Significant silicon accumulation by marine picocyanobacteria

The marine silicon cycle is thought to be intimately tied to the carbon cycle through its effect on the growth of diatoms. These unicellular algae form substantial blooms in cold, nutrient-rich waters. Their dense, siliceous cell walls promote the sinking of particulate matter, and all the carbon and nutrients contained therein. As such, diatoms are thought to be the primary organisms responsible for the low levels of dissolved silicon observed in the surface ocean and the export of mineral silica to depth. Here, we use synchrotron X-ray fluorescence microscopy to determine the elemental composition of individual diatoms and cyanobacterial cells from the eastern equatorial Pacific and the Sargasso Sea. We show that cells of Synechococcus, a small unicellular marine cyanobacterium that dominates in nutrient-depleted waters, can exhibit cellular ratios of silicon to sulphur, and silicon to phosphorus, approaching those detected in diatoms in the same location. Silicon accumulation was also observed in cultured Synechococcus strains. We estimate that the water column inventory of silicon in Synechococcus can exceed that of diatoms in some cases. We suggest that picocyanobacteria may exert a previously unrecognized influence on the oceanic silicon cycle, especially in nutrient-poor waters.