According to the plot of my story, terraforming earth-like planets turned out to be an incredibly difficult task and exact copies of the Earth with an atmosphere of oxygen are rare, so it was decided to genetically adjust the children of the colonists to local conditions, namely for breathing chlorine or hydrogen sulfide.

Why chlorine?

Typical nucleosynthesis processes in stars create more even elements than odd ones, and more light elements than heavy ones. Chlorine is a relatively heavy element, and odd. On average, it is more than 20,000 times rarer than oxygen, an element normally secreted by photosynthesis in the life-bearing inner system worlds. However, there are small regions of the galaxy in which the interstellar medium is rich in chlorine. This is especially true for second and third generation stars, which have a higher "metallicity" (percentage of elements heavier than hydrogen). In the case of chlorine worlds, the interstellar medium from which the star and its attendant planets were formed should also be especially enriched with the so-called elements of the Alpha process (sulfur, chlorine, argon, calcium, Titanium and chromium). If this relatively rare process of nucleosynthesis was common enough in the stars that formed the starting material, then the chlorine world might be possible.

Even if the starting materials are enriched in chlorine, the planetary process must follow a rather unusual series of events to create a chlorine world. Hydrogen chloride is volatile compared to water, and unlike water, it is not often combined in bulk with silicates (hydrated "wet" silicates are the main source of water in the formation of terrestrial planets). The band of material, cold enough to contain sources of hydrogen chloride, and at the same time warm enough to allow the formation of a rocky planet, is very narrow. Within this narrow strip, the planet retains only water in its hydrosphere (although there may be unusual concentrations of more refractory chloride minerals); outside of this strip, gas giants or ice worlds are the rule. Most often, the chlorine world is formed at the outer boundaries of the life zone of the system or even outside it. As the star ages and heats up, or as the dynamics between interplanetary material and planets bring their orbits closer to their original orbits, the chlorine world becomes possible.

The atmosphere of the mature chlorine world consists mainly of oxygen and nitrogen with a significant proportion of chlorine (about 5%). Chlorine is found primarily in the lower atmosphere because it is heavier than other primary gases and has a higher relative abundance in dry areas because it eventually reacts with water to produce hydrochloric acid again. Only the highly efficient and vigorous photosynthetic activity of plants and bacteria supports the supply of free chlorine. The atmosphere is opaque, but has a yellow-green tint from chlorine and various forms of chlorocarbons; on some chlorine worlds, the dense atmosphere also favors the growth of airborne life forms, which can give the lower atmosphere a cloudy, smoky appearance. Clouds are composed of different proportions of water droplets or hydrogen chloride. The weather is complicated by the fact that hydrochloric acid evaporates more easily than water and freezes much more easily. The rain is always sour, but its concentration varies considerably. Ice and snow are rare, and even very cold bodies of water rarely freeze. When they do this, the acidity rises sharply under the water ice due to the removal of water relative to hydrochloric acid.

Here we return to my question: These genetically modified colonists, breathing chlorine or hydrogen sulfide, can periodically leave their planets to meet other peoples of people, sometimes visiting planets with an atmosphere of oxygen. Therefore, when staying in a different atmosphere, they must be able to switch from one type of gas (oxygen) to another (chlorine or hydrogen sulfide), how to do this?

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    $\begingroup$ you're not going to be able to do this just by messing with the genes. you'd need to re-work the entire biochemistry down to a level that we don't really understand. Oxygen is really important, not just for breathing, and adding in a bunch of Cl is going to throw all the reactions into chaos. You might have better luck with sulfur---that's at least got the same charge as Oxygen---but its still pretty unworkable. Switching is not going to biologically feasible, you'll have to handwave $\endgroup$
    – Sol
    Commented Oct 30, 2020 at 20:34
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    $\begingroup$ NopeNopeNope. This would not require a change in respiratory system, but a fundamental change in Biochemistry. Down to which chemical(ATP) is used for cellular metabolism. $\endgroup$
    – user79911
    Commented Oct 31, 2020 at 5:50
  • $\begingroup$ Not sure that the large nucleosynthesis paragraphs are useful here $\endgroup$
    – Axel B
    Commented May 24, 2022 at 20:01

3 Answers 3


Frame challenge time.

t was decided to genetically adjust the children of the colonists to local conditions, namely for breathing chlorine or hydrogen sulfide.

