Award: OCE-1131046

Synechoccocus are a group of marine cyanobacteria that are dominant autotrophs in large regions of the global ocean. Despite their diminutive size, these microscopic plants are responsible for much of the oxygen we breathe, and their roles in ocean and global biogeochemistry are of significant interest. Synechococcus are known to require nitrogen, phosphorus, and a suite of trace metals and vitamins for growth. Unlike diatoms, Synechococcus are not thought to require the element silicon or be involved in the ocean cycling of silicon. However in 2012 our group published the surprising observation that Synechococcus cells collected from both the Atlantic and Pacific Oceans, as well as those grown in the laboratory, contain significant amounts of silicon. In many of the populations, cells contained more silicon than phosphorus, despite the broad incorporation of phosphorus into many cellular biochemicals such as nucleic acids, ribosomes and lipid membranes. Additionally, in some parts of the ocean we found that more silicon was associated with Synechococcus than with the diatoms that are universally thought to control the oceanic cycling of silicon. This project was a collaboration between 5 research groups to: assess more broadly the variability in the accumulation of silicon by Synechococcus; determine the cellular distribution and chemical nature of cellular silicon; identify the genetic basis for silicon accumulation; and quantify the contribution of Synechococcus to pools of particulate silicon in the western North Atlantic Ocean. The group at Bigelow Laboratory for Ocean Sciences was primarily responsible for measuring silicon in Synechococcus cells collected during 4 research cruises in the Atlantic Ocean. To do this we used a technique called Synchrotron X-ray Fluorescence microscopy, or SXRF. This approach utilizes a synchrotron facility (in our case, the Advanced Photon Source at Argonne National Laboratory) to generate very intense X-rays that allow us to quantify the elements in individual Synechococcus cells. We found silicon contents to be highly variable in natural cell populations, but the average content among cells at different locations in the ocean were similar. Cells collected deeper in the ocean had less silicon than cells from surface waters. We also used a second synchrotron X-ray technique, X-ray Absorption Near-Edge Spectroscopy (XANES), to study the chemical form of the silicon in samples of cultured Synechococcus. We found that silicon is bound to oxygen in the cells, in a chemical form that is comparable—but not identical—to the form of silicon in diatom shells, also known as opal. The form in Synechococcus appears to be somewhat more crystalline than diatom opal. Our results indicate that silicon is accumulated by most if not all Synechococcus cells in the ocean. A postdoctoral researcher and four undergraduate researchers were trained as part of this research project. Additionally, our collaboration with physicists at Department of Energy national laboratories results in improved methods that benefit researchers in other fields such as biomedical sciences. Last Modified: 04/11/2016 Submitted by: Benjamin S Twining