Martin Ekman lowered a water sampler into the dark waters above Landsort Deep, the Baltic Sea's deepest point, where tiny organisms invisible to the naked eye are helping fight climate change. At just two micrometers wide—so small they’d stretch across a grain of sand—picocyanobacteria don’t seem like much. But together, they’re moving carbon from the surface to the deep sea in surprising amounts. A new study from Stockholm University shows these microscopic life forms don’t just float aimlessly. Some strains form sticky colonies that clump together, grow heavy, and sink—carrying carbon with them.
This matters because the ocean absorbs about half of all carbon dioxide removed from the atmosphere by photosynthesis, mostly through phytoplankton. When these organisms die and sink, they take that carbon down with them, locking it away in deep waters for decades or longer. This natural process, called the biological pump, helps regulate Earth’s climate. But scientists have long puzzled over how such tiny cells—too small to sink on their own—could contribute meaningfully to this system.
The answer lies in their behavior. The research team, led by Ekman and including scientists from the Marine Ecological Lab, collected samples at different depths in the Baltic Sea, from the sunlit surface down to 75 meters. Using sediment traps and advanced filtering, they separated larger particles from smaller ones. They found that certain strains of Synechococcus, a type of picocyanobacteria, were more likely to form colonies and appear in sinking material. One strain, in particular, was frequently found in the large fraction (>10 micrometers) and in sediment traps—proof it was aggregating and descending.
Genetic analysis confirmed that aggregation isn’t random: it’s a trait tied to specific strains. Some strains stayed mostly as single cells and remained near the surface, while others formed compact colonies that sank rapidly. The study also revealed seasonal shifts—colony-forming strains became more dominant in summer, suggesting they thrive under warmer, more stratified conditions.
This finding could reshape how we predict carbon movement in warming oceans. As climate change leads to warmer, more nutrient-poor seas, picocyanobacteria are expected to grow more abundant. If more of them can form sinking colonies, they could help pull more carbon from the atmosphere—naturally.
"Given that picophytoplankton are expected to increase in abundance in a warmer and more nutrient-poor ocean, understanding mechanisms allowing their biomass to be exported to the deep ocean is important for predicting future atmospheric CO2 levels," Ekman says. The study, published in The ISME Journal, highlights a small but mighty player in Earth’s climate system—one that’s already doing its part, quietly, in the dark waters of the Baltic.