Sequestration of atmospheric carbon in the ocean is intimately tied to the fate of silicon. This element is required for the growth diatoms, tiny single celled plants whose dense silica skeletons cause them to sink to deep waters, taking with them carbon and associated nutrients they accumulated near the surface. However, it has recently been shown that tiny photosynthetic bacteria, the picocyanobacteria, also accumulate silicon. Because these organisms are so tiny, they are less likely to sink. Nonetheless, material budgets suggest that a nontrivial amount of the carbon sinking to deep waters from the surface must be in the form of picocyanobacteria. The presence of Si inside these cells, if it increases the density of aggregates that include them, could be one reason for this apparent paradox. To determine the maximum potential contribution of the picocyanobacterium, Synechococcus, to the oceanic silicon cycle, we studied a region where they are among the major contributors to biomass and primary production. We measured inventories of Si in cyanobacteria and diatom cells, estimated from cell abundance and single cell measurements of cellular Si made using Synchrotron X-ray Fluorescence microscopy (SXRF). We found that picocyanobacteria and diatoms have very different impacts on the Si cycle. On average, about half of the biogenic Si in the picocyanobacterial size fraction is in living cells, whereas most of the Si in the diatom fraction is in non-living detritus. This suggests that Si is rapidly recycled back to the dissolved phase from picocyanobacteria, possibly by protozoan grazers, while the Si detritus produced by diatoms persists for much longer in the environment, allowing it to accumulate. As a consequence, the amount of biogenic Si in the bacterial size fraction is only 16% of total biogenic Si even though specific Si uptake rates in that fraction are similar to that in the diatom fraction. The amount of biogenic Si in the picocyanobacterial size fraction is also relatively constant across sites and times, possibly due to relatively constant productivity and control of populations by protozoan predators and viruses. The uptake of Si and biogenic Si for the diatom size fraction is much more variable by comparison. What is less clear is the mechanism and possible fitness value of Si uptake by the picocyanobacteria. Si uptake by Synechococcus varied with concentration of dissolved silicon in a complex manner, suggesting at least two mechanisms of uptake into or onto the cell. We also observed substantial variability in Si uptake among six cultured strains of Synechococcus. However, when we looked at the genomes for Synechococcus generally we found few genes that could clearly be described as Si-related, and no correlation between the presence of those genes and Si uptake by the strains. There is also some indication from our experiments that Si might be leaking into the cell incidentally via a phosphate channel, as occurs in some land plants. However, once again, there is no apparent correlation between the presence of various types of phosphate uptake genes and Si uptake. In the field, picocyanobacterial community structure at the clade level was not correlated with rates of Si uptake rates or biogenic Si in the cyanobacterial fraction. The very wide variation in cellular Si even with a single culture suggests either substantial plasticity or polymorphism at the substrain level that would not be detected by these approaches. This uncertainty extends to the form of silica within Synechococcus cells. Freeze fracturing cells caused much of the Si to become dissolved, suggesting that much of the Si is not in the form of solid mineral silica. We used x-ray absorbance near edge structure spectroscopy (XANES) to probe cyanobacterial strains grown under different levels of Si. The XANES spectra clearly indicate differences between the form of Si in Synechococcus and that in diatoms, with the former exhibiting more features associated with crystalline forms or discontinuities between different mineral forms of Si. A much more complete library of Si mineral forms is needed to more fully interpret these spectra. Resolving these issues in the future could affect whether picocyanobacteria are possibly useful models for studying biological silicification so as to improve efficiency of silicon chip production methods. In addition to these findings, the funding will result in at least 9 publications, and there have been 8 presentations at major scientific meetings. Three graduate students, three undergrads and two post-docs were involved in the research at Stony Brook. Last Modified: 08/26/2016 Submitted by: Stephen B Baines