Future ocean warming may cause large reductions in Prochlorococcus biomass and productivity.

The cyanobacterium Prochlorococcus is Earth's most abundant photosynthetic organism and crucial to oceanic ecosystems. However, its sensitivity to a changing climate remains unclear. Here we analysed decade-long field measurements using continuous-flow cytometry from our SeaFlow instrument, collecting per-cell chlorophyll fluorescence and size data for ~800 billion phytoplankton cells across the tropical and subtropical Pacific Ocean to quantify the temperature dependence of cell division. Prochlorococcus division rates appear primarily determined by temperature, increasing exponentially to 28 °C, then sharply declining. Regional surface water temperatures may exceed this range by the end of the century under both moderate and high warming scenarios. Under these future conditions, our global ocean ecosystem model suggests a possible 17-51% reduction in Prochlorococcus production in tropical oceans. Even with the inclusion of hypothetical warm-adapted strains, models show significant production declines in the warmest regions, suggesting that thermal adaptation may not prevent negative impacts. These results highlight the potential vulnerability of Prochlorococcus-dependent marine ecosystems to future warming.