avatar - Why is Pandora's atmosphere so toxic to humans, yet still so conducive to life?

I wasn't able to put my finger on it. Avatar wiki says:

Pandora's toxic atmosphere is unbreathable by humans due to the presence of high concentrations of carbon dioxide (>18%) and hydrogen sulfide (>1%).

However, entire planet seemed to be covered in plants, including water plants. Earth isn't half as green as Pandora is, but it still has pretty low levels of carbon dioxide (0.035%) even with all the emissions that come from human society.

I understand about the air toxicity due to other chemicals found in the atmosphere, but if the plants are performing photosynthesis, shouldn't it be enough to keep the atmosphere relatively free of carbon dioxide?

Answer

Alien planet, alien plants. Just because they appear to be similar to Earth plants doesn't mean they necessarily behave identically to Earth plants. It would be entirely possible for an alien plant to do the opposite of our plants: take in oxygen and expel carbon dioxide. The plants on Pandora are specially suited to that planet's ecosystem, so while they may perform something like photosynthesis they likely absorb and expel different types of chemicals and in different amounts than Earth plants.

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