Oxygen of the air is transformed into carbon dioxide by  combustion and by respiration of animals and plants.
The consumed air can be recycled during sunshine.
*  Place a water plant top-down into a 100-ml beaker full of water.
*  Place a 4-ml vial full of water over the end of the plant´s stem.
*  Supply the green plant with as much light as possible.
Observation: Gas bubbles are ascending from the end of the stem and from the leaves.
Explanation:  Green plants can make oxygen (nutrients) from carbon dioxide and water during sunshine.
This is called photosynthesis: Green plants can take up carbon dioxide from their environment. This gas is transformed into oxygen gas in the light.