SEATTLE (AP) — For years, scientists believed that Prochlorococcus — the tiniest and most plentiful phytoplankton in the ocean — would flourish as global temperatures climbed. But recent findings show these microscopic organisms, which play a crucial role in ocean ecosystems and climate regulation, could drastically decline in warmer seas.

A new report in the journal Nature Microbiology warns that Prochlorococcus populations could drop by up to 50% in tropical waters within the next 75 years if surface temperatures exceed around 82 degrees Fahrenheit (27.8 Celsius). Many tropical regions are already surpassing historical averages and are expected to frequently hit 86 degrees Fahrenheit (30 Celsius) in the decades ahead.

“They’re key species — incredibly important ones,” said François Ribalet, a research associate professor at the University of Washington’s School of Oceanography and the lead author of the study. “A decline in a cornerstone species brings changes throughout the entire food chain. The balance of the ecosystem will shift.”

Francois Ribalet, a research associate professor at the University of Washington’s School of Oceanography, holds a sample of Prochlorococcus on Friday, Sept. 5, 2025, in Seattle. (AP Photo/Annika Hammerschlag)

These tiny organisms are essential to marine ecosystems

Prochlorococcus inhabit approximately 75% of the ocean’s sunlit surface and generate roughly 20% of Earth’s oxygen through photosynthesis. More importantly, according to Ribalet, they transform sunlight and carbon dioxide into essential nutrients at the base of the marine food chain.

“In tropical waters, nearly half of the energy produced originates from Prochlorococcus,” he explained. “Many species depend on their existence.”

While other types of phytoplankton may partially offset the loss, Ribalet cautioned they aren’t exact substitutes. “This ecosystem evolved around very specific interactions,” he said. “Changing that alters a finely tuned system.”

Earlier projections that Prochlorococcus would benefit from warming waters were largely based on findings from laboratory settings. Ribalet’s team, however, gathered field samples across the Pacific over a ten-year span to track changes more accurately.

Using over 100 research cruises — totaling the equivalent of six global circumnavigations — the scientists counted approximately 800 billion cells, collecting data at one-kilometer intervals. In his Seattle lab, Ribalet demonstrated a device called the SeaFlow, a high-tech instrument fitted with laser systems that allowed the team to count the microbes in real-time. “We have counted more Prochlorococcus than there are stars in the Milky Way,” he said.

The SeaFlow, a custom instrument created by University of Washington scientists, uses laser technology to identify Prochlorococcus cells. It sits in a lab in Seattle, Friday, Sept. 5, 2025. (AP Photo/Annika Hammerschlag)

Experts caution about widespread ecological effects

Paul Berube, a researcher at the Massachusetts Institute of Technology who studies this microbe but did not take part in the study, praised the extensive research. He explained that Prochlorococcus has a compact genetic structure, making it less resilient to sudden environmental changes.

“They’re at the very bottom of the ocean’s food chain, supporting everything above — small organisms, fish, and eventually humans,” Berube said. “Altering such a fundamental species is bound to have far-reaching effects.”

The research team explored whether more heat-resistant strains of Prochlorococcus might evolve to cope with warmer conditions. But Ribalet said their models showed that even these strains would likely not survive the most extreme warming scenarios if greenhouse gas emissions remain unchecked.

He emphasized that the study doesn’t consider other stressors like plastic pollution and that the outlook may be more dire than presented. “We leaned toward a best-case estimate,” he said. “Reality could be far worse.”

Francois Ribalet, research professor at the University of Washington, works with the SeaFlow instrument, used to study Prochlorococcus, in his Seattle laboratory on Friday, Sept. 5, 2025. (AP Photo/Annika Hammerschlag)

Steven Biller, an associate professor from Wellesley College, called the projected losses “alarming but entirely feasible.” He noted that Prochlorococcus make up part of the ocean’s “invisible forests,” microscopic life that goes largely unnoticed but is critical to global survival.

“Roughly half of the world’s photosynthesis happens in the ocean, and Prochlorococcus is a major contributor,” Biller explained. “The scale of this potential disruption is staggering.”

All three scientists — Biller, Berube, and Ribalet — agreed that while some microbial species might step in to replace lost functions, the long-term threats to ocean biodiversity and fisheries remain significant.

“The causes of global warming are clear. Scientists unanimously agree,” Ribalet said. “We need urgent action to cut carbon emissions.”

He hopes the study increases awareness of tropical ocean zones, which could serve as early indicators of large-scale ecological changes — and perhaps guide solutions.

“This is one of those times I want to be proven wrong,” Ribalet added. “But the evidence is compelling.”