Physiological and Stoichiometric Studies of Marine Diazotrophs

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Over the last decade, marine N2 fixation has emerged as an important component of the nitrogen (N) cycle of oligotrophic marine ecosystems, and the number of recognized diazotrophic microorganisms has increased dramatically. The planktonic cyanobacterium Trichodesmium was once thought to be the predominant diazotroph in the open ocean, and it is still thought to account for possibly half of the N2 fixation inferred by several geochemical indices. However, we now know that cyanobacterial symbionts of diatoms, and other components of the bacterioplankton also contribute substantially to this process. Current efforts are underway to explicitly represent N2 fixation in marine biogeochemical models, but key parameterizations required for these models are relatively primitive. Photosynthetic parameters for both CO2 and N2 fixation by Trichodesmium have only been coarsely characterized and are non-existent for other photosynthetic diazotrophs. Geochemical analyses that have assessed the extent of excess N formation (i.e. excess relative to P) have assumed a non-Redfield N:P ratio for diazotrophs, the presumed source of the excess N. The value assumed greatly affects the derived rate of N2 fixation, especially when extrapolated to the global ocean. However, the N:P ratio of diazotrophs is poorly constrained. The principle aim of this project will be to systematically evaluate factors that limit diazotrophic growth and N2 fixation in situ. The investigators will examine two photosynthetic marine diazotrophs, Trichodesmium IMS 101 and coccoid cyanobacteria, as model organisms. The investigators will define the fine scale controls of light and P on growth and diazotrophic activity of the model organisms, their interaction with light and nutrients, and their responses with respect to photosynthetic parameters and cellular N:P stoichiometry. The data and ideas produced in this study will increase understanding of the complex interactions between light and nutrients with respect to the globally significant process of nitrogen gas fixation. A better understanding of the physiological controls on N2 fixation in Trichodesmium and diazotrophic coccoid cyanobacteria will in turn lead to a better understanding of controls on global N2 fixation as a whole. Results derived from this study are being sought by and will be of direct use to the marine ecosystem and biogeochemical modeling community, which is attempting to forecast ocean C cycle dynamics. This project will support training of two senior female graduate students, as well as undergraduates. The investigators will also participate in the COSEE-West program, which disseminates recent findings in oceanography to high school teachers.