Warming seas threaten key phytoplankton species that fuels the food web, study finds

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https://www.seattlepi.com/news/article/warming-seas-threaten-key-phytoplankton-species-21036934.php

Warming seas threaten key phytoplankton species that fuels the food web, study finds

By ANNIKA HAMMERSCHLAG,
Associated Press
Updated Sep 8, 2025 8:56 a.m.

SEATTLE (AP) — For decades, scientists believed Prochlorococcus, the smallest and most abundant phytoplankton on Earth, would thrive in a warmer world. But new research suggests the microscopic bacterium, which forms the foundation of the marine food web and helps regulate the planet’s climate, will decline sharply as seas heat up.

A study published Monday in the journal Nature Microbiology found Prochlorococcus populations could shrink by as much as half in tropical oceans over the next 75 years if surface waters exceed about 82 degrees Fahrenheit (27.8 Celsius). Many tropical and subtropical sea surface temperatures are already trending above average and are projected to regularly surpass 86 degrees Fahrenheit (30 Celsius) over that same period.

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Prochlorococcus inhabit up to 75% of Earth’s sunlit surface waters and produce about one-fifth of the planet’s oxygen through photosynthesis. More crucially, Ribalet said, they convert sunlight and carbon dioxide into food at the base of the marine ecosystem.
“In the tropical ocean, nearly half of the food is produced by Prochlorococcus,” he said. “Hundreds of species rely on these guys.”

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