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A concept I've encountered a few times is aliens with "vastly" different biology than terrestrial creatures. In this case, I'm interested in replacing the parts that use oxygen with something a bit milder (a worse oxidizer).

I'm looking for something that would, biologically, serve the same function as oxygen in a human. It should be a gas in a reasonable temperature range around liquid water, and it should be reasonably produced in sufficient quantity by natural processes although I'm not concerned with the actual manner in how it is produced.

Bonus points if it's something that sticks around better in lighter gravity.

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    $\begingroup$ keep in mind oxygen is the most common oxidizer in the universe, and will be even more likely around liquid water. $\endgroup$ – John Apr 12 '18 at 23:14
  • $\begingroup$ What does "milder" mean? Oxy is position #8 on the periodic table. The only gases "lighter" than it at room temperature are nitrogen, helium, and hydrogen. Try visiting here and seeing if any of those oxidizers would work for you. $\endgroup$ – JBH Apr 12 '18 at 23:17
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    $\begingroup$ @JBH By "milder" I mean "worse at oxidizing." I've seen that page but, being neither a chemist nor a biologist, I can't tell if something would even be remotely viable. I know Chlorine Tetraflouride would be a really bad choice, but... that's about it. $\endgroup$ – Andon Apr 12 '18 at 23:29
  • $\begingroup$ @John I'm aware, and that's outside of what I'm worrying about in this question. Liquid water is simply the temperature range, not necessarily the location. $\endgroup$ – Andon Apr 12 '18 at 23:30
  • $\begingroup$ Anaerobic bacteria use a whole range of oxidisers (and reducers): iron compounds, sulphur compounds, nitrogen compounds, methane. Google "redox tower". Here's an example diagram of some of the chemicals used: rstb.royalsocietypublishing.org/content/363/1504/2755 $\endgroup$ – DrBob Apr 16 '18 at 18:42
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Fluorine and Chlorine are the only common gaseous oxidizers that don't contain Oxygen, to the best of my knowledge. However, neither are especially common in the universe. Furthermore, because they've got 7 electrons in their outer shells, they don't tend to form terribly complex compounds. Oxygen is common, forms lots of different compounds, and is a good oxidizer.

A more probable oxidizer would be Sulfur. Sulfur reducing bacteria can use it in place of Oxygen, so it's use is definitely biologically possible. However it won't be a gas, or even a liquid, in environments with liquid water. Sulfuric acid would be liquid, and makes a great oxidizer, but it contains oxygen, so any environment with sulfuric acid will probably contain oxygen, as well.

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    $\begingroup$ Somewhat tangental: Could a creature not "breathe" via consuming sufficient quantities of an oxidizer? IE, eat a brick of sulfur, and the body stores it and uses it as needed? $\endgroup$ – Andon Apr 12 '18 at 23:38
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Disclaimer: I love this question, but I don't have the PhD to answer it well. I'm going to take a shot at it nonetheless. I won't laugh at me if you won't laugh at me, OK?

We're looking for a way to replace CHON, an acronym for Carbon, Hydrogen, Oxygen, and Nitrogen, the four most common elements in Terrestrial life. Our goal? To create a biome that isn't based on Oxygen (I need to ignore "mild" aka "worse at oxidizing" 'cause, frankly, I'm not even sure if I can make this believable work).

Basically, you need a liquid to replace water that isn't based on oxygen. For no reason other than the feel of sunlight on my left cheek, I'm going to try Fluorine.

As a test, H2F (fluoronium) is an acid. From a ridiculous POV, so is water (the "universal solvent"). Humans don't dissolve when they drink water, so let's assume a creature that won't dissolve when drinking fluoronium.

So, let's take on glucose. All creatures need energy, right? Can I make a glucosy something using fluroine? I'm guessing the basic problem is the CH2OH part of the molecule. What is X for XH2FH? I'm thinking phosphorous. PH2FH.

And I'm not going to take on proteins 'cause if by now you think I'm doing anything other than pulling interesting associations out of an etherial hat... I'm so far out of my depth... But I'm having fun, so ppfffffhpt!

So, now we have PHFN. My spidey-sense is telling me we can't use Nitrogen with Fluorine, so we need something a bit more adept: let's use chlorine! (It actually works well with Fluorine. I think...) That gives us PHFCl and, I'm guessing and hoping... the building blocks of life...

On a planet not dissimilar to Venus. Hot and acid prone. The creature would be much heavier than humans (heavier molecules), and would likely favor yellow over red for the basic wine of life. (You know... blood...)

So, fluorine...

  • It is an organic oxidizer
  • It is gaseous at room temperature
  • It exists on my planet vs. oxygen in the same ratio that oxygen is to fluroine on Earth.
  • The molecules it creates would be heavier, which is good for lighter gravity.

And though students of chemistry and the very Angels in Heaven are probably weeping over this answer (or laughing, it might be hard to tell the difference with all the tears), I give you my favored candidate:

Fluorine

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    $\begingroup$ I don't think you can get PH2FH. Flourine and Phosphorous don't bond that way. Phosphorous has five valence electrons, and flourine has 7, so you could only maybe do PHF. You also can't make aromatic phosphorous rings, to the best of my knowledge. The only stable aromatic structures I've heard of are C6 or Si6-based. $\endgroup$ – ckersch Apr 13 '18 at 15:13
  • $\begingroup$ @ckersch Excellent! However, please note my disclaimer. I was completely shooting in the dark. $\endgroup$ – JBH Apr 13 '18 at 22:05
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    $\begingroup$ @ckersch Boron can also form aromatic structures in anoxic environments. $\endgroup$ – Logan R. Kearsley May 31 at 22:07
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The element whose properties are closest to oxygen is right below it in the periodic table: sulfur.

