Award: OCE-1335012

Intellectual Merit: As part of a broader collaborative project with investigators from Stony Brook University and the Bigelow Laboratory for Ocean Sciences, the Dauphin Island Sea Lab and University of California Santa Barbara team confirmed the presence of significant amounts of Silicon in picocyanobacteria through a series of laboratory experiments and field observation. The previously-unknown occurrence of Silicon (Si) in these organisms fundamentally changes our assumption regarding which phytoplankton groups contribute to the biological cycling of Si in a vast majority of the ocean (e.g. subtropical gyre systems). Culture experiments with pure clones from the cyanobacteria genus Synechococcus demonstrate that cellular Si levels and the rate of Si accumulation respond proportionately to changes in the concentration of silicic acid in the external medium. Si per cell is also inversely related to growth rate, implying growth rate dilution, and our studies demonstrated that additional dissolved silicic acid availability does not enhance cellular growth rates. While Si uptake increased linearly with increasing dissolved silicic acid (up to 500 micromolar), which implies diffusive transport of Si into cells at environmentally relevant silicic acid concentrations, there also was evidence that Si may pass through a phosphate transporter under low-silicic acid conditions. Within the cell, Synechococcus had a significant proportion of Si which was water-soluble and the concentrations were high enough (i.e. ~2 millimolar) to imply the existence of a large pool of organically bound silicic acid, as has been hypothesized for diatoms (the main phytoplankton group which uses Si). These culture data have assisted in the development of an operational model for Si accumulation in these organisms. In the field, the picoplankton size class (which includes Synechococcus spp.) was observed to contribute a measurable, and at times significant, proportion of the total biogenic silica standing stock and to its rate of production in the Sargasso Sea. Prior to this study, the fundamental assumption in this system was that diatoms were the only group with a significant contribution to the standing stock and production rate of biogenic silica in these systems. The picoplankton standing stock was consistent in space and time; however, the rate of biogenic silica production in this size-class was more variable. When accounting for differences in the quantity of biogenic silica in each size, the rate of production in the picoplankton was similar to or exceeded that of the diatom size class in many stations. Our results suggest picoplankton may have a small, but relatively stable, contribution to biogenic silica in this region, which underlies a more dynamic microplankton biogenic silica pool driven by diatoms. However, in the Sargasso Sea, the magnitude of biogenic silica production rates in the picoplankton size class are unlikely to increase current estimates of how much biogenic silica is produced per unit area annually. Such a lack of change is due to the selection of standard filtration sizes during studies in the early 1990s, which have been used since, and were small enough to capture nearly all the picoplankton cells considered in this study; therefore, current annual production rates already include some or most of the picoplankton contribution. Broader Impact: During the life of this project, two senior personnel and two technicians were supported. One undergraduate student, from an under-represented ethnic group, was trained during the first two years of the award and presented work at a society meeting. This was the first exposure for the undergraduate to oceanography and this individual has since earned a M.S. degree in Marine Science and is pursuing a Ph.D.; therefore, this project was significant in helping recruit a new oceanographer and provide opportunity for a promising young scientist. A second undergraduate,...