Where Does Oxygen Come From?

The microscopic organisms phytoplankton make a big change to the Earth’s atmosphere!

Table of Contents

Phytoplankton produce oxygen through photosynthesis, allowing organisms like us to breathe and survive.


We often imagine the deep blue sea as a long, unchanging expanse. In reality, this intricate ecosystem varies wildly from coast to coast, as well as from the surface to the hidden depths. Ocean currents are massive forces, blending and churning water. Rains rushing off the coasts bring with them floods of silt, dirt, and soil, leading to a fresh influx of nutrients. In the warm, shallow waters, coastal reefs and kelp forests are havens of life. What we think of as plain blue is actually swirled with white and green and changes a lot through its vast range.

A satellite image of the ocean off the coast of Namibia
A satellite image of the ocean off the coast of Namibia, Credit: Wikimedia/European Space Agency

Most of the green comes from a group of organisms known as phytoplankton

Is phytoplankton a plant?

If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck… right? Just like land plants, phytoplankton are green, filled with chlorophyll , and turn carbon dioxide into oxygen!  Yet, there are some key differences. Phytoplanktons are not plants. Instead, there are two kinds: diatoms and cyanobacteria. These are protists and bacteria respectively.


Diatoms are eukaryotes . Eukaryotes have cell membranes and are relatively complex. Plant and animal cells fall under this category, making diatoms more similar to plants. Diatoms are also famous for their beautiful, crystalline snowflake shapes!

Diatoms under the microscope
Diatoms under the microscope, Credit: Wikimedia/Rovag

The second kind of phytoplankton is a prokaryotes . These cells do not have cell membranes and are simpler. They are actually bacteria that can photosynthesize! The large majority of these are known as cyanobacteria.

Cyanobacteria under the microscope
Cyanobacteria under the microscope, Credit: Wikimedia/Willem van Aken, CSIRO

Cyanobacteria also have the special distinction of being one of the oldest organisms on Earth. In the first two billion years of the Earth’s existence, it was not a hospitable place to live! There was very little oxygen, and most of the atmosphere was carbon dioxide and nitrogen. Cyanobacteria photosynthesized until the whole planet changed, and the atmosphere became 21% oxygen!

How do plants make oxygen?

Most of the oxygen on Earth comes from a process known as photosynthesis. Three essential components are necessary for photosynthesis. Carbon dioxide, sunlight, and a pigment known as chlorophyll. Plants photosynthesize to make carbohydrates, which provide them the energy to grow, flower, and make seeds.


Chlorophyll is responsible for giving plants their distinctive green color. It performs a very unique function. Chlorophyll traps sunlight and converts light energy to chemical energy. This powers the reaction of photosynthesis!


Carbon dioxide and water are the raw materials needed to make carbohydrates. Carbohydrates are large molecules made of carbon and hydrogen. Using the energy from sunlight, plants combine carbon dioxide and water. Combining the carbon dioxide and water means getting rid of the oxygen present. Life-giving oxygen, which is so important to us, is actually a waste product for plants!

The ins and outs of photosynthesis
The ins and outs of photosynthesis, Credit: Wikimedia/At09kg, Wattcle, Nefronus

In short, plants combine water and carbon dioxide using energy from the sun to produce carbohydrates. Oxygen is a waste product of this reaction.


Phytoplankton are full of chlorophyll too! Even though they are not the same organisms as plants, they still photosynthesize in the same way, to produce the same things.

Where does our oxygen come from?

Approximately half of all the oxygen on Earth comes from phytoplankton in the ocean!  Larger kelp and seaweed are also ocean oxygen producers . This brings the total ocean oxygen production up to 50-80%. The smallest photosynthetic organism, Prochlorococcus, produces 20% of the world’s oxygen all alone!

Where the world’s oxygen comes from! (https://oceanservice.noaa.gov/facts/ocean-oxygen.html)

The remaining oxygen comes from land plants, particularly tropical forests, grasslands, and savannahs.

Phytoplankton and the Food Chain

Phytoplankton are primary producers and are at the base of all oceanic food chains. The energy which supports life on Earth comes from the sun. Plants convert sunlight to other forms of energy, supporting organisms which cannot photosynthesize.


Imagine a hungry lion. It feeds on deer which roam in the savannah, and the deer feed on grass. The grass converts sunlight into chemical energy. The lion would not exist without the grass.


Phytoplankton are the grass of the ocean! Phytoplankton are fed on by slightly larger, non-photosynthetic plankton. These are eaten by smaller fish, then bigger fish, then by predators like sharks or dolphins!

An example of a food chain in the ocean
An example of a food chain in the ocean, Credit: 123rf.com/blueringmedia

Phytoplankton occupy the category of primary producer. Then come the primary consumers, secondary consumers, tertiary consumers, and quaternary consumers. Rarely do ecosystems have more than four levels, as energy is lost along every step of the food chain.


From changing the atmosphere of prehistoric Earth to still being the largest producer of oxygen, we have a lot to thank phytoplankton for! The role of these organisms is definitely much larger than their tiny size would suggest.


Chlorophyll: A green pigment that traps sunlight


Eukaryotes: Simple cells with no cell membrane


Producer: An organism that uses sunlight to create carbohydrates


Prokaryotes: Complex cells with cell membranes

Flesch Kincaid Grade Level: 7.6


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  • Yamini Srikanth

    Yamini's (he/they) interests lie in environmental education, science communication and trying to build a better world. When not languishing in front of his laptop, they can be found outside, poking at any insect, bird or plant. They love making science accessible, especially to those who aren't encouraged to pursue it. Yamini hopes that the young women who read Smore love learning from their articles and get just a little bit more excited about science!