Diatoms, unicellular, eukaryotic photoautotrophs, are one of the most ecologically successful and functionally diverse organisms in the ocean. In addition to contributing one-fifth of total global primary productivity, diatoms are also the largest group of silicifying organisms in the ocean. Thus, diatoms form a critical link between the carbon and silicon (Si) cycles and understanding the factors that impact silica production in these organisms will have implications for biological pump efficient and primary productivity. Three consecutive field campaigns in the California Current Upwelling System were conducted to explore the impact of Si and iron (Fe) limitation on diatom growth and silica production. In this highly dynamic region, we found that taxonomic composition and activity of the eukaryotic phytoplankton assemblage (dominated largely by diatoms) was driven as much, if not more, by allogenic (advection, mixing) factors as autogenic factors (biological/ecological interactions). We also found that Si limitation appears to be a major driver of diatom viral infection and mortality, placing a role for diatom viruses in silicon biogeochemistry for the first time. Previous studies suggest Fe limitation increases silica production and leads to secondary Si limitation of diatom growth and productivity. We explored the relationship between Fe limitation and silica production using manipulative, deckboard incubations during our 2014 field campaign. Under Fe limitation, we observed an increase in the drawdown of Si compared to nitrogen suggesting increased silica production in Fe limited diatoms. To explore the molecular basis for this response, we sequenced the eukaryotic metatranscriptomes and analyzed the expression of genes involved in silicon uptake and nitrogen assimilation. We found a down-regulation of the genes encoding the silicon transporters, the proteins responsible for the uptake of silicon from the extracellular environment into the cell and an up-regulation of genes involved in nitrogen assimilation. At the same time, we found that cellular silica production was not higher under Fe limitation, contradicting previous studies demonstrating that Fe limitation leads to increased silica production. We suggest that the increase in cellular Si:N observed under Fe limitation is rather explained by a decrease in nitrogen assimilation. Results from this project have been communicated through scientific meetings and peer-reviewed publications. Data from all field expeditions are available through the Biological and Chemical Oceanography Data Management Office (https://www.bco-dmo.org/project/ 558198). This project directly supported an early career female research professor and a PhD student, but samples collected during field expeditions also provided data for an additional PhD student and a postdoctoral researcher. Numerous undergraduate students participated in this project, two of which were able to participate in field campaigns and get their first field-based experience. Both of those undergraduates subsequently enrolled in PhD programs in marine microbiology. As part of our Broader Impact efforts to introduce the nature and process of science to middle and high school students, we contracted with Tilapia Film, LLC to produce an educational video on "Asking Testable Questions" as part of the ?Tools of Science" (ToS; http://toolsofscience.org). The ToS aims to introduce science and engineering practices using real-world examples and practicing scientists. Last Modified: 11/16/2018 Submitted by: Kimberlee Thamatrakoln