Nutrient assimilation in marine cyanobacteria

Prochlorococcus and its contribution to new production in the Sargasso Sea

This is 4-year project funded by NSF Biological Oceanography. Our main collaborator on this project is Michael Lomas from the Bermuda Institute of Ocean Science.

 

The cyanobacterium Prochlorococcus marinusis 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 Prochlorococcusisolates cannot assimilate NO3-. However, Lomas' group has used flow cytometry and stable isotope tracers to demonstrate direct NO3- assimilationby Prochlorococcusin the Sargasso Sea. In support of these findings, our research 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 Prochlorococcusare capable of NO3- assimilation and can contribute to new production in many oceanic regions – but the overall significance is yet unknown.

This leads to our specific research questions:

  1. Is NO3-a quantitatively important nutrient source for Prochlorococcusand does Prochlorococcus contribute to new production, sensu Dugdale and Goering?
  2. What is the influence of seasonal and vertical variation in nitrogen substrates (NH4+, urea, NO2-, and NO3-) on the genome content of Prochlorococcus and oxidized nitrogen uptake rates?

Our overarching hypothesis is that cell-specific NO3-assimilationrate 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). To answer our questions, we propose the combined use of high-sensitivity nutrient measurements, a flow cytometric assay developed by Lomas to quantify nitrogen assimilationin 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, we will quantify NO3- assimilation and the distribution of NO3- assimilation genes in Prochlorococcusthrough three full seasonal cyclesand 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 our hypotheses. We 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 proposal will be a more complete understanding of the nutritional ecology of Prochlorococcus in field assemblages. We have selected to conduct our study in the Sargasso Sea, because of the wealth of necessary supporting data and logistical infrastructure that this site provides, and because we have already shown that Prochlorococcusis capable of nitrate assimilation in this region.

 

Our proposed research will have broader impacts on many levels. First, the potential for previously unrecognized widespread productivity by Prochlorococcus fueled by NO2- and NO3- assimilation has significant implications for our understanding of the biogeography of Prochlorococcusand 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. Thirdly, little is known about nutrient assimilation rates of any taxonomic groups in the ocean. Thus, the directed experiments that we have proposed are key to the success of ecosystem models as more information is required for specific rate processes and physiological preference for specific nitrogen substrates.

Some relevant papers:

Casey et al.: Prochlorococcus contributes to new production in the Sargasso Sea deep chlorophyll maximum

Martiny et al.: Widespread metabolic potential for nitrite and nitrate assimilation among Prochlorococcus ecotypes