Where does oxygen come from and where does it go?

Colloquially, when we talk about making oxygen, we are usually talking about the oxygen cycle and making the kind of oxygen we breathe. But believe it or not, there is only one way to create atomic oxygen — nuclear fusion in stars much more massive than our Sun. 

Massive stars, about eight times larger than our Sun, undergo advanced stages of nuclear fusion, combining hydrogen into helium and helium into heavier elements and so on, eventually creating oxygen. Once a massive star exhausts its nuclear fuel, it collapses and undergoes a supernova explosion. This cataclysmic event expels the star’s outer layers, dispersing the oxygen and other heavy elements into space. Oxygen is the third most common element in the galaxy, but the kind we breathe, molecular oxygen, is exceedingly rare.

How does oxygen get on the Earth?

The oxygen and other elements expelled by supernovae form new stars and planetary systems. So, while our Sun itself is not massive enough to produce oxygen through fusion, the oxygen present in the solar system was inherited from earlier, more massive stars that lived and died before the Sun was born. 

In other words, the Earth is a condensed ball of dust, some of that dust being oxygen. And since our Sun is not big enough to produce more, a finite amount is available. Though a passing comet or the impact of a meteor might add small amounts of oxygen to the Earth, it does not compare to the amount lost in space. Earth’s gravity is not strong enough to hold all the gases, so as it hurtles through space, it leaves behind trace amounts of gases. You could say that the Earth is evaporating. 

But don’t worry, oxygen is also the most abundant element in the Earth’s mantle, making up 44.8% of its composition. The problem is that only a small percentage of oxygen is breathable. (Sorry. Did I say don’t worry?)  

Where does breathable oxygen come from?

For billions of years, oxygen on Earth was locked into unbreathable forms like sand and water. It wasn’t until the evolution of phytoplankton and photosynthesis that the kind of oxygen we breathe was formed. It’s called molecular oxygen, which is two atoms of oxygen combined with the chemical formula of O2. Oxygen by itself, atomic oxygen (O1), is a free radical. It doesn’t like being alone and runs around trying to make friends with anything, like hydrogen, which makes water, or carbon, which produces carbon dioxide, or silicon, which makes sand. It’s very rare to find oxygen all by itself. 

Once photosynthesis evolved, it still took billions of years to fill the atmosphere with enough air to breathe. I explain this more in the article: How much oxygen do plants produce? But here is the basic summary: Almost all breathable oxygen comes from cyanobacteria in the ocean. Cyanobacteria is a type of phytoplankton, formerly called blue-green algae. And since algae and plants reabsorb almost all the oxygen they produce, it’s only because the cyanobacteria usually sink to the ocean floor before decomposing that there is any leftover oxygen being produced. And since oxygen is a free radical, it gets quickly reabsorbed out of the atmosphere. So, there is very, very little excess, a fraction of a fraction of a percent.

A weird fun fact: it is thought that cyanobacteria actually powers all plants. Billions of years ago, it formed a symbiotic relationship with plants and became a chloroplast—the plant’s internal factory that performs photosynthesis.

Where does oxygen go?

In one sense, oxygen doesn’t go anywhere; it just gets combined into different things. Oxygen is a highly reactive element and will combine with many other elements, like water to form rust. From humanity’s point of view, we use oxygen for energy, so it gets combined with petroleum in our machines and combined with sugar (glucose) in our bodies. This is what the formulas look like:

Combustion (energy production, in this case using methane):
CH4 + 2 O2 → Energy + CO2 + 2 H2O

Cellular respiration (food consumption by both plants and animals): 
C6H12O6 + 6O2 → Energy + 6CO2 + 6H2O 

In both cases, the end product is energy (the desired outcome) with the byproducts of water and carbon dioxide. These are the ideal equations. The reality is much more messy. The fuel used in combustion is not pure, nor are the engines operating at 100% efficiency, so a lot of byproduct pollutants are created, like carbon monoxide, nitrogen dioxide, and sulfur dioxide. Notice they are all consuming oxygen without creating anything useful.

