You don't need to put algae in cities. They can be basically anywhere to absorb CO2.
Trees in cities tend to be carefully chosen for the environment. Are we in a climate where we need to put salt on the road in the winter? Choose trees that can tolerate some salt in the ground.
As much as it sucks, until we reduce the need for cars, northern rural areas are going to need to use salt for roads to be usable. Of course, if global warming gets worse it won't be an issue
Besides the already stated fact that global warming will only make winters worse, there are better ways like cleaning the snow (ok, that's radical) or using abrasives like sand or gravel.
And the oceans are incredibly vast, so they provide most of the world's oxygen! Obviously it's hard to get a precise number but 50-70% is the accepted range.
There are many reasons to plant trees in the city but local oxygen supply isn't one of them. Mostly trees look nice, and make people feel better by their presence. They also have a significant cooling effect, something a steamy tank full of warm algae definitely won't help with on a summer day.
Local oxygenation is important, conversion at the source pretty much always is.
Moreover it doesn't at all imply in lue of trees and importantly oxygenate at the same rate day and night since they're independently lit ideally 24/7/365.
You really think those massive, experimental water tanks won't require more maintenance, because you have to trim trees once ever few years? Or because their roots might grow too much?
Well of course, you can't give working class people any money for working, you can only give them a slave-wage. That's why all manufacturing was outsourced to very underdeveloped countries when NAFTA was first put into place.
You can easily get away with exploiting people who have no other choice but to work for a dollar per year, but it's much more difficult to do that to someone's neighbor in their community.