Scientists reported on September 8, 2025, in Nature Microbiology that Prochlorococcus, the world’s smallest and most plentiful phytoplankton, may face a serious decline in tropical oceans during the next 75 years. This microscopic bacterium is important because it produces about one-fifth of the planet’s oxygen and supports hundreds of marine species. The study, led by researchers at the University of Washington, explained that populations could decrease by as much as half if sea surface temperatures exceed 82 degrees Fahrenheit (27.8 Celsius). Tropical waters are already rising above normal levels and are expected to surpass 86 degrees Fahrenheit (30 Celsius) more often in coming decades. Experts described this organism as a keystone species, meaning its decline would bring changes to the structure of the food web and the biodiversity of the oceans. The research contradicted earlier assumptions from laboratory studies that warmer waters would help Prochlorococcus to grow.

The scientists reached their conclusion after more than 100 research cruises across the Pacific Ocean during the past decade. They used an advanced device called SeaFlow, which counts cells with a blue laser while seawater flows through. This allowed the team to measure about 800 billion cells in real time, a number larger than the total stars in the Milky Way. Experts such as François Ribalet emphasized that other phytoplankton may partly replace the lost productivity, but the substitutes cannot fully take the role of Prochlorococcus. Specialists described the results as alarming, incontrovertible, and supported by meticulous data. They warned that the microbe’s streamlined genome makes it less adaptable to rapid environmental shifts. Scientists also noted that plastic pollution and other stressors could worsen the decline. The findings provide an unprecedented warning about the fragile balance of ocean systems and a formidable challenge for global climate action.