- The oxygen cycle is a fundamental biogeochemical cycle that describes the movement of oxygen within and between the Earth’s atmosphere, biosphere, lithosphere, and hydrosphere.
- Oxygen is essential for life, playing a crucial role in cellular respiration, photosynthesis, and the formation of vital compounds such as water (H₂O), carbon dioxide (CO₂), and ozone (O₃). Although the atmosphere contains about 21% oxygen in the form of O₂, the balance of oxygen in Earth’s systems is constantly maintained and regulated through a series of interconnected biological, geological, and chemical processes.
- At the core of the oxygen cycle are two complementary biological processes: photosynthesis and respiration. In photosynthesis, green plants, algae, and cyanobacteria use sunlight to convert carbon dioxide and water into glucose and oxygen. This oxygen is released into the atmosphere as a byproduct. Conversely, aerobic respiration—performed by most living organisms—consumes oxygen to break down glucose, releasing energy, carbon dioxide, and water. Together, these processes form a closed loop that recycles oxygen and maintains its levels in the environment.
- Another important contributor to the oxygen cycle is the hydrosphere. A significant amount of dissolved oxygen exists in oceans, lakes, and rivers, where it supports aquatic life. Oxygen enters water bodies through diffusion from the atmosphere, photosynthesis by aquatic plants and phytoplankton, and wave and wind action that mixes air into the water. Aquatic organisms, including fish and microbes, utilize this dissolved oxygen for respiration. Depletion of oxygen in aquatic systems—often due to nutrient pollution and eutrophication—can lead to dead zones, where most marine life cannot survive.
- The lithosphere also plays a role in the long-term oxygen cycle. Over geological timescales, oxygen becomes locked in oxidized minerals, such as iron oxides (rust), and in the form of carbonate rocks like limestone. Volcanic activity and weathering of rocks can release oxygen-containing compounds back into the environment. Similarly, burial of organic carbon in sediments effectively removes carbon from the cycle and leaves more free oxygen in the atmosphere, contributing to the long-term rise in atmospheric oxygen over Earth’s history.
- In the stratosphere, a special form of oxygen known as ozone (O₃) forms a protective layer that shields life from harmful ultraviolet (UV) radiation. Ozone is both produced and destroyed through complex photochemical reactions involving solar UV light. This ozone-oxygen cycle is essential for maintaining the stability of the atmosphere and protecting biological organisms from DNA damage caused by UV rays.
- Human activities have influenced the oxygen cycle in various ways. Deforestation, fossil fuel combustion, and industrial pollution reduce oxygen production and increase CO₂ levels. Ocean warming and acidification also reduce oxygen solubility in water, threatening marine ecosystems. While atmospheric oxygen levels have not changed dramatically on a global scale, localized oxygen depletion and ecological imbalances are of growing concern.
