The University of Belgrade’s Institute for Multidisciplinary Research invented an urban photo-bioreactor. Liquid 3 cleans the air and serves as a bench in the Belgrade city centre.
It's a tank filled with microalgae, a solar panel on the top generates electricity for USB charging ports for citizens and a pump that pumps air into the tank where the algae process the CO² and produce Oxygen.
https://balkangreenenergynews.com/liquid-tree-to-combat-air-pollution-in-belgrade/
It's not meant to replace trees. But it could be integrated into architecture.
As someone who has a backyard pond, I can tell you algae is super resilient and essentially can't be stopped. It even survives cold Canadian winters. I'm with you, it doesn't need to replace trees, but it can be utilized for other purposes.