All oxygen on earth originally came from being created in stars, or perhaps from other cosmic sources such as formation during the rapid expansion period of the Big Bang before the stars formed. Oxygen exists as molecular oxygen (O2) in the atmosphere because the element oxygen was part of the composition of our planet when it was created and was later broken out of molecular combinations with other molecules by certain chemical processes.
The abundant atmospheric oxygen on Earth is primarily a result of the photosynthetic activity of plants, algae, and some types of bacteria. These organisms use sunlight, water, and carbon dioxide to carry out photosynthesis, a process that converts light energy into chemical energy and produces oxygen as a byproduct. This oxygen is then released into the atmosphere, contributing to the overall composition of the Earth’s atmosphere.
Photosynthesis is mostly balanced by respiration:
Photosynthesis: CO2 + H2O + h𝜈 → O2 + CH2O(biomass) Respiration: O2 + CH2O(biomass) → CO2 + H2O
Photosynthesis converts sunlight, carbon dioxide, and water into glucose and oxygen, while respiration oxidizes glucose to produce energy, carbon dioxide, and water.
If these two reactions were always balanced, however, the concentration of O2 would have not changed. If abiotic processes deplete O2 to form iron oxides, and other compounds, for there to be persistent O2 in the Earth’s atmosphere, the biosphere as a whole must continually produce it in excess to balance that loss.
Over billions of years, the accumulation of oxygen from photosynthesis gradually increased the amount of atmospheric oxygen on Earth. Early in Earth’s history, the atmosphere had very little oxygen, but as photosynthetic organisms evolved and their populations grew, they started releasing larger amounts of oxygen. This buildup of oxygen in the atmosphere allowed for the development of more complex life forms that rely on oxygen for respiration, including animals and humans.
The Great Oxygenation Event
It is worth noting that the current percentage of atmospheric oxygen (20.9%) is a result of a balance maintained through various processes, such as photosynthesis, respiration, and chemical reactions. Any significant disruption to these processes would have the potential to alter the composition of atmospheric oxygen.
Current Sources of Atmospheric Oxygen
Approximately 21% of Earth’s atmosphere is composed of oxygen. The sources are primarily two natural processes: photosynthesis and the breakdown of molecules containing oxygen.
1. Photosynthesis: The main source of atmospheric oxygen is photosynthesis, which is primarily carried out by plants, algae, and some types of bacteria. During photosynthesis, carbon dioxide (CO2) from the atmosphere and water (H2O) from the soil are converted into oxygen (O2) and glucose (C6H12O6) through a series of chemical reactions.
This NASA video shows the yearly average cycle of plants both at sea and on land:
2. Atmospheric Dissociation: The second major source of atmospheric oxygen comes from the dissociation or breakdown of molecules containing oxygen. Some of these molecules include water vapor (H2O) and carbon dioxide (CO2) present in the atmosphere. Sunlight, cosmic rays, and other high-energy processes play a role in breaking down these molecules, releasing oxygen into the atmosphere. The smaller percentage of atmospheric oxygen is generated through various chemical reactions. This includes the dissociation of water vapor into hydrogen and oxygen molecules in the upper atmosphere, as well as volcanic emissions and lightning discharges.
In addition to these natural sources, human activities also contribute to the oxygen levels in the atmosphere. Industrial processes, burning of fossil fuels, and deforestation reduce the vegetation and plant cover, which can impact the balance of oxygen production and consumption. However, the vast majority of atmospheric oxygen comes from natural sources, primarily photosynthesis.
O2 Percentages From Land vs Ocean Sources
The current sources of atmospheric oxygen can be broken down into three main categories: terrestrial plant photosynthesis, oceanic phytoplankton photosynthesis, and atmospheric chemical reactions.
Here is what various sources have said about the percentages over the years:
1. Terrestrial Plant Photosynthesis:
2004 June: 50% “Half of the world’s oxygen is produced via phytoplankton photosynthesis. The other half is produced via photosynthesis on land by trees, shrubs, grasses, and other plants.” {NatGeo}
2. Oceanic Phytoplankton Photosynthesis:
2023 November: Over 50% “Marine organisms produce over half of the oxygen that land animals currently need to breathe.” {NOAA}
June 2015: 50-85% “Phytoplankton contribute between 50 to 85 percent of the oxygen in Earth’s atmosphere. They aren’t sure because it’s a tough thing to calculate” {Earthsky}
3. Atmospheric Chemical Reactions:
Atmospheric chemical reactions contribute around 1-5% of the total atmospheric oxygen.
It’s important to note that these percentages can vary depending on factors such as environmental conditions, geographic location, and time scales.
The lesson here is obviously that the Earth’s atmosphere, specifically its levels of oxygen, has changed dramatically in the past and we should take heed and do what we can to keep it able to sustain life as we know it.
