Collaborative Research: Prochlorococcus and its contribution to new production in the Sargasso Sea

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The cyanobacterium Prochlorococcus marinus is ubiquitous in the oligotrophic subtropical and tropical oceans and can contribute up to 82% of the primary productivity in certain regions. In contrast to most other phytoplankton, cultured Prochlorococcus isolates cannot assimilate NO3-. However, Lomas' group has used flow cytometry and stable isotope tracers to demonstrate direct NO3- assimilation by Prochlorococcus in the Sargasso Sea. In support of these findings, Martiny and colleagues have shown that Prochlorococcus cells residing in the mixed layer carry genes for NO2- and NO3- assimilation, and that these genes are functional and expressed in field populations. The combined results suggest that uncultured lineages of Prochlorococcus are capable of NO3- assimilation and can contribute to new production in many oceanic regions - but the overall significance is yet unknown.The overarching hypothesis of this project is that cell-specific NO3- assimilation rate is a function of both the ambient nutrient concentrations and the metabolic potential of the cell (i.e. presence of genes encoding for NO2- and NO3- assimilation). The specific research questions of this project are:1) Is NO3- a quantitatively important nutrient source for Prochlorococcus and does Prochlorococcus contribute to new production?2) What is the influence of seasonal and vertical variation in nitrogen substrates (NH4+, urea, NO2-, andNO3-) on the genome content of Prochlorococcus and oxidized nitrogen uptake rates?To answer these questions, PIs will use the combination of high-sensitivity nutrient measurements, a flow cytometric assay developed by Lomas to quantify nitrogen assimilation in specific taxonomic groups, and metagenomics and a qPCR assay to determine the occurrence of nitrite (nirA) and nitrate reductase (narB) genes associated with Prochlorococcus. Using these tools, they will quantify NO3- assimilation and the distribution of NO3- assimilation genes in Prochlorococcus through three full seasonal cycles and over the entire euphotic zone. In addition, these direct measurements will be augmented by manipulative mesocosm experiments (reciprocal transplant and nutrient addition experiments) to explicitly test aspects of their hypotheses. The PIs hope to achieve a mechanistic understanding of direct (variations in the concentration of nitrogen species) and indirect controls (genomic adaptation in Prochlorococcus) on NO3- assimilation rates. One of the most exciting outcomes from this project will be a more complete understanding of the nutritional ecology of Prochlorococcus in field assemblages. The PIs have selected to conduct this study in the Sargasso Sea, because of the wealth of necessary supporting data and logistical infrastructure that this site provides, and because they have already shown that Prochlorococcus is capable of nitrate assimilation in this region.Broader Impacts: This project will have broader impacts on many levels. First, the potential for previously unrecognized widespread productivity by Prochlorococcus fueled byNO2- and NO3- assimilation has significant implications for the understanding of the biogeography of Prochlorococcus and its role in oceanic carbon and nitrogen cycles. Secondly, it will exemplify how genome evolution, cell physiology, and environmental variables interact to shape the biogeochemical role of bacteria in the ocean. The PIs will present their results in a manner that can be easily incorporated in biogeochemical and ecosystem models, including those related to changes in regional and global biological processes in response to climate-driven variability. This project will provide research opportunities for graduate (one at UCI) and undergraduate students. This project will also establish an internationally linked education outreach program to provide high school teachers and students in California and Bermuda with hands-on experience in marine microbial ecology and its relationship to global ocean processes and biogeochemistry.