For decades, scientists believed Prochlorococcus, the smallest and most abundant phytoplankton on Earth, would thrive in warmer oceans. However, a new study has overturned this assumption. Now, researchers warn these microscopic organisms could face a dramatic decline as global temperatures rise.

Study warns of steep drop

The study, published Monday in Nature Microbiology, predicts that Prochlorococcus populations could shrink by 50 percent in tropical oceans over the next 75 years if sea surface temperatures rise above 82 degrees Fahrenheit (27.8 Celsius).

Currently, many tropical waters are already warmer than average. Moreover, experts project these temperatures will regularly exceed 86 degrees Fahrenheit (30 Celsius) in the coming decades.

“These are keystone species — very important ones,” said François Ribalet, lead author and research associate professor at the University of Washington. “And when a keystone species decreases in abundance, it always has consequences on ecology and biodiversity. The food web is going to change.”

Crucial role in ocean ecosystems

Prochlorococcus inhabit up to 75 percent of Earth’s sunlit surface waters. Through photosynthesis, they produce nearly one-fifth of the planet’s oxygen.

Ribalet explained their critical role in marine life. “In the tropical ocean, nearly half of the food is produced by Prochlorococcus,” he said. “Hundreds of species rely on these guys.”

Although other types of phytoplankton may move in to replace them, Ribalet warned they cannot fully compensate. “Evolution has made this very specific interaction,” he said. “Obviously, this is going to have an impact on this very unique system that has been established.”

Decades of assumptions challenged

For years, scientists assumed Prochlorococcus would thrive in a warming world. However, those predictions were based on limited lab data.

To challenge this, Ribalet and his team collected ocean samples for ten years. They completed more than 100 research cruises across the Pacific, which was equivalent to six trips around the world.

As a result, they counted about 800 billion individual cells. In Ribalet’s University of Washington lab, a custom machine called SeaFlow tracked microbes in real time using a piercing blue laser.

“We have counted more Prochlorococcus than there are stars in the Milky Way,” Ribalet said.

Experts highlight global risks

Paul Berube, a research scientist at MIT who studies Prochlorococcus, praised the scale of the data, calling it “groundbreaking.”

“They’re at the very base of the food web, and they feed everything else — the fish eat the things that eat the phytoplankton and we eat the fish,” Berube said. “When changes are being made to the planet that influence these particular organisms that are essentially feeding us, that’s going to have big consequences.”

Meanwhile, Ribalet’s team also tested whether Prochlorococcus might adapt to hotter waters. Even so, their models showed that a heat-tolerant version of the microbe could not survive extreme warming if greenhouse gas emissions continue to rise.

Ribalet noted that the study used conservative projections. Importantly, it did not include other threats such as plastic pollution or ocean acidification. “We actually tried to put forth the best-case scenario,” he said. “In reality, things may be worse.”

A warning for the future

Steven Biller, a professor at Wellesley College, called the findings “scary but plausible.”

He described Prochlorococcus as part of the ocean’s “invisible forests” — tiny organisms that most people never notice but are essential for human survival.

“Half of all photosynthesis is happening in the oceans and Prochlorococcus is a really important part of that,” Biller said. “The magnitude of the potential impact is kind of striking.”

Although other microbes may partially offset the loss, scientists warn that biodiversity and fisheries will face serious risks.

Therefore, Ribalet urged governments to act decisively to reduce greenhouse gas emissions. “We know what drives global warming. There is no debate among the scientific community,” he said.

He hopes the research brings attention to tropical oceans, which may serve as early warning systems for future ecological disasters.

“For the first time, I want to be wrong. I would love to be wrong,” Ribalet said. “But these are data-driven results.”