Influence de la biodisponibilité des nutriments sur la fixation de N2 et réponse de Crocosphaera watsonii face à la limitation en fer

• Main Author: Jacq, Violaine
• Format: Doctoral or Postdoctoral Thesis
• Language: French; English
• Published: Université Pierre et Marie Curie 2014
• Subjects: Fixation de N2; limitation en fer; Crocosphaera watsonii; poussières désertiques; Atlantique subtropical Nord; métaux traces; N2 fixation; iron limitation; Saharan dust; subtropical North Atlantic; trace metals; North Atlantic
• Online Access: https://archimer.ifremer.fr/doc/00855/96699/105208.pdf; https://archimer.ifremer.fr/doc/00855/96699/

Despite the biogeochemical importance of N2 fixation, which represents the largest source of newly-fixed nitrogen to the open ocean, some uncertainties remain about its controlling factors. Iron (Fe) is widely suspected as a key controlling factor due to the high Fe content of the nitrogenase complex involved in the biological N2 fixation and to its low concentration in oceanic surface seawaters. N2 fixation rates associated with unicellular N2 fixing cyanobacteria (UCYN) were estimated to be ~50% of the total N2 fixation at global scale, but as they have been recently discovered few studies have been conducted on these organisms. We performed culture experiments in order to quantify for the first time the response of an UCYN, Crocosphaera watsonii, to Fe limitation. Reduction of ambient Fe concentration led to significant decreases in growth rate and N2 fixation rates per cell and we observed an adaptive strategy to Fe limitation with a cell volume reduction. Then, the enhancement of growth and activity of C. watsonii under Fe limitation condition after artificial Saharan rain addition highlighted that at least a part of the Fe released by the dust is bioavailable. In subtropical North Atlantic, an important contribution of N2 fixation to new production was observed and we showed that primary production and N2 fixation were globally P-limited. We revealed that trace metals play a key role in controlling N2 fixation in this area. Saharan rain addition stimulated N2 fixation, presumably by supplying these nutrients. All these results contribute to our knowledge of the control of oceanic N2 fixation and provide new insight about interactions between Fe, nitrogen and carbon biogeochemical cycles. La fixation de N2, ou diazotrophie, est un processus biogéochimique majeur en raison de son apport en azote nouveau dans la couche de surface de l’océan. Cependant ses facteurs de contrôle restent mal connus. Le fer, dont les concentrations de surface sont faibles, est un élément potentiellement limitant de la fixation ...