Forgotten: genetically modified algae-like organisms that would float at the habitable-ish altitude due to having a gas bubble. These organisms should consume s

Forgotten: genetically modified algae-like organisms that would float at the habitable-ish altitude due to having a gas bubble. These organisms should consume sunlight and transform gaseous substances into something more solid/liquid to rain it down onto the surface, thus making the atmosphere less thick and more transparent. Bonus points for binding and removing chlorine and leaving oxygen intact.