JF - the Science Show was talking about current trials, but they did seem to be saying that you don't get much bang for your buck with algae - its early days and it sounds like a great idea. Perhaps high rise algae farms are the go...but doesn't it need sunlight to grow?

Candide,
Your thinking on the subject is very advanced. No matter how tall or how deep, I don't see a a problem in a reactor designed for the purpose.

I consider the problem is censorship of coal face information and debate concerning crucial need or not and R&D viability.