Effects of clay minerals on the aggregation and subsequent settling of marine Synechococcus

Cyanobacteria are considered to play an important role in the oceanic biological carbon pump, especially in oligotrophic regions. But as single cells are too small to sink, their carbon export has to be mediated by aggregate formation and possible consumption by zooplankton producing sinking fecal pellets. Here, we study the aggregation and subsequent settling of the ubiquitous marine picocyanobacterium Synechococcus as a model organism in roller tanks and investigate the effects of the clays kaolinite and bentonite in a series of concentrations. We found that (1) Synechococcus cells formed aggregates as large as 1.4 mm in diameter that sank at velocities of up to 440 m d^-1 and 660 m d^-1 with and without ballasting clays, respectively; (2) clay minerals increased the number and reduced the size of aggregates, and their ballasting effects increased the excess density and sinking velocity of aggregates; (3) kaolinite, with its lower cation exchange capacity, resulted in smaller and slower sinking aggregates compared to bentonite; and (4) based on our experimental conditions, Synechococcus could potentially export carbon 2-3 times of that contained in their cells via aggregation, likely due to the scavenging of transparent exopolymer particles and dissolved organic matter. The presented results have relevance for our understanding of the role of cyanobacteria in the ecology and biogeochemistry in today's ocean and also elucidate mechanisms of carbon export in the early Proterozoic ocean.