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So for a project I'm working on I need plants which make the air around them toxic to Earth life. A way I had thought of to make this was to make the plants photynthesize with oxygen and release a gas toxic to most non-extremophiles. Would plants that use oxygen instead of carbon dioxide to photosynthesize be possible?

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    $\begingroup$ Is there a reason you want to use oxygen photosynthesis to make things toxic? Needing protection against herbivorous predators is perhaps a simpler reason to spew toxic gases around ^^. $\endgroup$
    – Tortliena
    May 28 at 23:25
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    $\begingroup$ I am wondering if when you say "photosynthesize" you mean something quite different from standard photosynthesis. What do you mean by photosynthesis as you use it here? $\endgroup$
    – Willk
    May 29 at 0:54
  • $\begingroup$ It will not be a local effect, most gasses mixes into the atmosphere too fast. also plants already use oxygen for respiration. $\endgroup$
    – John
    May 29 at 11:45

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Oxygen, said in layman term, is high up in the ladder of free energy for chemicals, while oxides are lower. Because of this oxidation is a spontaneous reaction while the opposite isn't.

Using oxygen as photosynthesis start material would mean finding something with oxygen in it which has an even higher free energy and is still a gas.

As far as I know there is nothing like this. If you want a toxic gas released by plants, just go with CO2 released during respiration: since also plants breathe, they also release it.

Additionally, early photosynthetic organism did precisely create an atmosphere which was toxic for the other living beings, because the oxygen they released forced the anaerobic organism out of their way.

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  • $\begingroup$ Fluorine is. Chlorine has a similar oxidation potential to oxygen so could conceivably be made this way. $\endgroup$ May 29 at 9:00
  • $\begingroup$ @SeanOConnor you can't make chlorine or fluorine out of oxygen, since they are all different chemical species $\endgroup$
    – L.Dutch
    May 29 at 11:38
  • $\begingroup$ You'd need to oxidise fluoride or chloride that the plant had absorbed via its roots. $\endgroup$ May 29 at 15:27
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You could try fluorine...

There are very few gases with higher oxidation potential than oxygen. Fluorine is one. It's lethal if at all concentrated.

Your plant would need lots of fluoride in the soil, so would have to grow near fluoride containing rocks, like fluorite ('ite' not 'ide').

Chlorine is also an option; take common salt and oxidise it to chlorine gas.

But why?

Photosynthesis is special because it takes a low energy gas (CO2) and releases a higher energy gas (O2). Most really toxic gases aren't higher in energy than O2. You can probably get whatever you want using normal photosynthesis.

For example, you could A) make something nasty out of carbon dioxide (e.g. carbon monoxide), or B) make toxins without photosynthesis the way many plants do, except gaseous (e.g. sarin, hydrogen cyanide), only using normal CO2 -> O2 photosynthesis indirectly to power it. This option is how plants make 99% of the most lethal poisons that they do make.

An outside option is nitrogen

There's lots of nasty nitrogen containing compounds that could be made if a plant could use it as an input for photosynthesis.

E.g. Nitrogen + water -> ammonia + oxygen (toxic, corrosive and maybe explosive).

Nitrogen + CO2 -> cyanide + oxygen.

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