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A race of mine terraforms planets to cater to their need for a higher atmospheric oxygen content of about 45% (They are insect like, who would've imagined), so I was wondering how it would affect a planet if such a terraforming program ran longer than it supposed to be. At some point the high oxygen content would lead to a lot more burning and similar things are not very nice to living things wouldn't it?

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    $\begingroup$ I believe one of the highest Oxygen concentrations (about 60%) came during the Carboniferous Period during which the planet saw wildfires that could rage for months, the largest insect life the world has ever seen and enough fauna to basically give us every ounce of coal we see today. $\endgroup$ – Darius Arcturus Dec 1 '19 at 13:01
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    $\begingroup$ I should also have commented on how Vascular plants came to dominate. We no longer see what was once referred to as ‘scale trees’, they’re now extinct and they were actually more akin to ferns or moss. $\endgroup$ – Darius Arcturus Dec 1 '19 at 13:11
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    $\begingroup$ @DariusArcturus I think it peaked a lot lower than 60%... a lot closer to 30%. I'm pretty certain that 60% would be rapidly self-correcting. $\endgroup$ – Starfish Prime Dec 1 '19 at 14:33
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    $\begingroup$ @Darius Arcturus: We have fires that burn for months nowadays. And most coal comes from flora - that is, plants. Fauna = animals. $\endgroup$ – jamesqf Dec 1 '19 at 18:20
  • $\begingroup$ Haha, oh god that was meant to be Flora. My bad. I’m not very helpful. $\endgroup$ – Darius Arcturus Dec 1 '19 at 22:03
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Oxygen oxidizes.

As oxygen content increases, the problems oxygen poses on our own world would be more problematic.

  1. Minerals exposed to the environment would tend to form the oxide. This led to an enormous problem during the emergence of life: iron was bound as the oxide. Higher levels of oxygen might tend to convert reduced nitrogen and sulfur compounds to the oxides, changing bioavailability.

  2. Reduced carbon oxidizes in contact with oxygen.

2a. When this goes fast it is fire. It can go slowly too. Oxidation of dead biomass on the surface converts it to CO2 and levels could rise unless the process of oxygenation consumed CO2.* The amount of non CO2 carbon on the surface would decrease more quickly, fire or no fire.

2a. Mutation. degradation. Oxygenaton of live biomass would break down and degrade proteins faster, which would need to be replaced by the organism. Oxygen free radicals attack DNA and presumably any other carbon-based molecule used for coding; organisms would be selected for antioxidants / repair ability.

Lifeforms that live on the surface of earth have lots of mechanisms to defend themselves against the corrosive effects of oxygen: epithelia with defenses, mutation repair mechanisms, strategies to withstand fire. Over evolutionary time these would increase.

.* The oxygenation of the surface would depend on where the oxygen was coming from. Currently, nearly all surface O2 all comes from CO2 (with a tiny percentage from photolysis of water). O2 locked into carbonate rocks (like limestone; CaCO3) is not available to be released as oxygen via photosynthesis. As non-O2 oxygen sources are depleted, the terraforming process will be less able to put O2 into the environment, unless it is something outré like creation of matter from limitless energy sources,

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    $\begingroup$ I'm reasonably certain that O2 from photosynthesis mostly comes from photolysis of water inside chloroplasts (and the like). They don't split CO2, but cleave the double bonds into singles and glom the results together into carbohydrates, etc. (grossly oversimplifying, but you get the idea) $\endgroup$ – Starfish Prime Dec 1 '19 at 18:27
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    $\begingroup$ @StarfishPrime - I suppose that since the energy comes from a photon then technically it is photolysis, but all the enzymatic jiggery pokery in plants makes it something pretty different from plain radiation and water molecules in the upper atmosphere. $\endgroup$ – Willk Dec 2 '19 at 1:07
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    $\begingroup$ Sure, but the key thing is that the O2 doesn't come from CO2 in either case, which is what I was correcting. $\endgroup$ – Starfish Prime Dec 2 '19 at 9:14
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    $\begingroup$ @StarfishPrime - you are right! How is it I still get that turned around? There will probably be a lot of water on an oxygenated world and lots of things besides carbon that can use those hydrogens and be reduced from the oxide as a way to store energy. The sky is the limit for oxygenation! $\endgroup$ – Willk Dec 2 '19 at 13:15

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