Fate of N2 fixation

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion EGU Abstract While we now know that marine N 2 fixation is a significant source of new nitrogen (N) in the marine environment, little is known about the fate of this production, despite the importance of diazotrophs to global carbon and nutrient cycles. Specifically, does new production from N 2 fixation fuel autotrophic or heterotrophic growth, facilitate car-5 bon (C) export from the euphotic zone, or contribute primarily to microbial productivity and respiration in the euphotic zone? For Trichodesmium, the diazotroph we know the most about, the transfer of recently fixed N 2 (and C) appears to be primarily through dissolved pools. The release of N appears to vary among and within populations and, probably as a result of the changing physiological state of cells and populations. The 10 net result of trophic transfers appears to depend on the complexity of the colonizing community and co-occurring organisms. In order to understand the impact of diazotro-phy on carbon flow and export in marine systems, we need a better assessment of the trophic flow of elements in Trichodesmium communities dominated by different species, various free and colonial morphologies, and in various defined physiological states. Ni-15 trogen and carbon fixation rates themselves vary by orders of magnitude within and among studies highlighting the difficulty in extrapolating global rates of N 2 fixation from direct measurements. Because the stoichiometry of N 2 and C fixation does not appear to be in balance with the stoichiometry of particles, and the relationship between C and N 2 fixation rates is also variable, it is equally difficult to derive global rates of one 20 from the other. A better understanding of the physiology and physiological ecology of Trichodesmium and other marine diazotrophs is necessary to understand and predict the effects of increased or decreased diazotrophy in the context of the carbon cycle and global change.