Apparently it takes 7-8 trees to make enough oxygen for a human

But. Algae is where the oxygen is at. Producing much more than trees. "According to research, algae can be significantly better at filtering carbon dioxide than trees, with some studies showing algae as up to 10-50 times more efficient at capturing carbon dioxide compared to terrestrial plants, meaning it can remove much more CO2 per unit area than a tree can; in certain controlled bioreactor environments, algae can even be considered up to 400 times more efficient than trees at carbon capture."