It’s easy to miss the ocean’s smallest inhabitants, yet some play outsized roles in keeping the planet running. One of the most remarkable is Prochlorococcus – a microscopic, single-celled powerhouse so tiny that billions can fit into a single drop of seawater.
The tiny microbes occupy the ocean surface, particularly in tropical and subtropical oceans. They create food using sunlight and carbon dioxide – a process called photosynthesis – and work diligently to sustain much of the ocean.
Now, scientists are concerned. As the Earth warms, recent studies reveal that Prochlorococcus may not be coping with increasing temperatures. And that could have a ripple effect through the whole food chain – from plankton to fish to whales.
Tiny microbes power the oceans
Prochlorococcus is not just any microbe. It’s the most common photosynthesizing organism in the ocean. It helps supply food for tiny animals that are eaten by bigger animals, all the way up the food chain.
These microbes also contribute about 5 percent of all the photosynthesis on Earth – meaning they help take in carbon dioxide and release oxygen, just like trees do.
More than 75 percent of surface ocean waters are packed with Prochlorococcus. That’s why scientists once thought these microbes would thrive in a warmer world. After all, they already live in the warmest parts of the ocean.
Microbes meet their limit
But it turns out there’s a limit. Prochlorococcus prefers temperatures between 66°F and 86°F (19°C–30°C). Once the water gets warmer than that, things start to fall apart. Cell division slows significantly, and populations drop.
In the warmest parts of the ocean, temperatures are expected to rise even more in the next 75 years. Climate models predict many areas will go past that upper limit. That could put Prochlorococcus at serious risk in places where it’s always been most common.
The study was led by François Ribalet, an associate research professor of oceanography at the University of Washington.
“For a long time, scientists thought Prochlorococcus was going to do great in the future,” said Ribalet. “In the warmest regions, they aren’t doing that well, which means that there is going to be less carbon, less food for the rest of the marine food web.”
Tracking microbes across oceans
Over the last 10 years, Ribalet and his team sailed over 150,000 miles and gathered information from nearly 100 research cruises.
They analyzed around 800 billion Prochlorococcus-sized cells. Instead of just growing the microbes in labs, they studied them directly in the ocean using a tool called SeaFlow – a machine that fires a laser through seawater to track tiny cells in real time.
“I had really basic questions,” Ribalet said. “Are they happy when it’s warm? Or are they not happy when it’s warm?”
The team built a model to measure growth and compare conditions like sunlight, nutrients, and temperature. In the end, temperature turned out to be the big factor. Above 86°F (30°C), the microbes’ cell division rate dropped to just one-third of what it was at 66°F (19°C).
Missing defenses against heat
In tropical waters, nutrients are scarce. The ocean looks clear and blue in those areas because there isn’t much in it. But Prochlorococcus survives there because it is tiny and efficient.
Over millions of years, it has shed unnecessary genes and retained only the ones it absolutely needs. That adaptation helped it live in nutrient-poor water.
But that evolution came with a trade-off: the microbe no longer has the genes to protect itself from heat stress.
“Their burnout temperature is much lower than we thought,” Ribalet said. Earlier climate models expected the microbes to keep growing in warmer water. But now it’s clear they cannot.
Another microbe steps in
Another microbe called Synechococcus lives in the same waters. It’s bigger and better at handling heat, but it needs more nutrients.
If Prochlorococcus populations shrink, Synechococcus could become more dominant. Still, it’s unclear whether the rest of the food web would adjust.
“If Synechococcus takes over, it’s not a given that other organisms will be able to interact with it the same way they have interacted with Prochlorococcus for millions of years,” Ribalet said.
Global warming and productivity
The researchers looked at different climate scenarios – from mild to severe. In the tropics, a modest increase in temperature could cut Prochlorococcus productivity by 17 percent. In the worst-case scenario, it could drop by 51 percent.
Globally, the declines range from 10 percent to 37 percent, depending on how much the planet warms.
“Their geographic range is going to expand toward the poles, to the north and south,” Ribalet said. “They are not going to disappear, but their habitat will shift.” That shift could change entire ecosystems in the tropics and subtropics.
Microbe survival depends on diversity
At the same time, there are limits to what this study could capture. The team sampled broad areas and could not test every single strain. It is possible that a heat-tolerant version of Prochlorococcus exists out there, just waiting to be discovered.
“This is the simplest explanation for the data we have now,” Ribalet said. “If new evidence of heat-tolerant strains emerges, we would welcome that discovery. It would offer hope for these critical organisms.”
That hope might come. But for now, the ocean’s smallest worker is sending a clear message – it is feeling the heat. And that could mean trouble for the rest of us who depend on a healthy ocean to survive.
The full study was published in the journal Nature Microbiology.
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