Wikipedia has this to say about its biological role:

Sulfur oxidizers can use as energy sources reduced sulfur compounds, including hydrogen sulfide, elemental sulfur, sulfite, thiosulfate, and various polythionates (e.g., tetrathionate). They depend on enzymes such as sulfur oxygenase and sulfite oxidase to oxidize sulfur to sulfate. Some lithotrophs can even use the energy contained in sulfur compounds to produce sugars, a process known as chemosynthesis. Some bacteria and archaea use hydrogen sulfide in place of water as the electron donor in chemosynthesis, a process similar to photosynthesis that produces sugars and utilizes oxygen as the electron acceptor.

If oxygen were not present in a world, sulfur would be the next candidate for an oxidizer.

Elemental sulfur is usually solid, but it easily forms gases such as H2S. This is a gas, slightly heavier than O2, and with the characteristic smell of rotten eggs. So any planet with plenty of hydrogen and sulfur (which is not hard... it is one of the most common elements) should have gaseous sulfides.

It also has a cool theme song.

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    $\begingroup$ Sulfur makes a lot of sense, but I don't think hydrogen sulfide is an oxidizer. Most sulfur reducers on earth produce it, rather than consuming it. $\endgroup$ – ckersch Apr 13 '18 at 15:58
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I am shocked an amazed that nobody already mentioned anaerobic respiration--the actual usage of oxidizers-that-are-worse-than-oxygen by actual real organisms on Earth!

Fluorine is not a weaker oxidizer than oxygen, so that's out.

Chlorine is, and the remaining halogens but it's hard to imagine a situation where there would be enough free chlorine in the environment to replace oxygen (although you could, in some types of environments, use chlorine as a secondary option, along with some other oxidizer). There are, however bacteria that use halogenated organic compounds as oxidizers, producing halide ions / acids and hydrogenated organics as output products. So, you could have aliens breathe simple halogenated organics like methyl chloride ($CH_3Cl$), producing CO2, methane and hydrochloric acid instead of CO2 and water as respiration products.

Other electron receptors that are actually used by Earthling organisms include:

  1. Fe(III) ions; e.g., ferric oxide ($Fe_2O_3$) or magnetite (iron (II,III) oxide, $Fe_3O_4$) which gets reduced to Fe(II) ions (e.g., $FeO$) and water. Manganese, cobalt, and uranium compounds can also serve as oxidizers, but all of them are rather difficult to breathe, so, moving on...

  2. Nitrate, nitrite, or free nitrogen oxide gasses. These are produced in large quantities by nitrogen-fixing organisms because they are actually needed in terrestrial biochemistry, and when the organisms containing them die, they can be reclaimed by decomposers as oxidizers. Hal Clement's Nitrogen Fix posits a world in which nitrogen-fixing organisms go into overdrive and eliminate all free oxygen from the atmosphere--which is reasonable, because reacting nitrogen with oxygen is itself slightly exothermic, and could be used as a defense mechanism by plant-analogues to avoid oxygen poisoning (whereas, on our world, oxygenic photosynthesizers just went ahead and poisoned everything...). Using nitrate as an oxidizer produces about half as much energy as straight oxygen, with diatomic nitrogen and water as wastes.

  3. Fumaric acid (C4H4O4), and various other organic oxidizers. Fumarate oxidation produces succinic acid (C4H6O4) as a byproduct. Theoretically, this can be further reduced down to methane and CO2, or all the way to methane and water. In an alien biosphere, simpler organic molecules, like acetylene, might be used as weak oxidizers, producing methane as a byproduct.

  4. Sulfate, sulfite, free sulfur dioxide or sulfur trioxide gas, or elemental sulfur. These all produce hydrogen sulfide and (with the exception of elemental sulfur) water as byproducts. Sulfur trioxide would be expected as a gaseous oxidizer on sulfuric acid worlds (like a cooler, smaller Venus), in which case it would likely only be reduced to sulfuric acid, rather than all the way to water and hydrogen sulfide.

  5. Carbon dioxide. This only works if you can figure out a really strong source of reducing potential to work into your food source; on Earth, it only gets used in the presence of excess hydrogen gas. The ultimate waste products are methane and water (possibly with a detour through acetic acid).

And all of the gaseous options (various nitrogen oxides, sulfur dioxide, sulfur trioxide above 45C, simple desaturated or halogenated organics, and carbon dioxide) all stick around better in lower gravity than oxygen does.

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The strongest allotropic oxidizers (that are somewhat stable in our atmosphere) are, in order of decreasing effect, Ozone (O${_3}$), Oxygen(O${_2}$), Florine(F${_2}$), Chlorine(Cl${_2}$) and Bromine(Br${_2}$), of these the first four, O${_3}$, O${_2}$, F${_2}$, and Cl${_2}$ are all very much gases around 0°C and F${_2}$ and Cl${_2}$ are both heavier than O${_2}$ meaning that they're more compatible with a world with lower surface gravity. Br${_2}$ is a liquid at standard temperature and pressure only becoming a gas around 60°C at sea-level. Depending on the prevailing conditions on the world you wish to create any of the three halogens is a potential candidate as a non-oxygen oxidiser for biological systems. Broadly speaking any oxidiser that can replace oxygen in a mirror respiration pathway can also reasonably replace oxygen in a mirror photosynthesis pathway but unfortunately I'm not a biochemist so I can't even take a stab at what those pathways would look like.

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