The most important source of atmospheric oxygen is photosynthesis, performed by plants, algae, and certain bacteria. During this process, these organisms convert sunlight, carbon dioxide, and water into glucose and oxygen. The oxygen produced is then released into the atmosphere as a byproduct. This is critical for sustaining life on Earth, as most living organisms, including humans, rely on oxygen for respiration. Photosynthesis not only replenishes atmospheric oxygen but also plays a fundamental role in the carbon cycle, helping regulate the levels of carbon dioxide in the atmosphere and providing energy for the ecosystem.