On September 8, 2025, scientists reported in Nature Microbiology that Prochlorococcus, the world’s smallest and most common phytoplankton, may face a major decline in tropical oceans during the next 75 years. This microbe is important because it produces about one-fifth of the planet’s oxygen and supports hundreds of marine species. The University of Washington study found that populations could decrease by half if sea temperatures rise above 82 degrees Fahrenheit (27.8 Celsius). Tropical waters are already heating and are expected to exceed 86 degrees Fahrenheit (30 Celsius) more often in the future. Experts warned that the loss of this keystone species would change the food web and reduce biodiversity. The research also contradicted earlier studies that predicted growth in warmer waters.

The scientists reached their conclusion after more than 100 research cruises across the Pacific Ocean in the past decade. They used a device called SeaFlow, which measures cells with a blue laser, recording about 800 billion in real time. Specialists described the results as perilous, unequivocal, and supported by intricate data. Other phytoplankton may partly replace the productivity, but not fully. Because of its simple genome, the microbe adapts slowly. Experts said the findings are a momentous warning and an imperative call for stronger climate action.