On September 8, 2025, scientists reported in Nature Microbiology that Prochlorococcus, the smallest and most common phytoplankton, may face a big decline in tropical oceans in the next 75 years. This microbe is crucial because it makes about one-fifth of the world’s oxygen and supports many marine species. A study from the University of Washington showed that the population may fall by half if sea temperatures go above 82 degrees Fahrenheit (27.8 Celsius). Tropical waters are already warming and may often reach 86 degrees Fahrenheit (30 Celsius). Experts warned that the loss of this keystone species could cause severe changes to the food web and reduce biodiversity. The research also went against older studies that predicted growth in hotter waters.

The scientists made the conclusion after more than 100 research trips in the Pacific Ocean in the past ten years. They used a tool called SeaFlow, which counts cells with a blue laser, and recorded about 800 billion in real time. Specialists said the results are inevitable and based on precise data. Other phytoplankton may help, but not enough. Because of its simple genome, this microbe adapts slowly. Experts called the findings an urgent reminder for stronger action on climate change.