Trees and forests dominate the conversation about oxygen production, but the oceans are the planet’s primary oxygen factory.

Reliable sources, including NOAA and the Smithsonian, estimate that roughly 50% (or more) of Earth’s atmospheric oxygen is produced by the oceans. The main contributors are phytoplankton — microscopic drifting plants, algae, and cyanobacteria that perform photosynthesis in the sunlit upper ocean layer.

One single tiny species, Prochlorococcus, is responsible for up to 20% of all oxygen in the biosphere — more than all tropical rainforests combined.

Estimates sometimes range as high as 50–80% (or even 50–85% in some older popular accounts), depending on how net production is calculated.

Land plants and trees (including all forests) account for the remaining ~50%, with tropical rainforests contributing only about 28% in many assessments.

Important note: The oceans produce a huge amount of oxygen, but marine life (respiration and decomposition) also consumes roughly the same amount. The net contribution keeps the atmosphere balanced at ~21% oxygen over long timescales.

The imbalance in public attention comes down to simple visibility and messaging:

Trees are tall, photogenic, and easy to photograph or campaign around (“save the rainforest” resonates emotionally).

Phytoplankton are invisible to the naked eye — you can’t hug a bloom or plant one in your backyard.

Deforestation is a direct, visible human impact. Ocean threats (warming, stratification, nutrient changes) feel more abstract and global.

As one straightforward explanation puts it: phytoplankton “don’t have good PR” compared to charismatic trees.

Phytoplankton populations are sensitive to ocean warming, which can increase stratification and reduce nutrient upwelling.

Some recent modeling and field studies show regional shifts or modest declines in biomass in certain areas, and ocean deoxygenation is occurring (roughly 2% loss per decade in marine waters over recent decades).

However, the overall atmospheric oxygen level remains stable — the system has large buffers, and phytoplankton have shown resilience in past warm periods.

The oceans quietly do the heavy lifting for every other breath you take.

Protecting ocean health (reducing warming, pollution, and overfishing) is therefore one of the most effective ways to safeguard this invisible but essential oxygen engine.

There is a purely chemical (abiotic) way oxygen can be produced in the oceans — separate from photosynthesis by phytoplankton or trees.

This process is called “dark oxygen” production and was first reported in a 2024 study published in Nature Geoscience. It occurs in the deep sea, thousands of metres below the surface where no sunlight reaches.

Polymetallic nodules (potato-sized mineral lumps rich in manganese, nickel, cobalt, copper, and other metals) lie scattered across large areas of the abyssal seafloor, especially in the Clarion-Clipperton Zone of the Pacific.

These nodules can generate small electrical voltages (up to ~0.95 V measured on their surfaces, and potentially higher when clustered together).

The voltage is enough to drive seawater electrolysis — splitting water molecules (H₂O) into hydrogen gas (H₂) and oxygen gas (O₂) without any light or living organisms involved.

In lab and in-situ experiments, oxygen levels in sealed chambers rose significantly (sometimes more than tripling background concentrations) over two days when nodules were present.

The nodules essentially act like natural “geo-batteries,” with different metal layers creating an electrochemical potential that powers the reaction.

This chemical oxygen production is localized to the seafloor and does not contribute meaningfully to the global atmospheric oxygen we breathe (which is still overwhelmingly from surface phytoplankton photosynthesis). However, it could be important for:

Local oxygen supply to deep-sea life in otherwise oxygen-poor sediments.

Our understanding of how life might exist on other ocean worlds (e.g., icy moons like Europa or Enceladus).

It adds one more layer to why the oceans are far more chemically and biologically complex than the simple “trees produce oxygen” narrative suggests.

The discovery also ties into practical debates: the same nodules are targeted for deep-sea mining for battery metals, so any impact on this process is being carefully studied.