As for photosynthesis, since the formula is the exact opposite of cellular respiration, you might think we can just make more oxygen. The problem is that photosynthesis, and life in general, is very inefficient. So, though it appears to be a process that can be recycled over and over, a lot of hydrogen and oxygen is lost in the process. Some hydrogen escapes into space. And some oxygen is effectively bonded forever to other compounds. For example, the bond of carbon dioxide is effectively permanent under Earth-like conditions.¹ So, for plants to create more oxygen, they must break the bond of more water — NOT carbon dioxide.

Eventually, plants will use all the water, and photosynthesis will stop. Luckily, as I mentioned, that will take billions of years. However, photosynthesis is a slow process, and the Earth may run low on breathable oxygen. It’s happened before, like in the Permian–Triassic extinction event. And it may be happening again now as humanity burns through tremendous amounts of fossil fuels. 

Summary

That’s the short version of where oxygen comes from and where it goes. There is a finite amount of oxygen left over from a supernova that seeded our solar system. The breathable form is created from photosynthesis. However, some oxygen is lost in the process, so plants must keep getting more oxygen from water. Eventually, plants will burn through all the water. Photosynthesis will stop, and all the oxygen will be locked into unbreathable forms. Fortunately, that will be billions of years in our future unless humanity’s meddling with burning fossil fuels and destroying the environment that produces the oxygen (plants and algae) upsets the balance. 

If you are curious to know more about where oxygen comes from and where oxygen goes (the oxygen cycle), I go into a deep dive below.

Oxygen reservoirs non-breathable

Oxygen is part of most compounds, like water and sand. A breathable form may be released through various processes, as shown below.

Oxygen reservoirs breathable

Terrestrial reservoirs of oxygen

These are storehouses of breathable oxygen or molecular oxygen (O2). It took billions of years to create this biosphere. Under normal circumstances, the amount of oxygen would ebb and flow naturally. However, due to global warming, fossil fuel burning and deforestation, these reservoirs now become sources of oxygen.

• Air (Atmosphere). Currently, humans are burning more oxygen than can be replaced. A loss of at least 21.23 gigatons per year.²
• Land (Lithosphere)
  • Soil outgassing. Think desertification.
• Water (Hydrosphere)
  • Ocean outgassing. The ocean is currently losing about 1.5−3.1 gigatons of oxygen each year.³ As the waters warm, they can’t hold as much gas.
  • Glacial outgassing. Same problem as above.
• Plants and animals (Biosphere). Think deforestation. The plants and microbes are what make the oxygen.

Extraterrestrial reservoirs of oxygen

At the moment, these are insignificant sources of oxygen, but I’ve added them for fun.

• Comets. Theoretically, a major source of water in the Earth’s formation. No comet is known to have hit the Earth in modern times. But the Earth may sweep up a little oxygen and ice as it passes through their tails.
• Meteors. Similar to comets.
• Exoplanets and moons. Molecular oxygen is known to exist in a few other places, like Jupiter’s moon, Europa. We could also make oxygen from the water. It also makes good rocket fuel.
• Sun. As mentioned, massive stars are where oxygen is created and then dispersed via supernova. Our star, the Sun, is too small to fuse oxygen.

Oxygen sources (renewable breathable air)

Biogenic sources of oxygen (organic processes)

• Photosynthesis
  • Oceans
    • Primarily phytoplankton (5000 known species), particularly cyanobacteria
    • All other aquatic plants and algae
  • Lakes and rivers
    • Aquatic plants and algae
  • Land
    • Terrestrial plants and algae
• Processes other than photosynthesis (microbial)
  • Ammonia-oxidizing archaea 
  • Methanotrophy
  • Chlorate respiration 

Abiotic sources of oxygen (inorganic processes)

Most processes below that create breathable oxygen involve breaking apart (decomposing) water, but other chemicals and even rocks can also be broken apart.

