| Summary: | Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2009 Trichodesmium spp. are considered the dominant nitrogen (N) fixing cyanobacteria in tropical and subtropical oceans, regimes frequently characterized by low iron (Fe). Limited information exists about what levels of Fe limit Trichodesmium N fixation. I developed a diagnostic for Fe limitation using quantitative reverse transcription PCR (qRT-PCR) of the Fe stress response gene isiB, which encodes for flavodoxin a non-Fe containing substitute for ferredoxin. I determined that high isiB gene expression corresponded to cell-specific reductions in N fixation rates in both phylogenetic clades of Trichodesmium grown on varying levels of Fe. Using these laboratory-determined thresholds, I assessed Fe limitation of Trichodesmium from the Sargasso Sea, equatorial Atlantic Ocean and Western Pacific Warm Pool in conjunction with other analytical measurements (N, phosphorus (P) and dissolved Fe (<0.4μm filtered)). I found widespread Fe limitation in Trichodesmium from the Pacific Ocean and minimal expression in the North Atlantic Ocean. I also found an inverse correlation between isiB expression and dissolved Fe:P ratios in seawater and data suggesting that most dissolved Fe in seawater, including organic ligand-bound Fe, is available to Trichodesmium. These data support and refine previous model predictions and demonstrate, in situ, the importance of Fe to the marine N cycle. Henry Houghton Fund, the Center for BioInorganic Chemistry, the National Science Foundation (grants OCE- 0220945 and OCE-0623499), a National Defense Science and Engineering Graduate Fellowship and the WHOI Academic Programs Office.
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