ROLE OF SYNECHOCOCCUS TO BIOGENIC SILICA CYCLING IN THE NORTH ATLANTIC

Recently, the ubiquitous marine cyanobacterium, Synechococcus, has been found to accumulate Si from the environment, raising questions about the role of these organisms in the Si cycle. We used synchrotron x-ray fluorescence microscopy (SXRF) to measure quotas of Si within Synechococcus cells at various stations in the North Atlantic, so as to assess their contribution to standing pools of biogenic Si (bSi). Station averages of Si cell quotas ranged widely from 17 to 600 amol cell^-1, which spanned the range that has been observed previously in the equatorial Pacific. Assuming a typical growth rate of 0.6 d^-1 for Synechococcus in the mixed layer, uptake rates of Si measured by ³²Si uptake suggest cell quotas of 19-500 amol cell^-1, in excellent agreement with SXRF measurements. The concentration of Si in these cells amounted 0.3-2.3 nmol L^-1 within the mixed layer, which constituted 50% of the biogenic Si within the <3.0 um size fraction. While only a fraction of the total bSi in the water column was contained within the <3.0 µm size fraction, specific Si uptake rates for this fraction were similar to or larger than those in the >3.0 µm fraction. In contrast, living diatom Si only contributed ~20% to the >3μm bSi pool. These observations suggests that Si incorporated by Synechococcus is more rapidly recycled to the dissolved phase than is diatom silica. While the inventory of bSi in Synechococcus cells may approach that in living diatoms, these cyanobacteria do not contribute proportionally to the detrital bSi pool.