Climate change threatens ocean’s most abundant microbe, study finds

One of the ocean’s smallest but most important microbes could see steep declines as global warming pushes seas to higher temperatures, new research shows.

Prochlorococcus, a microscopic bacteria that performs as much as 5% of the world’s photosynthesis and produces around 20% of the oxygen in the oceans, struggles to survive in waters warmer than 28°C. Scientists had previously thought the microbe would thrive in a hotter world, but the study published in Nature Microbiology reveals its limits are closer than expected.

If global temperatures rise by 2°C, Prochlorococcus productivity could fall by about 17%. A 4°C rise could see declines as steep as 51%. While some losses might be offset by other organisms, the changes could ripple across marine ecosystems.

“More concerning than the oxygen reduction are the cascading effects through marine food webs,” said lead author Dr François Ribalet.

Prochlorococcus has co-evolved with countless other microbes, and its decline could disrupt the nutrient cycles on which marine life depends.

Other microbes, such as Synechococcus, may expand into niches vacated by Prochlorococcus, with models predicting global increases of 5–20%.

But they cannot fully replace its unique ecological role. One group at risk is SAR11, a type of bacteria that relies on Prochlorococcus for survival and makes up a quarter of all plankton.

Overall, the findings highlight how warming seas threaten not just visible ocean life, but the microbial foundations of marine ecosystems.

By Sabina Mammadli