ISTANBUL

Ocean warming threatens to devastate populations of Prochlorococcus, microscopic organisms that account for 5% of all global photosynthesis, with their productivity potentially declining by 37% worldwide under severe climate scenarios.

New research published Monday in Nature Microbiology, an online-only journal, reveals that these tiny marine microbes can only tolerate water temperatures up to 86F (30C) before their reproduction rates crash dramatically, contrary to scientists' previous expectations.

The study by the University of Washington found these organisms multiply most efficiently in water between 66F-84F (18.8C-28.8C), but above 86F, cell division rates plummet to just one-third of optimal levels.

Climate models predict tropical and subtropical ocean temperatures will exceed this threshold within 75 years.

Under moderate warming scenarios, Prochlorococcus productivity could drop 17% in the tropics and 10% globally. High-warming scenarios could cause a 51% tropical decline.

"Their burnout temperature is much lower than we thought," said Francois Ribalet, who led the study, which analyzed 800 billion cells across 150,000 miles (241,401 kilometers) during nearly 100 research cruises in 10 years.

Researchers said that Prochlorococcus form the foundation of marine food webs, supporting everything from small sea creatures to whales across more than 75% of ocean surface waters.

These cyanobacteria supply nutrients throughout marine food chains despite their microscopic size.

"For a long time, scientists thought Prochlorococcus was going to do great in the future, but in the warmest regions, they aren't doing that well, which means that there is going to be less carbon — less food — for the rest of the marine food web," said Ribalet.