A new study from Stockholm University finds that tiny photosynthetic bacteria in the Baltic Sea can form clumps large enough to sink, pulling carbon from the surface into deeper water. Researchers sampled picocyanobacteria at multiple depths, used sediment traps, and combined genetic analysis with size fractionation to trace which strains exported the most carbon. They found that a strain's tendency to aggregate directly controls how much biomass leaves the surface layer. This matters because picophytoplankton are expected to become more common as oceans warm and become more stratified. If these small cells dominate, their clumping behavior will play a bigger role in how much CO2 gets stored in the deep ocean versus re-released. The study also relies on over 30 years of monitoring data from the Baltic, giving the findings a solid long-term baseline. The research is published in the ISME Journal and points to a need to check whether similar aggregation-driven carbon export happens in open ocean regions, not just coastal seas.
