Dimensions: Collaborative Research: Functional Diversity of Marine Eukaryotic Phytoplankton and Their Contributions to the C and N Cycling

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Intellectual Merit: Phytoplankton form the basis of the marine food web and thus are a crucial element in the biological pump whereby CO2 from the atmosphere is sequestered in the deep ocean. For decades, biological oceanography focused on the eukaryotic phytoplankton. In the 1970-80s, it was discovered that the single celled picocyanobacteria are numerically dominant in the oceans and are responsible for a large fraction of ocean photosynthesis. What ensued was the growth of a new paradigm in which the picocyanobacteria dominate upper ocean biology and biogeochemistry. In fact, new data support the classic view that the eukaryotic phytoplankton are disproportionately important in both N and C cycling, even in regions where very small cells dominate the biomass and where the eukaryotes themselves are in the "pico" size fraction. In oligotrophic environments, where very small cells dominate, in situ recycling appears to supply most of the nitrogen (i.e., ammonium) required for primary production. New nitrate supply must balance N loss from the system, however, and new data suggest that this nitrate is an important contribution to phytoplankton N, even in the oceanic deserts. It is not known whether the nitrate supply in these systems is used by the entire assemblage or predominantly by larger phytoplankton (essentially entirely eukaryotic). On the basis of still quite limited molecular surveys, we now recognize that the diversity of both large and small eukaryotic phytoplankton is greater than previously thought and that the most abundant and widespread eukaryotes are probably not in culture and may not be closely related to known cultivated organisms. This project will investigate the taxonomic, genetic and functional diversity of eukaryotic phytoplankton at two North Atlantic sites (subarctic and subtropical) in two seasons.The PIs will use diagnostic microarrays for community analysis based on functional genes (both DNA and RNA) and next generation sequencing (i.e., transcriptomics using 454 technology) to identify the players, both in terms of community composition and activity, and to explore the functional diversity of the natural assemblage. In order to identify which groups are active in C and N assimilation and which N source is being utilized by the different size and functional groups, both filter-separated and flow cytometry-sorted samples will be used to 1) measure 13C primary production and 15N assimilation by incubations with isotope tracers, 2) measure the natural stable N isotope signatures of different taxonomic groups and 3) link the molecular diversity to the functional diversity in C and N transformations. Using flow cytometry linked to mass spectrometry, these investigators have found an unexpectedly strong differentiation in the form of N assimilated by prokaryotes and eukaryotes, with eukaryotes being more dynamic.Integration: This project will investigate the taxonomic, genetic and functional diversity of eukaryotic phytoplankton and to link this diversity and assemblage composition to the carbon and nitrogen biogeochemistry of the surface ocean. Taxonomic diversity will be investigated by identifying the components of the phytoplankton assemblages using molecular, chemical and microscope methods. Genetic diversity will be explored at several levels, including direct sequencing of clone libraries of key functional genes and metatranscriptomic sequencing and microarray analysis of size fractionated/sorted phytoplankton assemblages. Using natural abundance and tracer stable isotope methods, genetic and taxonomic diversity will be linked to functional diversity in C and N assimilation in size- fractionated and taxon-sorted populations.Broader Impacts: The broader impacts of this project include contributions to fundamental research and education: 1) continued development of new advanced methods of isotope analysis in environmental samples, with increasing breadth of applications in biogeochemistry and biodiversity; 2) undergraduate teaching in foundation courses on climate and environmental science to recruit freshmen and sophomore students into the science majors; 3) undergraduate research experience through internships and senior thesis research (a requirement at Princeton) for upper level undergraduates; 4) training the next generation of microbial ecology/ biogeochemistry researchers through classroom and research experience at the graduate level. In addition, a new module will be created (The Forests and Deserts of the Ocean) for the Princeton outreach program for middle school teachers (QUEST, Questioning Underlies Effective Science Teaching).