Belgrade has an innovative tool in the fight against dirty air – this so-called “liquid tree”. It’s Serbia’s first urban photo-bioreactor, a solution for tackling greenhouse gas emissions and improving air quality. It contains six hundred litres of water and uses microalgae to bind carbon dioxide and produce pure oxygen through photosynthesis. “The microalgae replaces two, 10-year-old trees or 200 square metres of lawn,” said Dr Ivan Spasojevic, one of the authors of the project from the Institute for Multidisciplinary Research at the University of Belgrade. “The system is the same because both trees and grass perform photosynthesis and bind carbon dioxide.