[whswhs]

GURPS Space lists a variety of planetary types defined in large part by atmosphere. Two of them are Ocean and Garden. An Ocean world has liquid water and a carbon dioxide/nitrogen atmosphere; a Garden world has photosynthesizing organisms that break up water into oxygen and hydrogen, combining the hydrogen with carbon dioxide to produce organic matter. Ocean is near chemical equilibrium; Garden is not.

All the other planetary atmospheres seem to be near chemical equilibrium. I suppose this is natural enough; the ones in our solar system are apparently near chemical equilibrium. But if we're going to suppose that other planetary types can have life, that life ought to be capable of energy capture and of creating disequilibrium states with its waste products, and perhaps closed cycles exploiting those waste products as a fuel.

What could the resulting atmospheres be like? If you start out with, say, a hydrogen based atmosphere, and life forms native to that atmosphere do something analogous to photosynthesis, breaking up an available liquid or gas, what are the waste products, and what kind of closed chemical cycle might they create? Would the atmosphere be as different from a gas giant atmosphere as Garden is from Ocean? What if you start out with, oh, an ammonia world? Are there new possible atmospheric types?