The ocean's smallest and most abundant creatures can't cope with the warming sea.

Measuring less than one-thousandth of a millimeter, Prochlorococcus are giants. Discovered at the end of the last century, they are responsible for much of the turquoise color of tropical seas. Ahead of terrestrial plants, they are also the primary photosynthetic living beings, metabolizing light to generate organic carbon, the basis of marine ecosystems. As a byproduct, they release 5% of the oxygen available for breathing (their ancestors were the protagonists of the Great Oxidation Event that filled the planet with this element millions of years ago). However, they do not tolerate heat well , and the seas are increasingly experiencing more of it. A study published in Nature Microbiology estimates that, by the end of the century, the abundance of these cyanobacteria will be reduced by half. This decline will trigger cascading effects that are still unknown.

For more than 10 years, a group of oceanographers and marine biologists has traveled some 150,000 nautical miles (around 277,000 km) in around 100 voyages studying phytoplankton (microscopic organisms that float on the surface of the sea). They sought to estimate the abundance of the main cyanobacteria according to latitude and, in particular, to measure the impact of temperature on the cell division and multiplication process of Prochlorococcus . Up to 100,000 cells (they are unicellular organisms) can be found per cubic millimeter of water. To count them, they used a larger version of a device found in any clinical analysis laboratory or hospital: a flow cytometry device.

“Counting such small organisms requires specialized equipment,” says François Ribalet , professor of oceanography at the University of Washington (USA) and first author of the research. “We used a continuous flow cytometer called SeaFlow that shoots laser light at the cells as they pass by. Each Prochlorococcus cell contains chlorophyll that fluoresces when hit by the laser, creating a unique optical signature that we can detect and count,” he adds. Ribalet compares it to an automated microscope that can process tens of thousands of samples per second. “Over the past decade, we’ve analyzed more than 800 billion Prochlorococcus cells this way!”

The results of this voyage confirm that Prochlorococcus like a bit of warmth. They don't exist at the poles or in the coldest seas. In fact, their abundance increases with latitude; the closer to the equator, the more of these cyanobacteria. Their cell division peaks (their replication rate) in the tropical Atlantic and Indian Oceans, where their populations would double every 10.5 hours if the preceding populations didn't die. They also found that the key factor for their multiplication isn't the available nutrients (nitrogen and phosphorus) or the amount of light, but the thermometer: they found that the cell division rate increased exponentially as the sea surface temperature approached 28°C, but they collapsed after that point.

“We haven't directly observed population declines during our 10-year study, as we sampled different locations each year rather than monitoring fixed sites,” Ribalet clarifies. However, he adds, “when comparing measurements of similar temperature ranges in different years and locations, the pattern of thermal sensitivity is remarkably consistent: populations are systematically lower in the warmest waters we encountered.”

By combining their field work with laboratory culture experiments, they have found that Prochlorococcus accelerate their cell division and multiplication starting at 19°C, at a rate that becomes exponential around 28°C. However, once this threshold is exceeded, what was a thermal optimum degenerates into thermal stress: when the water reaches 30°C (as has happened this summer on the Spanish Mediterranean coast ), that growth rate drops to a third, and beyond that, the population begins to decline.

“Phytoplankton is the grass of the sea, the forests of the ocean,” compares Xosé Anxelu G. Morán, a professor at the Gijón Oceanographic Center (IEO/CSIC). Within this group, the main group is Prochlorococcus . “They are so small and each one produces so little chlorophyll that they went unnoticed with traditional microscopy,” he explains. It wasn’t until 1986, when MIT microbiologist Sallie Chisholm’s team used flow cytometry, that these cyanobacteria became visible. “If this and the rest of the phytoplankton didn’t exist, there would be no primary production. Life in the sea depends on zooplankton eating them, fish larvae eating zooplankton, small fish eating…” comments Morán, who was not involved in the study.

Well, according to Ribalet's team's predictions, in the future there will be seas so warm that Prochlorococcus will have disappeared from them. Using the data accumulated over the past decade, they fed a climate model with two alternative scenarios. One, the most optimistic, predicts a carbon dioxide (CO₂) accumulation of 650 parts per million (ppm) by the end of the century. Today it's 424 ppm . The other, the most pessimistic, raises the concentration to 1,370 ppm, which would imply even more pronounced warming.

Whatever happens with emissions, at best, the abundance of Prochlorococcus in tropical seas will drop by 17%. And at worst, it could be reduced by 51%. That's the average. "Our models predict that the most severe declines will occur in the warmer tropical regions, particularly the Western Pacific Warm Pool (around Indonesia, the Philippines, and Papua New Guinea), parts of the central Pacific, the warmer areas of the Indian Ocean, and the Arabian Sea," Ribalet notes.

With heat, cellular metabolism tends to accelerate. This led scientists to believe that, with global warming, good times would come for cyanobacteria. “ Prochlorococcus is a perfect photosynthetic machine to obtain solar energy and convert it into chemical energy,” recalls Laura Alonso, from the Biotechnology and Marine Molecular Ecology Department at the AZTI research center. With a very small genome (DNA requires both nitrogen and phosphorus), its cellular machinery requires very little, “which is why it thrives in regions with few nutrients,” Alonso adds. But that same simplicity, shaped over millions of years of evolution, sets its limits: “ Prochlorococcus are not adapted to new temperatures,” the researcher adds. In laboratory cultures with one strain, Alonso and her team verified how heat reduced the availability of RNA necessary to express genes into proteins.

Neither the study's authors nor other experts on Prochlorococcus are clear about what will happen when they start to become scarce. Their role as oxygen generators isn't among the problems, "since other phytoplankton, such as Synechococcus , will compensate for the loss," says Ribalet. The concern has to do with their role in the food chain. Without these cyanobacteria, the gap would be filled by others, which are much larger. And this detail would have consequences that Morán, from the Oceanographic Center of Gijón, summarizes: "A small organism, like zooplankton, that eats a smaller one [ Prochlorococcus ], won't be able to eat a larger one [ Synechococcus ]." For Alonso, from AZTI, what happens is unpredictable: "Until this in situ study, almost all of them have been done with isolated cultures in the laboratory. Beyond their photosynthetic work, we don't know much about their interactions with other organisms."