Effects of iron limitation on the expression of metabolic genes in the marine cyanobacterium Trichodesmium erythraeum IMS101

Authors

  • Tuo Shi,

    1. Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA.
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    • Present address: Department of Ocean Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA.

  • Yi Sun,

    1. Department of Molecular Biology and Biochemistry, Waksman Institute of Microbiology, and
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  • Paul G. Falkowski

    Corresponding author
    1. Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA.
    2. Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854, USA.
      *E-mail falko@marine.rutgers.edu; Tel. (+1) 732 932 6555 ext. 370; Fax (+1) 732 932 4083.
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*E-mail falko@marine.rutgers.edu; Tel. (+1) 732 932 6555 ext. 370; Fax (+1) 732 932 4083.

Summary

Iron deficiency in axenic cultures of Trichodesmium erythraeum IMS101 led to significant declines in both nitrogen fixation rates and photochemical energy conversion efficiency, accompanied by downregulation of genes encoding the major iron-binding proteins, including psbA and psbE of photosystem II, psaA and psaC of photosystem I, petB and petC of the cytochrome b6f complex, and nifH. However, the iron-starved cultures remained viable and expression of the metalloprotein genes was partially or fully restored within 3 days following the addition of iron. Both physiological and molecular responses revealed that expression and synthesis of the nitrogen fixation and photosynthetic machinery follow the hierarchy of iron demand; that is, nitrogen fixation was far more susceptible to iron limitation than photosynthesis. Consequently, the nifH transcript exhibited a 1–2  day shorter half-life and two to three times faster degradation rate than that of the photosynthetic genes. Our results suggest that the changes in gene expression are related to the redox state in the shared photosynthetic/respiratory pathway which, when faced with short-term iron deficiency, signals Trichodesmium to selectively sacrifice nitrogen fixation to conserve iron for photosynthetic and respiratory electron transport. The observed functional and compositional alterations represent the compromises in gene expression and acclimation capacity between two basic metabolic pathways competing for iron when it is limiting.

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