where spiders and scorpions crawl. You must be right about the temperature difference and I am curious about how changes in temperature would affect the metabolic rate of the creatures. $\endgroup$
    – aadv
    Commented Jul 25, 2019 at 0:37

Alright, I'm gonna frame challenge myself.

In three different ways, in fact.

First: hydrogen may have low solubility in water, but it also diffuses much faster than oxygen, due to both its lower mass and physically smaller size. That means tracheal tubes delivering air directly to internal tissues, without the need for blood transport, will be more efficient for hydrogen breathers, and support creatures of larger size than that system can on Earth. But, it gets even better: if we assume that glucose is the primary food molecule used for energy, only one gaseous waste molecule (methane) is produced for every two dihydrogen molecules consumed (cf. aerobic respiration of glucose, which produces CO2 waste in one-to-one ratio with O2 consumed). That means there is half as much volume of waste gas being expelled into the trachea as there is hydrogen being taken up, which in turn means that, even without active pumping, the trachea will be operating at constant negative pressure just due to regular metabolic processes, thus further improving the efficiency of hydrogen transport from the environment! So, we may not need blood to transport hydrogen at all.

Second: There are ways for land creatures to make use of ammonia after all: rather than primarily breathing their metabolic reducer, they can drink it. Using up ammonia internally will screw with the chemical balance in their cells, but metabolizing proteins does that for us as well, and we have a perfectly good means of dealing with it: urine. So, "hydrogen"-breathing land creatures could in fact avoid having to breathe hydrogen after all, as long as they always have ready access to a supply of drinking "water" (water/ammonia solution) to replenish their ammonia stores. They would then concentrate and expel hydroxic acid (i.e., pure water) in the equivalent of a kidney to maintain a consistent water/ammonia ratio in their bodies.

Third: Even if they do breathe hydrogen, and they do need to transport chemical reducing potential in their blood, their blood doesn't necessarily need to transport the same complete dihydrogen molecules that they breathe in. As L.Dutch pointed out in the comments, any acid will do as a proton carrier; I initially dismissed this idea, as it's not really the protons that we care about carrying, it is the chemical reducing potential--i.e., the electrons. But there is the seed of a workable idea there! Because, you see, as long as suitable electrons can be delivered to metabolic reaction sites, there will be plenty of free protons (or at least hydronium and ammonium ions) floating around to recombine with them, and we need not bother with guaranteeing that any particular electron stays with the same proton that it was inhaled with. Thus, the lung and/or blood tissue of a hydrogen breather can take in atmospheric hydrogen and use it to reduce an intermediate molecule like NAD+ or NADP+ into NADH/NADPH, and then transport those molecules throughout the body, with hydrogen being reconstituted "on the spot" when the intermediate molecule is used in reduction metabolism.

  • $\begingroup$ 1/2 - Hydrogen won't react directly with glucose. Hydrogenating glucose is how we produce artificial sugars like sorbitol, and it requires a platinum catalyst, so you can't "burn" glucose with hydrogen like you can with oxygen. In addition, the output of the reaction isn't CO2 and Water, it's sorbitol (the hydrogen is completely used up). There is no organic chemistry reaction that can split glucose with pure hydrogen in an exothermic manner. Methanogens on Earth use CO2 and H2 to reduce to methane and water, so rather than glucose, CO2 would have to be your creature's "food." $\endgroup$
    – stix
    Commented Jul 25, 2019 at 17:22
  • $\begingroup$ 2/2 In real organisms, the glucose is used in several steps, but ultimately what it is doing is running pumps that move electrons around inside mitochondria to build ATP (the Krebs cycle). Your creature's biology will need a different energy carrier than glucose (and so you'll likely need a completely different ecosystem than on Earth). $\endgroup$
    – stix
    Commented Jul 25, 2019 at 17:23
  • $\begingroup$ @stix Who cares if hydrogen doesn't react spontaneously? We don't rely on uncatalyzed fires in our cells anyways. Of course the reaction will be catalyzed. Hydrogen doesn't burn with CO2 or acetic acid, either, but that doesn't prevent methanogens from existing. And of course it won't produce CO2 and water, it'll produce methane and water. And even if we stick solely with known terrestrial biochemical pathways, glucose can be fermented into acetate, butyrate, CO2, and excess H2, which with additional H2 added can be inputs to methanogenesis. $\endgroup$ Commented Jul 25, 2019 at 17:51
  • $\begingroup$ @stix And anyway, all of that is just for proof of concept. In reality, yeah, they'd probably use some other energy-storage molecules optimized for the ammonia-rich reducing environment and for energy density with regard to hydrogen reduction reactions. But very nearly anything that releases energy through reduction of carbon compounds will still have that same negative-pressure balance property. $\endgroup$ Commented Jul 25, 2019 at 17:53

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