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I just started thinking about a fictional universe where humans have colonized more or less any habitable planet (there is a huge concentration of habitable planets) and I'm trying to write down different planetary systems with different environments and stuff.

I thought about a planetary system where most of the planets have humans but, since the first expeditions found this planet and started setting up colonies, they found out something strange. On this planet there are no plants or any vegetation. The oxygen came from deep inside the planet, where atoms in water molecules are separated in some way I haven't yet thought of (but this is another problem for the future me).
Now, in the air of this planet there is also a gas that kill any plant they try to farm. I'm thinking about some toxic interaction with photosynthesis. The only plants that are farmed here grow in greenhouses and they cost a lot. Wood and paper are very expensive, as are vegetables. This problem persists until someone finds a plant, from another planet or created in a lab, that can live there. The question is:

  • Is there anything like the gas I thought about that exists and that I can refer to? Remember that this thing is toxic only for plants.
  • If the thing with the photosynthesis is acceptable, how can the "new plant" grow without photosynthesis?
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Frameshift: Andrew Brēza had the right idea but got it backwards.

It's not that the planet has a surplus of oxygen. Rather, it has a major deficit in carbon. The air isn't toxic at all, it's lacking CO2. Animals (including humans) don't use CO2, they won't care. Plants need CO2 to grow.

You either need to import all the carbon your plants need, or you need to have massive facilities to gather the dregs of CO2 that get exhaled by the inhabitants.

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    $\begingroup$ That's so elegant! And it also offers possible plot developments - where did all the carbon go? Since its lack is unlikely in the extreme. There is something like that in Timothy Zahn's Spinneret, where a whole planet is mysteriously lacking all metals. $\endgroup$ – LSerni May 31 at 7:39
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    $\begingroup$ I like the worldbuilding implications of the frameshift. And it deftly avoids the issues of too much oxygen. The lack of carbon also makes it fairly obvious why there is no life on the planet. $\endgroup$ – Tim Seguine May 31 at 10:02
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    $\begingroup$ Small caveat. We mammals may not require oxygen to live, but we do need it to remind us to breathe. One of the real dangers with putting someone in a pure oxygen environment is that our breathing reflex is not triggered by lack of oxygen, but by buildup of too much CO2. With pure oxygen, this takes so long to build up in the lungs that the patient breathes too infrequently, and can easily asphyxiate, because... they have too much oxygen! $\endgroup$ – PcMan May 31 at 10:14
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    $\begingroup$ @PcMan - it is true that we need CO2 to remind us to breathe, but we do not need it in the atmosphere. Our own endogenous CO2 reminds us to breathe. Humans do fine in an environment with no CO2. $\endgroup$ – Willk May 31 at 20:50
  • $\begingroup$ @PcMan Huh? Normally we inhale 20% O2 and exhale 16% O2 and 3.8% CO2. Breathing an atmosphere with the ambient CO2 removed would give exhaled air of 16% O2 and 3.8% CO2. Breathing pure oxygen would give exhaled air with 96% O2 and 3.8% CO2. $\endgroup$ – Loren Pechtel May 31 at 23:49
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Oxygen

You're looking for a gas that will not harm animals but will impede plant growth. I suggest oxygen. Lots and lots of oxygen. In fact, the atmosphere is overwhelmingly composed of oxygen and barely has any carbon dioxide. Check out this question from Biology StackExchange for some of the science, or this old journal article for even more science.

