In Belgrade, the capital of Serbia, a strange device stands in the street as if it were from a futuristic novel. A glass basin containing about 600 liters of water houses delicate algae that perform what trees have been doing since time immemorial: capturing carbon dioxide and releasing oxygen, converting light into life.

The device is called "Liqui3" or "LIQUID3," and it is the first urban living photobioreactor in Serbia. Developed by a team from the multidisciplinary research institute at the University of Belgrade with support from the United Nations Development Programme, the Serbian Ministry of Environmental Protection, and Starigrad Municipality, the idea is not to replace trees but to find a temporary alternative in those places where the city has taken up all the ground and space.

Modern cities are not lacking in buildings; they lack empty spaces. Every meter in them is accounted for: parking lots, roads, curbs, facades, billboards, pipes, cables. Here comes "liquid trees" as a harsh acknowledgment that some cities have taken over their soil to the point where they now seek nature inside a box.

According to project leaders, one Liqui3 device can sequester carbon equivalent to that of an adult tree or about 200 square meters of grass. In other estimates provided by the United Nations Development Programme in Serbia, the algae within the device could be equivalent to two trees ten years old or 200 square meters of grass. Project leaders also note that delicate algae may be up to 10 to 50 times more efficient than trees at capturing carbon dioxide.

According to "Urban Nature Atlas," a single Liqui3 unit can be installed on an area not exceeding about 3 square meters, while the full-scale project extends over approximately 6 square meters of street space. In this sense, "liquid trees" do not compete with forests but with congestion.

The idea works biologically simply. The delicate algae, microscopic organisms that naturally live in freshwater, use light and carbon dioxide in photosynthesis, then produce oxygen and biomass. This biomass can later be used as fertilizer.

Since the designers believed that nature should not be separate from daily life, the device functions as a seat with nighttime lighting and provides charging ports for phones.

Serbia, like many cities in Eastern Europe and the Balkans, faces a persistent problem of air pollution, especially during heating seasons and city density. Reports on the project indicate that about 59% of Serbia's population lives in urban areas, making indoor air solutions more urgent. The danger of pollution has spread to cover all parts of the city, not just being confined to factory corners at the edges of the country.

And yet, as science enthusiasts, we should not treat "the liquid tree" as a miracle. It is not a forest compressed into a glass, and it cannot replace the shade of a real tree, or a bird perched on a branch, or roots holding the soil. The tree is not just an oxygen machine; it is a small climate, shade, scent, soil, and habitat for life we cannot see, and a quietness that algae alone cannot provide.