The surface water of the marine environment has traditionally been viewed as a nitrogen (N) limited habitat, and this has guided the development of conceptual biogeochemical models focusing largely on the reservoir of nitrate as the critical source of N to sustain primary productivity. However, selected groups of Bacteria, including cyanobacteria, and Archaea can utilize dinitrogen (N 2 ) as an alternative N source. In the marine environment, these microorganisms can have profound effects on

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... community production processes and can impact the coupling of C-N-P cycles as well as the net oceanic sequestration of atmospheric carbon dioxide. As one component of an integrated 'Nitrogen Transport and Transformations' project, we have begun to re-assess our understanding of (1) the biotic sources and rates of N 2 fixation in the world's oceans, (2) the major controls on rates of oceanic N 2 fixation, (3) the significance of this N 2 fixation for the global carbon cycle and (4) the role of human activities in the alteration of oceanic N 2 fixation. Preliminary results indicate that rates of N 2 fixation, especially in subtropical and tropical open ocean habitats, have a major role in the global marine N budget. Iron (Fe) bioavailability appears to be an important control and is, therefore, critical in extrapolation to global rates of N 2 fixation. Anthropogenic perturbations may alter N 2 fixation in coastal environments through habitat destruction and eutrophication, and open ocean N 2 fixation may be enhanced by warming and increased stratification of the upper water column. Global anthropogenic and climatic changes may also affect N 2 fixation rates, for example by altering dust inputs (i.e. Fe) or by expansion of subtropical boundaries. Some recent estimates of global ocean N 2 fixation are in the range of 100-200 Tg N (1-2 × 10 14 g N) yr −1 , but have large uncertainties. These estimates are nearly an order of magnitude greater than historical, pre-1980 estimates, but approach modern estimates of oceanic denitrification.