We can stop using them once we've counteracted the overabundance of CO2 we currently have.

its because of co2 and other greenhouse gasses that global warming is happening. you dont really have to worry about it running out for plants anytime soon

they produce oxygen....think about it...

i figured they would, since its a bi-product of photosynthesis, but i dont know much about algae, so i was thinking, what if their respiration worked in some completely different fucked up way?

Consider their implementation. I mean, it's unlikely that there will be enough of them to starve all the world's trees of O2, especially when you consider how localized they'll likely be. If they're confined to city environments, where tree density is low and CO2 emissions are high, they may not be directly competing with trees and other vegetation.

but if they take up about 150x as much, then surely...?

Trees survived just fine for millions of years before we disrupted the carbon cycle. Nature's hardy.

true true, and i guess we could think of it as replacing the millions of trees humanity has destroyed to make room for civilization over time :/