Land.

50 trees take a lot of space. The idea is to put these boxes where trees can't fit.

Note that these algae are not taking the room of 50 trees, that's really not the way to look at it. We want more trees inside cities, and whether or not this happens is totally decorrelated to whether or not we see these algae boxes or advertisement in the bus stops.

how much more expensive is manufacturing and maintaining them than planting and watering a tree is?

I don't think it is hard to imagine that a box of algae takes less maintenance than a tree inside a city. Typically you don't plant a seed in a city to grow a tree. You grow very specific resistant species in a tree farm and then transplant them once they are tall enough in a city. This is a long and expensive progress. Trees require maintenance: they need to be trimmed, healed, they may have fallen leaves that need cleaning, they may need watering when it is too dry, they need removal if they are too damage.

I think a tree has probably a similar level of need than these boxes, so 50 trees will largely exceed that.

Hmm. These are valid concerns. I would personally rather have 50 trees instead of one of these. In any current cities do you know how to get 50 trees planted on a city block?

And why not fix whatever is keeping trees from growing, and then grow trees

I woild guess because that would require you to completely tear up the bitumen and anything underneath it like pipes and wires in order to make room for the roots. Trees are pretty big things y'know and it's not just what's above the surface that matters. You could put a tank like this in say, a train station platform that's raised well above the ground or on a building

Also a tub of algae isn't going to become a health hazard if it gets sick or infested and won't take decades to establish itself

And if these tanks are 50 times more "efficient" than a normal tree, how much more expensive is manufacturing and maintaining them than planting and watering a tree is?

That's without mentioning the resources invested into building those oxigen farms. The fact that this is done on a rich country already tells a lot. Solar panels aren't made with love and care, but minerals extracted from the earth.

Also, our problem isn't lack of O2 or excessive CO2. Our problem is a series of ideas and decisions we make about how we treat the world we're a part of. One of those ideas, is precisely efficiency above all other things. It's the same idea that makes market speculators fall in love with "line goes up". She even makes a point of saying how much more efficient than trees those are.

Sure, those are efficient in making the air safer for humans, but do they help with other local biodiversity? Do they help fixing the soil? Do they help the global ecosystem as a whole?

@stabby_cicada @MonkCanatella Lots of ideas for both algae and traditional trees in Bioshelters, Ocean Arks, City Farming: Ecology as the Basis of Design by Nancy and John Todd, both of whom are still around.

Years of links to urban and advanced agriculture at http://cityag.blogspot.com
which is also a listserv

Carbon capture is something we need. Except for real. Which will probably involve algae tanks orders of magnitude larger than this, and then tossing harvested algae down a mineshaft, never to be seen again.

As for these tanks, nah. As public art, they're not great, and their stated benefits are limited.