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EMBC Biogeochemical Processes and Climate change 2009 Nitrogen fixation in the Ocean from cyanobacteria Recent studies in fields of biological oceanography, molecular ecology, geochemistry and paleoecology all tend toward a same conclusion: the role of cyanobacteria in Nitrogen cycling in the oceans has been underestimated so far. A low concentration in 15N is the main indicator of the occurrence of biological fixation. Such situation has been observed in tropical and subtropical ecosystems, which are usually highly stable. Nitrogen fixation by cyanobacteria happens in the surface mixed layer of oceans. A specific index N* has been developed, which is “a linear expression of the relative regeneration of nitrate and phosphate” in ocean cycles. A positive value of N* is a signature of biological fixation of nitrogen before it reaches the sediments. Organisms which can fixate N2, making it available for other organisms, are called diazotrophs. N2-fixation is highly correlated to atmospheric carbon absorption. Paleology studies even draw a link between global evolution of the earth climate and the activity of these organisms. Trichodesmium, a non-heterocytous cyanobacteria, is the most studied cyanobacteria genus due to its ease of detection by satellite (light emission). Trichodesmium is found both as microscopic free filaments called trichomes (about 100 cells) and as macroscopic aggregates composed of several hundred trichomes. However, the accuracy of satellite evaluation is controversial. In order to better account for all organisms (other cyanobacteria) able of nitrogen fixation new molecular techniques have been developed, which track a gene associated with the activity of the enzyme Nitrogenase. Nitrogenase is a highly conserved enzymcomplex, which is irreversibly inhibited by molecular oxygen and reactive oxygen species. Diazotrophs have developed mechanisms and morphological traits to avoid the contact of those two molecules; some of these are explained here after: (i) fixing nitrogen under microaerobic conditions (eg. Plectonema spp., Phormidium, and some species of Pseudoanabaena, (ii) temporal separation of photosynthesis and N2-fixation (at night-time); in nonheterocystous filamentous Symploca and Lyngbya majuscula, and the unicellular Gloeothece, Synechoccus and Cyanothece. (iii) temporal and spatial separation of photosynthesis and N2-fixation (at day-time) in Trichodesmium. N2-fixation happens for about 6 hours in the middle of the photoperiod, while photosynthesis is inhibited. Cells can turn photosynthetic activity on or off within 10– 15 min. Individual cells modulate oxygen production and consumption during the photoperiod. (iv) spatial separation. N2-fixation in heterocysts (eg. Anabaena variabilis). Heterocysts are micro-anaerobic cells, characterized by a thick membrane that slows the diffusion of O2. Some species of picoplankton, heterotophic bacteria and diatoms (from endosymbiosis with cyanobacteria) also display such characteristcs. Synechoccus species have drawn the specific attention of scientists due to their high abundance in marine waters. Nitogen fixation by such organisms is thus to take into account for N-cycling. Nitrogen fixation by diazotrophs is limited by phosphorus and iron availability in the water, as these are both involved in nitrogenase enzyme complex. Understanding processes that control nitrogen fixation can allow us to evaluate the capacity of oceans to absorb carbon and, in the long run, to make predictions on consequences of global climate change. However, some aspects of nitrogen cycling are still to study. Specifically, the fate of fixed nitrogen is badly understood as cyanobacteria only undergo grazing by specialized copepods. Other hypotheses focus on the production of DON by extracellular release, viral lysis of cyanobacteria, and remineralization by bacterial exoenzyme activity. Camille Vogel, Maria Klein