Warming seas threaten key phytoplankton species that fuels the food web, study finds

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Francois Ribalet, a research associate professor at the University of Washington's School of Oceanography, holds a vial of Prochlorococcus on Friday, Sept. 5, 2025, in Seattle. (AP Photo/Annika Hammerschlag)

SEATTLE – For decades, scientists believed Prochlorococcus, the smallest and most abundant phytoplankton on Earth, would thrive in a warmer world. But new research suggests the microscopic bacterium, which forms the foundation of the marine food web and helps regulate the planet’s climate, will decline sharply as seas heat up.

A study published Monday in the journal Nature Microbiology found Prochlorococcus populations could shrink by as much as half in tropical oceans over the next 75 years if surface waters exceed