These substances are so toxic to humans that genetic engineering is completely impractical here. You'd practically need to build a completely different organism. Very few organisms are tolerant of these substances at all.

It is, by comparison much, much, much easier to simply use enclosed living spaces and specialist environmental suits when forced to go outside these.

For a culture capable of interstellar space flight this capability would, in any case, be a necessity they had long ago mastered. It's a trivial thing for them and so natural they would not think of it as an issue. It's almost certain that huge enclosed spaces could be maintained by such a technology with no issue. It is comparatively simple to genetically engineer bacteria to aid in atmospheric processing to maintain an internal atmosphere for such a city-sized space compared to completely redesigning the human to make something that can live outside.

There is another cultural issue : the genetic changes would make them, essentially, non-human. This could be both good and bad from a story point of view as, traditionally, humans are somewhat irrational about tiny little genetic differences between races, cultures, etc. and there could be a lot of clashes between your new species (humans v2.0) and original humans v1.0. That in itself could make a good basis for a number of stories, but it may not be what you want (or it may ?). It is unlikely Humans v2.0 would think of itself as Human v1.0 after a while and might even start thinking 2.0 is better than 1.0. Lot's of possibilities there for "fun".

  • $\begingroup$ Bad answer, you did not answer the main part of the question (changing the human respiratory system and its biochemistry for both variants of the atmosphere, with the ability to switch from one to another gas for breathing) and concentrated on describing "why do I need this" (stories) although this absolutely unimportant and was created only so that no one would ask why it is necessary from the point of view of evolution. You just need to. $\endgroup$ Commented Oct 31, 2020 at 6:11
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    $\begingroup$ @FrenchThompson3 Frame challenges on WB SE are a normal and acceptable method of answering. People do not have to answer the way you want. They can provide objections to the fundamental idea or alternatives. You need to get used to this - it's how the site works. Frame challenges are considered valid because they can inform posters of fundamental flaws in their approach which defeat the overall objective of worldbuilding - creating a believable self-consistent world. $\endgroup$ Commented Oct 31, 2020 at 6:27

Both chlorine and oxygen are oxidizing agents. Oxygen is necessary for humans because we use its property as an oxidizer to break down nutrients in a process called oxidative phosphorylation. https://en.m.wikipedia.org/wiki/Oxidative_phosphorylation

Sometimes chemicals other than oxygen are used. For a list, look in this link where it says, "Examples of electron acceptors in oxidation." What you're looking for is where it says, "dehalorespiration." https://en.m.wikipedia.org/wiki/Anaerobic_respiration

Maybe you could get your fictional species (they would have to be totally different from humans at this point) to use chlorine if you could come up with some new sort of oxidative enzyme. https://en.m.wikipedia.org/wiki/Oxidative_enzyme

The problem is really that chlorine gas would interact with the water in your body to produce hydrochloric acid and hypochlorous acid. There are some kinds of bacteria that are used to strip chlorine from drinking water. Maybe if you could have your species be engineered to where their lungs are coated with a genetically modified version of those bacteria, the bacteria could somehow convert the chlorine gas to something less harmful, which the genetically modified human cells can use with the new enzymes to produce ATP....????

I'm thinking it would be easier to upload your consciousness into a robot or something.

Note: I'm not an expert, as I'm sure any expert can tell. I just got sucked down a rabbit hole a while back when scientists thought they found phosphine on Venus.


As explained in other answers, it would require so much changes in the human body to breathe chlorine or hydrogen sulfide, that those new people will hardly deserve the name of "human".

But you could build on symbiosis with another organism (let's say a local micro-organism) which could live embedded in human cells. It would then convert those local exotic gases in something usable by human cells (like Adenosine triphosphate). This would cut a shorter, simpler path from the gas to the existing human biochemistry by using an "already existing" adapted indigenous biochemistry.

I'm inspiring from mitochondrions, for which there is a theory stating that they appeared when an outside micro-organism started living in symbiosis inside other cells.

It has the advantage of keeping the existing oxygen-based lung system, which would be useful for travelling to other oxygen-based planets. You would still need to figure out how to render chlorine & sulfide-based gases not toxic anymore.


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