I've read that forests are important because they absorb carbon dioxide and release oxygen to the atmoshpere 1. But other places say that this is false for a mature forest:
"On average, then, this mature forest has no net flux of carbon dioxide or oxygen to or from the atmosphere, unless we cut it all down for logging," Sarmiento said. "The ocean works the same way. Most of the photosynthesis is counterbalanced by an equal and opposite amount of respiration." 2
It seems that phytoplakton are the responsibles of carbon dioxide fixing and oxygen emission.
Tiny ocean plants called phytoplankton contribute 50 to 85 percent of the oxygen in Earth’s atmosphere.3
So are there any solid studies that show what are the "lungs" of the planet, if any?