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Old 11-08-2019, 10:31 AM   #3
Join Date: Jun 2005
Default Re: a science question: atmospheric disequilibrium

Originally Posted by malloyd View Post
The purpose of photosynthesis is after all to extract the stuff you build your body from out of the environment. Since life is going to be made mostly of carbon, the likely atmospheric interactions are inhale CO2 and exhale oxygen or inhale methane (or some other light alkane) and exhale hydrogen (or water vapor if you have an oxygen source). I suppose you might extract nitrogen from ammonia, exhaling hydrogen, or oxygen from water vapor (again exhaling hydrogen), sulfur from H2S (exhaling hydrogen). There aren't really very many plausible atmospheric gases that you could break apart and dump a gas other than oxygen or hydrogen, and hydrogen is probably generally uninteresting - an atmosphere that doesn't already have *lots* of it probably means a world that doesn't have enough gravity to hold onto it long enough to change the atmosphere dramatically.
The basic reaction on Earth is that you split hydrogen off of water, combine it with carbon dioxide to get carbohydrate and water, and dump the oxygen from the water into the atmosphere. You can do that instead by splitting hydrogen off of hydrogen sulfide. It's not obvious to me (a) what the starting organic molecule is, (b) what element you are adding to it to get organic monomers that can be the basis for life, and (c) what element you're dumping into the environment after you split a source molecule to enable (b), in the various cases you discuss.
Bill Stoddard

A human being should know how to live fast, die young, and leave a beautiful corpse. Specialization is for insects.
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