Oxygen constitutes approximately 21% of Earth’s atmosphere. It is the second most abundant gas, after nitrogen, and is vital for respiration by many organisms.
Photosynthesis, which is carried out by plants, algae, and cyanobacteria, is the principal generator of oxygen in the atmosphere. During photosynthesis, these organisms use sunlight to convert CO2 and water into oxygen and carbohydrates. This process releases oxygen into the atmosphere, refilling the oxygen supply and sustaining life on Earth.