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Keywords:

  • Combined nitrogen;
  • Diazocytes;
  • Marine cyanobacteria;
  • Microbiology;
  • Nitrogen fixation

Abstract

Trichodesmium is a marine filamentous diazotrophic cyanobacterium and an important contributor of “new” nitrogen in the oligotrophic surface waters of the tropical and sub-tropical oceans. It is unique in that it exclusively fixes N2 at daytime, although it belongs to the non-heterocystous filamentous segment of the cyanobacterial radiation. Here we present the first quantitative proteomic analysis of Trichodesmium erythraeum IMS101 when grown under different nitrogen regimes using 2-DE/MALDI-TOF-MS. Addition of combined nitrogen (equation image) prevented development of the morphological characteristics of the N2-fixing cell type (diazocytes), inhibited expression of the nitrogenase enzyme subunits and consequently N2 fixation activity. The diazotrophic regime (N2 versus equation image cultures) elicited the differential expression of more than 100 proteins, which represented 13.5% of the separated proteins. Besides proteins directly related to N2 fixation, proteins involved in the synthesis of reducing equivalents and the generation of a micro-oxic environment were strongly up-regulated, as was in particular Dps, a protein related to iron acquisition and potentially other vital cellular processes. In contrast, proteins involved in the S-adenosylmethionine (SAM) cycle, synthesis of amino acids and production of carbon skeletons for storage and synthesis of amino acids were suppressed. The data are discussed in the context of Trichodesmium's unusual N2-fixing physiology.