About half of the oxygen generated on Earth each year is produced not by forests, but by microscopic organisms inhabiting the sunlit layer of the ocean. These are phytoplankton—algae, cyanobacteria, and other drifting organisms capable of photosynthesis. Despite the short lifespan of individual cells, their sheer numbers allow the marine system to rival terrestrial vegetation in scale.
As author Lachlan Brown notes, the claim that the ocean provides every second breath we take requires clarification. The atmosphere is constantly mixing, and a significant portion of the oxygen within it has accumulated over hundreds of millions of years. Furthermore, marine organisms and decomposition processes consume nearly as much oxygen as phytoplankton produce.
Phytoplankton use sunlight to create organic matter from carbon dioxide, releasing oxygen as a byproduct. These organisms live primarily in the upper layers of the ocean where light penetrates. In exceptionally clear water, the productive zone can extend to a depth of about 200 meters, though it is significantly shallower in murky areas. Phytoplankton also require nitrogen, phosphorus, iron, and other nutrients to grow; the distribution of these substances depends on wind, currents, and the upwelling of deep water.
The ocean covers about 71% of Earth’s surface, allowing even tiny cells to drive a massive volume of photosynthesis. Under favorable conditions, phytoplankton multiply rapidly, and the color of the water across hundreds of square kilometers can shift within days or weeks. While their total biomass is far smaller than that of forests, it regenerates much more quickly. A 1998 study showed that land and ocean contribute roughly equal amounts to the annual production of new biological material.
Scientists monitor phytoplankton from ships and via satellites that analyze ocean color. Launched in February 2024, NASA’s PACE spacecraft is capable of measuring hundreds of spectral bands, helping to distinguish between specific microorganism communities. However, satellites do not directly detect oxygen release. Researchers combine data on water color, temperature, light levels, and ecosystem conditions to estimate phytoplankton abundance and productivity.
Phytoplankton not only contribute to oxygen production but also form the foundation of most marine food webs. Furthermore, they transport carbon from the atmosphere to the deep ocean. During massive blooms, some organisms die and sink to the seafloor, where bacteria consume oxygen during decomposition, creating oxygen-depleted zones. Warming waters can also intensify ocean stratification and reduce the supply of nutrients to the surface, potentially impacting food webs and the carbon cycle.
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