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You might think its oxygen, but you would be wrong. Only aerobic organisms need oxygen. To anaerobic organisms oxygen is a deadly poison.

It might be nitrogen, because this is required to form amino acids: the building blocks of proteins. However only a small group of species of anaerobic bacteria can use this gas directly: nitrogen fixing bacteria. Other organisms can only use already fixed nitrogen: ammonia or nitrate, which are not gasses.

Sorry, but I'm having trouble thinking of a gas fitting your criteria, having already eliminated >95% of the gas in the atmosphere.

Perhaps it is the ozone in the ozone layer, that shields life on the surface of the earth from UV. But life below the surface and in the oceans doesn't depend on this.

I'm sorry, I can't think of any gas that all living things need to survive. Especially since viruses don't need any gas to survive.

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13y ago

What else can I help you with?