Prochlorococcus fixes enormous amounts of carbon in the surface ocean, but only a relatively small fraction of that carbon is stored long-term in the deep sea.
Each year, Prochlorococcus is estimated to fix roughly ~4 gigatons of carbon, converting dissolved CO₂ into organic matter through photosynthesis. However, carbon fixation and carbon sequestration are not the same process.
Most of the carbon processed by Prochlorococcus remains within the upper ocean, where it is rapidly recycled.
A significant portion enters the microbial loop, where dissolved organic compounds released by Prochlorococcus are quickly consumed and reprocessed by bacteria and other microorganisms. Viral lysis further accelerates this recycling by returning cellular material directly back into seawater.
Because of this rapid turnover, much of the carbon fixed by Prochlorococcus eventually returns to the atmosphere as CO₂ through respiration and decomposition rather than being permanently buried or transported into deep ocean reservoirs.
Only a small percentage becomes part of the biological carbon pump, the process that transfers carbon from surface waters into the deep ocean through sinking particles and organic matter.
This differs from larger phytoplankton such as diatoms, which often contribute more directly to long-term sequestration because their heavier silica-containing cells and aggregates sink more efficiently.
Prochlorococcus follows a different ecological strategy. Instead of promoting strong downward export of carbon, it helps sustain continuous carbon cycling within the upper ocean.
That does not make its role less important.
By controlling how carbon moves through microbial food webs, Prochlorococcus strongly influences:
- surface ocean productivity
- nutrient recycling
- microbial respiration
- and the balance between carbon retention and atmospheric return
Its activity therefore helps regulate the efficiency of the ocean as a whole carbon-processing system, even if much of the carbon it fixes is not stored permanently.
This distinction is increasingly important in climate research. Changes in warming, stratification, nutrient supply, or microbial community structure could alter how much Prochlorococcus-derived carbon remains in surface waters versus how much is transferred to the deep ocean.
In that sense, Prochlorococcus is less a long-term carbon vault and more a central regulator of how carbon continuously moves through the marine environment.
Despite fixing large amounts of carbon, only a minor share escapes recycling and reaches long-term storage.