Microscopic marine organisms generate more oxygen than rainforests, highlighting the crucial role oceans play in sustaining life on Earth.
When people think about the source of Earth’s oxygen, vast rainforests such as the Amazon often come to mind. However, scientists say that nearly half of the oxygen humans breathe actually comes from the ocean, produced by trillions of microscopic organisms known as phytoplankton.
According to scientific assessments cited by the United States National Oceanic and Atmospheric Administration (NOAA), these tiny marine organisms carry out photosynthesis on a massive scale across the world’s oceans. Floating in the sunlit surface waters, phytoplankton absorb carbon dioxide and use sunlight to produce energy, releasing oxygen as a byproduct.
Despite being invisible to the naked eye, phytoplankton are extraordinarily abundant. Researchers estimate that a single teaspoon of seawater can contain up to one million microscopic organisms. Among them is Prochlorococcus, the smallest known photosynthetic organism, which alone may generate up to 20 percent of the oxygen produced in Earth’s biosphere.
The findings challenge the popular belief that tropical rainforests are the planet’s primary oxygen producers. Scientists note that while forests generate large amounts of oxygen through photosynthesis, they also consume nearly the same amount through plant respiration and the decomposition of organic matter. As a result, mature forests contribute very little net oxygen to the atmosphere.
Experts explain that the Amazon rainforest actual contribution to global oxygen production is significantly lower than commonly claimed. While the rainforest accounts for a substantial share of land based photosynthesis, its overall contribution is estimated at roughly 6 to 9 percent of global oxygen production once oceanic sources are considered.
Researchers caution, however, that the often cited claim that the ocean produces half of Earth’s oxygen requires important context. Marine organisms, including phytoplankton, also consume oxygen through respiration and the decomposition of dead organic material uses additional oxygen.
The oxygen rich atmosphere that supports life today accumulated over hundreds of millions of years. Scientists explain that this occurred because a small fraction of organic carbon produced through photosynthesis became buried in ocean sediments rather than being broken down and returned to the atmosphere. This long-term storage allowed oxygen levels to gradually build up.
“If phytoplankton disappeared tomorrow, the immediate crisis would not be a sudden lack of breathable air,” scientists note. Instead, the collapse of marine food webs and disruptions to the global carbon cycle would trigger severe ecological consequences.
Understanding phytoplankton populations has become increasingly important as climate change alters ocean conditions. Since it is impossible to directly monitor the entire ocean, scientists rely heavily on satellite technology to track phytoplankton activity.
Satellites measure the color of ocean waters, with greener waters indicating higher concentrations of chlorophyll and therefore larger phytoplankton populations. Instruments such as SeaWiFS and MODIS have long provided global observations, while NASA’s Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) satellite, launched in 2024, offers more detailed insights into different phytoplankton communities.
Researchers say monitoring these microscopic organisms is critical because changes in phytoplankton populations can affect oxygen production, carbon storage, marine biodiversity and the health of global ecosystems.
Beyond Earth the findings also have implications for the search for life on other planets. Oxygen is considered one of the most promising biosignatures that astronomers look for when studying distant worlds. However, scientists caution that interpreting atmospheric oxygen requires understanding complex biological and geological processes, much like those operating on Earth.
As climate change continues to reshape oceans worldwide, researchers emphasize that protecting marine ecosystems is not only essential for biodiversity but also for maintaining the delicate natural systems that help sustain life on the planet.