Award: OCE-1524482

This project focused on an important group of photosynthetic algae, the diatoms. Diatoms are essential organisms at the base of the marine food web able to balance atmospheric carbon dioxide by sequestering the carbon they use for growth in the deep ocean when their cells sink. Diatoms are non-motile and as such their ability to grow and take up carbon dioxide can be heavily influenced by physicochemical dynamics that can move both the diatoms and the nutrients they require to different areas in the oceans. During the course of the project, a method to evaluate how the species that dominate diatom communities change in response to varying environmental conditions was refined. This refined method was used to evaluate diatom communities in dynamic regions associated with the transition from high nutrient coastal areas to low nutrient offshore areas from multiple coastal environments in the Pacific and Atlantic Oceans. As a result of these analyses, we have gained a better understanding of how diatom communities can change along physicochemical gradients in the North Eastern Pacific Ocean, including sampling from coastal-offshore transects under both upwelling and non-upwelling conditions as well as in the presence of eddies that transport packets of water from the coastal environment offshore. We also have similar analyses from a broad continental shelf associated with the passive margin of the North Western Atlantic Ocean where a driving physical factor influencing both plankton and nutrient movement is the boundary of the Gulf Stream. In addition to evaluating diatom species shifts in response to physicochemical gradients, the results of these analyses were also used to choose abundant and important diatom species to evaluate in iron limitation experiments. Iron is required for the structure and function of proteins essential to diatom growth. Iron concentrations are also known to be low in many areas of the ocean, but questions remain as to both what levels of iron limit diatom growth in the oceans as well as what forms of iron are bioavailable to different diatom species. Throughout the course of the project, multiple diatom species from temperate coastal and both coastal and offshore polar regions were grown under varied concentrations of iron from low values that limit growth to higher values that are considered replete for growth. Not only did these results provide useful information about the growth response of different diatom species to changing iron concentrations, but samples from these experiments were preserved to evaluate the gene expression response of the different diatom species to iron limitation. These samples are being used both to evaluate how different diatom species regulate their metabolism in response to iron limitation as well as to develop species specific quantitative methods for assessing levels of diatom iron limitation in situ. Additionally, as part of this project, natural mixed communities of diatoms have been grown in shipboard incubations with access to different forms of iron. In areas where iron was the main factor limiting growth, different iron sources led to distinct shifts in overall community productivity and diatom species composition. This project has provided valuable information about how communities of essential primary producers in the oceans respond to physicochemical forcing in multiple dynamic near-coastal transition zones. The project also focused extensively on iron limitation and bioavailability of different iron sources of both cultured diatom isolates and in situ mixed communities of diatoms and is expanding the number of species-specific quantitative methods available for assessing diatom iron limitation. Throughout the course of the project numerous young scientists gained valuable training in phytoplankton cultivation, trace metal biogeochemistry, bioinformatics, and molecular biology. The young scientists that received training opportunities and support through this project include two high school students, four undergraduate students, and two graduate students. Two of these young scientists are first generation college students, two are ethnic minorities, and three are women. Two outreach activities teaching kids about DNA barcoding and how to extract DNA from fruit using ?kitchen science? were held with the Virginia Aquarium Mentoring Young Scientists program, which is a program for middle school students in the Hampton Roads area. All members of the project volunteered for the Blue Crab Bowl the regional Ocean Sciences Bowl aimed at getting high school students interested in Ocean Sciences in all project years. Finally, participants in the project regularly visited high schools in the area to do outreach presentations about oceanography and careers in STEM fields. Last Modified: 11/30/2018 Submitted by: Phoebe Chappell