Tiny Baltic Sea organisms' colony-forming ability drives deep carbon transport

In the cold depths of the Baltic Sea, something small and invisible is doing work that matters for the entire planet's climate. Picocyanobacteria—organisms so tiny they measure less than two micrometers across—are ferrying carbon from the sunlit surface waters down into the darkness below. A team of researchers at Stockholm University has now shown that how efficiently these microscopic cells make that journey depends almost entirely on whether they travel alone or in crowds.

The discovery emerged from a careful study of the Baltic's Landsort Deep, a site where marine scientists have been watching the water column for more than three decades. Researchers collected samples from surface waters and at depths of 25 and 75 meters, then used sediment traps—devices moored below the sunlit zone—to capture the particles actually sinking toward the seafloor. They separated the collected material by size, isolating the tiniest cells on one filter and larger aggregates on another, then sequenced the genetic material in both fractions. What they found was striking: picocyanobacteria made up a substantial portion of the phytoplankton community, and their contribution to the exported biomass tracked closely with their abundance in surface waters. But not all strains behaved the same way.

Some varieties of Synechococcus, the dominant picocyanobacterium in the Baltic, appeared far more frequently in the larger particle fraction and in the sediment trap material. These were the colony-formers—cells that clumped together into aggregates large enough to sink. Other strains remained mostly as single cells, drifting in the water column without descending. The relationship was direct and measurable: the more a strain tended to aggregate, the more of its biomass ended up in the deep. This aggregation ability also showed seasonal patterns, suggesting it functions as an ecological trait that helps different strains adapt to changing conditions throughout the year.

Why this matters extends far beyond the Baltic. Phytoplankton, despite their invisibility, are responsible for roughly half of all photosynthesis on Earth. When they die and their bodies sink, they carry carbon with them—a process oceanographers call the biological pump. This mechanism is one of the ocean's most powerful tools for removing carbon dioxide from the atmosphere and storing it in the deep. In coastal seas like the Baltic, the remineralization of sinking phytoplankton also creates the anoxic bottom waters that define the ecosystem's character and constraints.

Picocyanobacteria have long puzzled researchers because individual cells are so small they should sink poorly on their own, yet evidence showed they were contributing significantly to carbon export. The Stockholm team's work provides the missing mechanism: these organisms don't need to be large individually if they can stick together. But here's the complication: as oceans warm and become more stratified—layered in ways that limit nutrient mixing—picocyanobacteria are expected to become more abundant. They thrive in these nutrient-poor, warmer conditions. Understanding which strains aggregate and which don't, and how that ability might shift under future ocean conditions, is now essential for predicting how much carbon will actually make it to the depths.

Lead researcher Martin Ekman emphasized that the variation among strains matters precisely because the future ocean will likely be dominated by these tiny organisms. If the strains that proliferate in warmer waters are poor aggregators, carbon export could decline. If they're good at forming colonies, the biological pump might remain robust. The team's work, published in The ISME Journal, was grounded in three decades of monitoring data from the Marine Ecological Lab—a resource that allowed them to check their findings against long-term trends. Co-author Rachel Foster noted that similar aggregation processes should now be investigated in other ocean regions where picocyanobacteria thrive, including the open ocean. The question of how these invisible workers will function in a changing world is no longer academic.