In situ Pico-Cyanobacterial Growth Rates in the Sargasso Sea Based on Cell-Specific rRNA Measurements

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The picoplanktonic unicellular cyanobacteria Prochlorococcus and Synechococcus are responsible for a significant fraction of the overall primary production in the world's oceans. Despite the close phylogenetic and ecological relationship of these two groups, their respective standing stocks are often inversely related on geographic, seasonal, and vertical scales. This project will (1) develop a convenient and robust method for measuring the in situ growth rates of natural populations of Prochlorococcus and Synechococcus, and (2) apply this method in the field in combination with more tradition growth rate measures in order to clarify the factors that are responsible for regulating the dynamics of these populations in the Sargasso Sea. This project will utilize flow cytometrically-determined group-specific ribosomal RNA (rRNA) content to estimate the in situ growth rates of Prochlorococcus and Synechococcus populations. Whole-cell hybridizations with fluorescently labeled 16S rRNA-targeted peptide nucleic acid probes will be used to measure cellular rRNA. Recent research has shown that when normalized to protein or light scatter (as a proxy for cell biomass), rRNA is a useful index of growth rate in a number of marine Synechococcus strains in the laboratory. This study intends to expand this initial work to include a greater diversity of Synechococcus strains as well as representative Prochlorococcus strains, growing under a range of environmentally relevant conditions. Once the relationship between cellular rRNA indexes and growth rate is established in the laboratory, the approach will be tested in the field by simultaneous application with more traditional (and more cumbersome) growth rate measurements. By so analyzing the growth rates of Prochlorococcus and Synechococcus under contrasting conditions of biomass dominance, the relative importance of physiological factors and food web processes in regulating the observed patterns of distribution will be clarified. Ultimately, this will lead to a better understanding of the forces that structure phytoplankton communities in general, and will increase our ability to predict the effects of environmental changes on these communities. This project will enhance undergraduate education by serving as a vehicle to provide undergraduate students with the opportunity to participate in oceanographic research cruises. Interested Marine Sciences majors (and students in related fields) will enroll in a semester-long seminar course covering relevant topics in oceanography. Those students who successfully complete the course will be given the opportunity to participate in a cruise during the University of Georgia's Maymester, for which they will receive research credit. This experience would help promising students appreciate the diverse and rigorous nature of biological oceanography, understand the process by which environmental scientists frame questions and design studies to answer them, experience the limitations inherent in carrying out and interpreting such studies, and learn about biological oceanographic processes in an exciting and meaningful way. The project will also enhance graduate education by supporting the operation of a flow cytometer devoted exclusively to the analysis of ecologically relevant microbes. Graduate students will gain the sort of hands-on experience with this instrumentation that is unavailable in typical staff-run flow cytometry facilities.