Towards an understanding of diazocyte development and diazotrophy in the marine cyanobacterium Trichodesmium

Abstract: Trichodesmium is a filamentous cyanobacterium of great ecological significance as it contributes significantly to primary production in oceans through an efficient nitrogen fixation strategy, including a unique cell type, the diazocyte. The aim of this thesis was to generate a deeper understanding of the structural and molecular background related to the development of diazocytes and the maintenance of diazotrophy in Trichodesmium erythraeum IMS101.Data obtained demonstrated a diurnal separation of basic physiological processes, with nitrogen fixation and photosynthesis confined to the light and cell division and diazocyte development to the dark period. Diazocyte development was preceded by cell division and expression of the main regulator of heterocyst differentiation, hetR. In non-diazotrophic cultures, subject to nitrogen depletion, a developmental program was initiated in a subset of cells that differentiated into mature nitrogenase containing diazocytes, while addition of combined nitrogen abolished development. A nitrogen depletion signal initially enhanced transcription of the global nitrogen regulator ntcA and was followed by elevated expression of coxBII (respiration) and glnA (primary ammonia assimilation) and pronounced sub-cellular rearrangements. Comparative proteomic analysis further revealed a clear shift in the cellular metabolism depending on the nitrogen regime, with 94 proteins being differentially expressed in diazotrophic (with diazocytes) as opposed to in non-diazotrophic cultures. Proteins up-shifted in diazotrophic cultures were related to supporting the nitrogenase enzyme with reducing equivalents and a lower oxygen tension. In addition, two highly divergent flavodoxins were identified with different expression patterns in response to combined nitrogen additions and the data discourage the use of flavodoxins as indicators of iron stress in Trichodesmium populations.