I want to try to get some numbers on Oxygen and other gas variance in biomes in this thread. We can start with realistic numbers and then tweak them to reach a balanced and fun state.
Anaerobic respiration would be done in the cytoplasm. It would consist of mutating your cytoplasm from being able to perform fermentation to being able to perform iron respiration instead, for example. A mitochondrion is only able to perform aerobic respiration.
For the time being, I think anaerobic respiration (other than fermentation) should not be a concern for us to implement, but it’s nice to keep in the back of the mind as an option and something we could add in the future that opens up some options for more exotic life.
The two main instances I can think of are accessibility to biomes and response to environmental change. Certain biomes could be restricted from the player if their species cannot tolerate those conditions (such as an acidic geyser/pool, or the ocean surface where water pressure drops dramatically). We could also have hazards appear in game, like bubbles of acid that if swum into will burst and damage the player. Lastly, there could be natural events like ocean acidification, in which case all life in the oceans needs to evolve some acid resistance or be forced to go onto land.
We could also diversify it a bit so there are a few ways to resist these factors. For example changing the internal pressure of your cell adapts you to a specific pressure. Mutating a contractile vacuole, on the other hand, uses up ATP/s but allows you to change your internal pressure and thus have your species exist in the deep and shallow ocean.
We could also cut salinity if need be, as I feel like it’s not as applicable to Microbial gameplay.