The small marine cyanobacteria Prochlorococcus are the most abundant photosynthetic organism on Earth. They make up a substantial proportion of the phytoplankton biomass and are an important contributor to marine ecosystem function by providing photosynthetically fixed carbon for other marine organisms. Global warming leads to a rise in sea surface temperatures, but the effects of future ocean temperature shifts on Prochlorococcus are not well understood. Ribalet et al. analysed data they collected over the past decade with SeaFlow (a flow cytometer developed for the continuous observation of small phytoplankton) across the tropical and subtropical Pacific Ocean to determine the in situ thermal sensitivity of Prochlorococcus. They generated in situ division rate estimates and report an initial increase in division rates of the cyanobacteria (at around 28 °C), which is followed by a sharp decline at high temperatures (by 31 °C). Moreover, a global ocean ecosystem model projects that increases in sea surface temperatures will lead to a decline in Prochlorococcus biomass production in tropical oceans or a near-complete collapse of the populations in the Western Pacific Warm Pool. Finally, hypothetical warm-adapted Prochlorococcus may delay but not prevent the negative impact of warming sea surface temperatures on Prochlorococcus populations.