Differential aggregation properties among Baltic Sea picocyanobacterial strains impact their export.

Picocyanobacteria contribute significantly to carbon export but the mechanism by which their biomass is transported to depth is unclear. By analyzing 16S rRNA gene sequences from sediment trap material and size-fractionated water samples from different depths in the Baltic Sea, we show that picocyanobacterial contribution to phytoplankton biomass export was largely proportional (70%-90%) to their surface water abundances. However, we also found that both cell aggregation (enrichment in >10 μm fraction) and sedimentation (trap enrichment) varied among Synechococcus ASVs (amplicon sequence variants). A robust linear relationship was identified between ASV enrichment in traps and ASV enrichment in the >10 μm fraction, suggesting that cell aggregation is a direct determinant of Synechococcus sedimentation. Seasonal abundance patterns were similar among the dominant aggregate-forming ASVs but distinct from those of single-cell ASVs, suggesting aggregate/colony formation being an important trait in determining distribution of Synechococcus strains and their response to environmental conditions. Correlating microscopy- and sequence-based abundances further allowed us to propose genotypes for the dominant Baltic Sea colonial picocyanobacterial morphotypes. Our results provide in situ data linking phenotypic characteristics of picocyanobacteria with sedimentation properties and several lines of evidence for a quantitative dependency between surface water picocyanobacterial aggregate/colony formation and sinking. These findings are important for our understanding of mechanisms involved in export of pico-sized phytoplankton biomass from the photic zone to depth and emphasize the need to investigate occurrence and environmental regulation of cell aggregation in other picophytoplankton-dominated waters (e.g. open ocean gyres).