COLLABORATIVE RESEARCH: Combining Flow Cytometry and Stable Isotope Techniques: A Method to Measure Phytoplankton- and Bacteria-specific Nitrogen and Carbon Uptake

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Multiple lines of evidence challenge the traditional view that phytoplankton are producers and bacteria are consumers in aquatic microbial communities as overly simplistic or incorrect. Many nutritional strategies previously attributed to one microorganism or functional groups are also or instead carried out by other groups of microorganisms. In the project, the investigators will use newly developed analytical approaches to reassess the functional roles of phytoplankton and bacteria in marine ecosystems. Traditionally nitrogen (N) uptake rates have been measured using 15N-tracer techniques that employ glass fiber filters (i.e. GF/Fs). What is seldom appreciated, however, is that a significant and variable fraction of the bacterial community is retained on these filters. As a result, uptake rates that are frequently assumed to represent autotrophic production are in fact confounded with heterotrophic bacterial uptake. Flow cytometry, when combined with other techniques, has a great, and largely untapped, potential for expanding our understanding of competitive interactions because microbial groups can be distinguished optically (either by inherent fluorophores such as pigments or by using fluorescent stains), sorted, and then collected for specific analyses. The investigators will combine culture and field study to complete the development of a coupled flow cytometric sorting - stable isotope method that to accomplish the following objectives:. Quantify the rate of phytoplankton-specific N and carbon (C) uptake. Quantify the rate of bacteria-specific N and C uptake. Conduct a small pilot field study to show the utility of the method in two systems the eutrophic Chesapeake Bay and the oligotrophic Sargasso Sea.The broader impacts will be far reaching by the development of a robust method to quantify phytoplankton-specific and bacteria-specific N and C uptake rates. The ability to further quantify taxa-specific uptake rates (e.g. diatoms vs. dinoflagellates vs. Archaea) has the potential to transform our understanding of N and C biogeochemistry in the marine environment. In addition, the project includes training opportunities for undergraduate and graduate students.