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    $\begingroup$ Extreme fire danger. $\endgroup$ – Loren Pechtel May 31 at 5:07
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    $\begingroup$ This is a great idea. The fire danger can be handwaved away; if there are no plants or trees, then there is not likely to be much on this planet which is flammable anyway. Alternatively, instead of increasing the proportion of oxygen in the environment, just take away the carbon dioxide. No carbon dioxide -> no photosynthesis -> no plants or trees. You could have nitrogen instead of oxygen and it would be just as effective without the fire risk. $\endgroup$ – kaya3 May 31 at 7:40
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    $\begingroup$ @LorenPechtel That's secondary. Oxygen toxicity is the biggest problem - well recognised as a risk for divers, and something you learn about when you're trained for different gas mixes $\endgroup$ – Graham May 31 at 9:04
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    $\begingroup$ @Graham oxygen toxicity has almost nothing to do with the percentage of oxygen in the air. It is all about the partial pressure of the oxygen. We are used to 20% oxygen at 1 bar. 80% oxygen at .25 bar will be very similar in its breathability. (80%, because your lung do need a bit of buffer gas) $\endgroup$ – PcMan May 31 at 10:08
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    $\begingroup$ That depends what they build things out of, and could be part of the story; it's a viable solution to the world building problem. $\endgroup$ – kaya3 Jun 1 at 7:25
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"Now, in the air of this planet there is also a gas that kill any plant they try to farm. I'm thinking about some toxic interaction with photosynthesis"

Difficult, but doable with a little handwaving. You need a volatile compound called pseudoatrazine tetrafluoride, a heavy, odorless and tasteless gas that is somehow synthesized deep in the soil, by those same bacteria that produce oxygen (the planet needs to have a very interesting microbic ecology). I'm afraid that means that the bacteria's energy source has to be pretty exotic - possibly they have evolved a way of feeding off radioactive decay. There are Terran microbes that almost do that already.

The reason it's called pseudoatrazine is because colonists have analyzed the gas and found that it has to be a molecule of the atrazine family, but they haven't been able to exactly pinpoint which. Also, atrazine-resistant strains aren't resistant to this molecule. Perhaps the molecule composition varies over time. They could probably riddle this out, but they haven't time.

The gas gets into humans and plants, where it breaks down. While mostly harmless for humans, the byproducts combine with D1 protein in plants, disrupting Photosystem II and effectively killing them.

The only solution is a gengineered plant that expresses the human cytochrome P450 (CYP1A2) genes with sufficient activity to effectively detoxify itself. Having a plant do this and yet keep a worthwhile yield as well as sufficient vitality in vivo is the reason it took time to produce it, and not all plants can be replaced with the resistant strain.

In the very long period, exterminating the dangerous soil microorganisms and replacing the oxygen cycle with a plant-based Earth-like cycle is the way to go, but doing this will take centuries.

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    $\begingroup$ If you had a thermosynthetic class of organism that actively produced multiple toxins to kill competitive plants so the heat of the sun bathed the organisms, you could totally explain why this killed all plants. This question would also cover where the O2 was coming from. worldbuilding.stackexchange.com/questions/96261/… $\endgroup$ – DWKraus May 30 at 23:34
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It would be easier with a pathogen.

The chemistry of a plant selective airborne toxic is tricky. A less tricky way to accomplish your end would be a pathogen. Humans and vertebrates generally have immune systems that can adapt in an individual level to reckon with new germs and so after an initial infection, humans are immune. Maybe there could even be a vaccine.

Plants are as durable as animals vs toxins, and maybe more. But as regards pathogens the plants have only intrinsic defenses. They must evolve new defenses. An alien pathogen could just sidestep whatever the plants have and plunder their resources. The greenhouses are really safe rooms. If the Bug gets in the plants are toast.

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How can the "new plant" grow without photosynthesis?

If there is CO2, your plant basically just needs energy to extract the carbon from it. If it doesn't get it from the sun, you could imagine that it exploits geothermy, for example.

The problem is if you choose to go the non-CO2 route. Because a plant is made of carbon, and to grow, it needs to find it somewhere (conservation of matter, unless it can generate it with say, nuclear fusion. But that would be one hell of a nuclear plant).
It could be parasitic (get it from other lifeforms), but that wouldn't solve the food problem. Or maybe it could feed on solid carbon you could find somewhere on your planet. Or you could say your planet's atmosphere contains another carbon-based gaz (but a lot of those are toxic).

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