In a sobering revelation for marine scientists and environmentalists alike, a major new study has found that warming sea temperatures could spell disaster for the world’s smallest and most abundant phytoplankton, Prochlorococcus. The findings, published on September 8, 2025, in Nature Microbiology and reported by the Associated Press, suggest that these microscopic organisms—long thought to be resilient in the face of climate change—are in fact highly vulnerable to rising ocean temperatures. The implications are profound, threatening not only the ocean’s intricate food web but also the planet’s ability to regulate its own climate.
For decades, Prochlorococcus has been something of a superstar in the marine world. These tiny bacteria, invisible to the naked eye, inhabit up to 75% of Earth’s sunlit surface waters. Through photosynthesis, they produce about one-fifth of the planet’s oxygen, making them unsung heroes of the air we breathe. “In the tropical ocean, nearly half of the food is produced by Prochlorococcus,” explained François Ribalet, lead author of the study and research associate professor at the University of Washington’s School of Oceanography. “Hundreds of species rely on these guys.”
But the new research paints a far more precarious picture for these essential organisms. According to the study, 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). That’s not some distant, hypothetical scenario—many tropical and subtropical sea surface temperatures are already trending above average and are projected to regularly surpass 86 degrees Fahrenheit (30 Celsius) during this period. The cascading effects of this decline could ripple through the entire ocean ecosystem.
“These are keystone species—very important ones,” Ribalet told the AP. “And when a keystone species decreases in abundance, it always has consequences on ecology and biodiversity. The food web is going to change.” That’s not just a matter of fewer fish or less seafood for human consumption; it’s about the very stability of marine life as we know it. Prochlorococcus sits at the base of the food web, converting sunlight and carbon dioxide into food for countless other organisms. Remove or weaken that foundation, and the whole structure becomes shaky.
What makes this study especially notable is its robust methodology. Previous assumptions that Prochlorococcus would thrive in warmer waters were based on limited laboratory data. Ribalet and his team, however, took a different approach. Over the course of a decade, they embarked on more than 100 research cruises—equivalent to six trips around the globe—collecting water samples at every kilometer. In total, they counted some 800 billion individual Prochlorococcus cells, using a custom-built device called the SeaFlow. This box of tubes, wires, and a piercing blue laser continuously pulled in seawater, allowing the team to count the microbes in real time. “We have counted more Prochlorococcus than there are stars in the Milky Way,” Ribalet remarked.
The data is as staggering as the implications. Paul Berube, a research scientist at the Massachusetts Institute of Technology who studies Prochlorococcus but was not involved in the work, called the breadth of data “groundbreaking.” He explained to the AP, “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. 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.”
One might wonder: Can Prochlorococcus simply evolve to withstand hotter seas? Ribalet’s team considered this, modeling a hypothetical heat-tolerant strain. The results were not reassuring. Even the most optimistic scenarios, the study found, would “not be enough to fully resist the warmest temperature if greenhouse emissions keep rising,” Ribalet said. And these projections are conservative—they don’t account for the additional stressors like plastic pollution or other environmental pressures. “We actually tried to put forth the best-case scenario,” Ribalet noted. “In reality, things may be worse.”
The uncertainty is unsettling, especially given Prochlorococcus’s pivotal role in oceanic and planetary health. Steven Biller, an associate professor at Wellesley College, described the projected declines as “scary but plausible.” He referred to Prochlorococcus as part of the “invisible forests” of the ocean—tiny organisms most people never think about, but that are essential to 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.”
There are, of course, other types of phytoplankton in the ocean. Could they step in to fill the void left by a declining Prochlorococcus? Ribalet and other experts urge caution. While some species might help compensate for the loss of oxygen and food, they are not perfect substitutes. “Evolution has made this very specific interaction,” Ribalet pointed out. “Obviously, this is going to have an impact on this very unique system that has been established.” The broader risks to biodiversity and fisheries are real and cannot be easily dismissed.
The study’s authors and other scientists agree on one key point: urgent action is needed to address the root cause of the problem. “We know what drives global warming. There is no debate among the scientific community,” Ribalet emphasized. “We need to curb greenhouse gas emissions.”
Ribalet hopes that the findings will bring more attention to tropical oceans, which could serve as natural laboratories for studying warming adaptations and as early warning signals for ecological collapse. “For the first time, I want to be wrong. I would love to be wrong,” he admitted. “But these are data-driven results.”
As the world grapples with the realities of climate change, the fate of Prochlorococcus serves as a stark reminder of the interconnectedness of life on Earth. The invisible forests of the ocean may not capture headlines like melting glaciers or raging wildfires, but their decline could have consequences just as far-reaching—and perhaps even more difficult to reverse.
