The Relationship Between Iron and Nitrogen Fixation in Trichodesmium spp.
MASSACHUSETTS INST OF TECH CAMBRIDGE
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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 0.4 microns 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 FeP 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.
- Inorganic Chemistry
- Physical and Dynamic Oceanography