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

Description from NSF award abstract:
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.

Related Background Publications:
Casey, J. R., M. W. Lomas, J. Mandecki, et al. 2007. Prochlorococcus contributes to new production in the Sargasso Sea deep chlorophyll maximum. Geophysical Research Letters 34:-

Martiny, A. C., S. Kathuria, and P. Berube. 2009. Widespread metabolic potential for nitrite and nitrate assimilation among Prochlorococcus ecotypes.