Development & Application of Molecular Diagnostics for Fe & P Limitation of Trichodesmium Nitrogen Fixation & Productivity

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Photosynthetic fixation of CO2 in the oceans accounts for approximately half of total primary production. A significant component of this primary production is due to the growth of oceanic cyanobacteria, such as the diazotroph Trichodesmium. For example, in the tropical and subtropical North Atlantic, Trichodesmium spp. are thought to be the most significant primary producers. Additionally, in this region Trichodesmium introduces the largest fraction of new nitrogen to the euphotic zone (~30mg N/m2/day), a value that exceeds the estimated flux of nitrate across the thermocline. Although there are many biogeochemical factors that may impact Trichodesmium productivity and N2 fixation (e.g. nutrients, light, temperature), recent studies have emphasized the critical importance of P and Fe bioavailability. To increase our understanding of how the bioavailability of these elements may constrain primary production and N2 fixation, these PIs developed specific molecular diagnostics for Fe and P limitation in Trichodesmium. The PIs have determined that axenic cultures of P-stressed Trichodesmium colonies induce the enzyme PhoA and Fe-stressed colonies express the protein IdiA. The PIs have also demonstrated that they can detect PhoA activity and IdiA expression in field populations. In this project, laboratory studies will be used to determine how these diagnostics correlate with decreases in both Trichodesmium productivity and N2 fixation. This work will result in the development and application of two well-tested molecular diagnostics for determining if P, Fe, or both, limit Trichodesmium C and N2 fixation. The assay for species-specific PhoA activity, called Enzyme Labeled Fluorescence, utilizes commercially available reagents, and as with IdiA heterologously-expressed in Escherichia coli, the PIs will provide the IdiA protein or antiserum to others. The impact of this research will be tools that can be used by investigators to quickly and easily identify P and Fe limitation in Trichodesmium species from any environment.