• Thermolysis (heat decomposition)
  • Volcanoes
• Chemical decomposition
• Radiolysis (radiation decomposition)
• Photolysis (light decomposition)
• Electrolysis (electricity decomposition)
  • Lightning
  • “Dark oxygen,” a new process discovered on the ocean floor. 

Manmade sources of oxygen

The following processes contribute insignificant amounts of breathable oxygen. Except for electrolysis, most methods are too energy-intensive to be productive. And, if the energy source is fossil fuels, then the process would consume more oxygen than it produces. However, in the future, I wouldn’t be surprised to see fusion reactors floating in the ocean powering electrolysis.

• Humans can duplicate most processes listed above, but the most commonly used are:
  • Electrolysis of water and other chemicals, like aluminum oxide.
  • Chemical decomposition of hydrogen peroxide.
• Possible but not practical
  • Electrochemical, like MOXIE, NASA’s Oxygen-generating experiment.
  • Artificial photosynthesis
  • Etc

By the way, there are many ways to separate and condense oxygen from the atmosphere, but these methods don’t generate any extra breathable air.

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Oxygen sinks (consumed breathable air)

Natural oxygen sinks

• Decomposition
  • Heterotrophic respiration
  • Methane released from active decomposition
  • Methane released from carbon sinks due to global warming. There are vast storages of methane in the polar circles.
  • Fire
  • Microbial oxidation
• Plants/Animals
  • Respiration
  • Photorespiration
  • Inefficiency of the enzyme rubisco during photosynthesis
  • Methane emissions (flatulence)
  • Sequestration of oxygen in organic matter. This is hard to measure, and it depends whether the oxygen comes from breathable air (via photosynthesis) as opposed to carbon dioxide.
    • Organic matter that escapes decomposition by getting buried or sinking to the bottom of the ocean (future fossil fuels)
    • Organic matter that doesn’t decompose back into oxygen. This requires a lot more research, but things like coral, seashells (maybe just the protein structure), teeth and bones all contain oxygen that becomes permanently bonded to another compound.
• Land
  • Chemical weathering (oxidation)
    • Rust, including iron ore.  
    • Carbonate rocks. This process turns rocks into carbon dioxide.
    • Silicate weathering. This process removes carbon dioxide from the atmosphere. A small amount of CO2 is made from breathable oxygen. It also exposes the rocks to more weathering.
• Ocean
  • Dead zones reabsorbing oxygen.
  • Serpentinization
• Air
  • Fixation of N2 by lightning
  • Oxidation of volcanic gases
  • Ozone formation
• Space.
  • A small amount of oxygen escapes the Earth’s gravity.
• Chemical
  • Too numerous to list, there are all kinds of natural chemical reactions in land, sea and air that consume oxygen.

Manmade oxygen sinks

Note: many of these processes permanently remove breathable oxygen from the atmosphere.¹

• Fossil fuels (oxidized)
  • Coal
  • Oil
  • Natural gas
  • Leaks and spills
• Fossil fuel byproducts
  • Photochemical oxidation (smog) 
  • Oxidation with pollution (acid rain, etc.)
• Biofuels
• Manufacturing. Oxygen is one of the most commonly used gases in manufacturing.
  • Paint and the chemical oxidation of the drying process.
  • Plastic
  • Chemicals of almost any kind.
  • Nitrogen fixation (fertilizer)
  • Enhancing the combustion process in manufacturing steel, glass, cement, etc.
  • Water treatment
  • And uncountable other things.
• Decomposition of manmade items. When items break down, they often consume oxygen through similar processes as already mentioned: combustion, rust, photo and chemical oxidation…
• Chemical weathering. Humans expose the earth by farming, deforestation, construction, etc., which then absorbs oxygen. Think rust.
• Pollution. I mentioned smog and acid rain, but almost any kind of pollution can create chemical reactions with oxygen. Think of old newspapers turning yellow from oxygen.